school

techniques

1. interleaving
   • if multiple topics/subjects, spend some time (e.g. 25-50 minutes) on each topic/subject. retention and transfer improves.
2. error journal
   • focus on mistakes and what you could be doing better. log any wrong answers, solve it, write a short sentence on what was done wrong/how it’s been fixed, and review the whole journal later.
   • check school/src/errorJournal.
   • for methods and routines to revise from the journal, check errJournalTech.md.
3. learning through creating
   • create something with what you just learned. for example, when programming or making psuedocode, make a project. when studying math, form your own math problem. this makes you engage with the material in a different manner and can expose weaknesses.
4. blurting
   • study the info then take it away. then, write everything you remember about the material. this identifies any knowledge gaps and strengthen memory by forcing the brain to retrieve info without looking at notes.
5. strategy studying
   • study different MCQ and SQ techniques.

syllabus links

quick links

To-Do

Assignments

English

• Speech writing practice on school matters: whether school days should be shorter or longer (Mon)

Computer Science

• Linear search and bubble sort worksheet
• Chapter 7 reading

Weak topics

Physics (MCQ)

• Pressure and fluid dynamics
• Thermal physics and surface radiation
• Optics and reflection
• Mechanics and equilibrium (forces, moments, etc.)
• Elasticity and Hooke’s law
• Dynamics, momentum, and impulse

Physics (SQ)

• Volumetric measurements and drop calculations
• Speed-time graphs and distance calculations
• Energy transfers and exam description terminology
• Hooke’s law and spring constant calculations

Chemistry (MCQ)

• Relative atomic mass and isotope calculation
• Acid-base theory and dissociation equations

Chemistry (SQ)

• Moles and quantitative chemistry
• Practical titrations and salt preparation
• Enthalpy and energetics
• Collision theory and exam terminology
• Experimental design and thermometric titrations

Error Journal Revision Techniques

Techniques and routines for getting the most out of the error journal (src/errorJournal), using mdBook notes, Anki, and a whiteboard.

Tool roles

Each tool does one job it’s best at. Don’t make them overlap.

• mdBook notes = knowledge source (explanations, error journal, worked examples)
• Anki = retention tool (spaced retrieval of facts, formulas, definitions)
• Whiteboard = generation tool (testing whether you can reconstruct knowledge from nothing)

Revision methods

1. Error journal → targeted drills

   • Most errors are setup errors, not knowledge gaps (wrong distance from pivot, total length instead of extension, missing graph areas).
   • Convert each error into 3–5 similar practice questions and redo them cold a few days later.

2. Active recall with formula conditions, not just formulas

   • For each formula ($P=\rho gh$, $F=kx$, $F=\frac{\Delta p}{t}$), make cards that test what each variable means and what it excludes.
   • Example: Front: “In $F=kx$, what is $x$?” → Back: “Extension = final length − unstretched length. NOT total length.”

3. Spaced repetition

   • Revisit each topic at increasing intervals (1 day → 3 days → 1 week → 2 weeks). Anki handles this automatically.

4. “Read the question twice” protocol (command words)

   • Highlight command words (describe / explain / state / calculate) before writing anything.
   • For “describe energy transfers” questions, always write: X energy → Y energy, because [mechanism].

5. Blurting / brain dumps

   • Study the note, take it away, write everything you remember. Check against the note and mark gaps in red. Repeat until the red disappears.

6. Teach it back (Feynman technique)

   • Explain concepts aloud in simple language (e.g., why pond shape doesn’t affect pressure, why momentum sign conventions matter). If you can’t explain why, you haven’t consolidated it.

7. Diagram-first practice

   • For moments, mirrors, and speed–time graphs, always redraw the diagram and label distances/areas before calculating.

The integrated workflow

1. Notes → Anki (one direction only)

Mine mdBook notes for cards the same day you write or update them:

• One idea per card.
• Make cards out of your errors first. Each “why it’s wrong” line in the journal is basically a card already.
• Use image occlusion for diagrams (speed–time graphs, moment setups, ray diagrams).
• The mdBook stays the “long explanation” — the card only tests the atom. If a card keeps failing, go back to the note to understand, not to re-memorize.

2. Whiteboard for blurting + problem solving

Use the whiteboard at the start of a session, before opening notes:

1. Pick a topic from a “Topics to work on” heading.
2. Write everything you know from memory — formulas, conditions, the trap you fell into last time, a worked example.
3. Check against the mdBook note. Mark gaps in a different colour; photograph the board.
4. Anything missed → becomes (or updates) an Anki card.

Also do full past-paper questions on the whiteboard, timed, no notes — this catches the setup errors flashcards can’t.

3. mdBook as the living record

• After every past paper/mock, log errors in src/errorJournal with why it’s wrong reasoning.
• “Topics to work on” headings double as the session planner — prioritise topics that appear across multiple files.
• Optionally note which errors already have Anki coverage.

Weekly rhythm

The loop: test → journal error → card the atomic fact → blurt the topic on the whiteboard → repeat.

Physics Multiple Choice Exam (Internal) Errors

Topics to work on

• Pressure and fluid dynamics
  • Distinguishing hydrostatic pressure factors ($P=\rho gh$) from total force ($F=P\times A$). Surface area and pond shape do not affect pressure at a given depth.
• Thermal physics and surface radiation
  • Fixed points: Recalling the standard reference points on the Celsius scale (0°C freezing point, 100°C boiling point) to calculate temperature differences.
  • Thermal absorption: Evaluating surface properties for radiant energy. Shiny white is the best reflector (slower temperature rise), whereas dull black is the best absorber.
• Optics and reflection
  • Plane mirror image properties: Calculating total distance between the object and its image ($d_{\text{total}}=d_{\text{object}}+d_{\text{image}}$) rather than just distance from the mirror surface.
• Mechanics and equilibrium (forces, moments, etc.)
  • Moments: Correctly identifying the perpendicular distance from the pivot point when setting up clockwise vs. anticlockwise moments ($\text{Moment}=F\times d$).
  • Gravitational effects: Differentiating intrinsic properties (mass, density) from gravity-dependent properties (weight, $W=mg$).
• Elasticity and Hooke’s law
  • Spring extension ($x$): Calculating extension ($x=L_{\text{final}}-L_{\text{initial}}$) rather than substituting total spring length into $F=kx$.
• Dynamics, momentum, and impulse
  • Vector directions in momentum: Assigning positive and negative sign values to opposite directions when calculating change in momentum ($\Delta p=m(v-u)$) for colliding/reversing objects before solving $F=\frac{\Delta p}{t}$.

Multiple choice questions

11. Which quantities, in addition to the acceleration of free fall, g, affect the pressure at the bottom of a pond of water?

    • Correct: A (the density of the water and the depth of the pond only)
      • Why it’s CORRECT: The pressure exerted by a fluid at a certain depth is calculated using P = ρgh, where:
        • P is the pressure
        • ρ is the density of the water
        • g is the acceleration of free fall
        • h is the depth of the pond
    • Chose D (the depth of the pond, the density of the water, and the surface area of the pond).
      • Why it’s WRONG: Surface area does not matter. The total force on the bottom of the pond depends on the area, but pressure is force per unit area (P = F/A). Since the area cancels out in the derivation, the surface area or shape of the pond has no effect on the pressure at the bottom.

12. What is the temperature difference between the fixed points on the C temperature scale?

    • Correct: B (100°C)
      • Why it’s CORRECT: The Celsius scale is defined by two reference points, the temperature at which pure water freezes/pure ice melts, set at 0°C, and the temperature at which pure water boils/steam condenses, set at 100°C. To find the difference between these two fixed points, it’s simply 100°C - 0°C = 100°C.
    • Chose A (10°C).
      • Why it’s WRONG: It does not match the answer to the difference between the temperature where pure water boils/steam condenses and the temperature where pure water freezes/pure ice melts.

13. Four thermometers, with their bulbs painted different colors, are placed at equal distances from a radiant heater. Which thermometer shows the slowest temperature rise when the heater is first switched on?

    • Correct: D (shiny white)
      • Why it’s CORRECT: When the heater is switched on, it emits thermal radiation (infared radiation). The rate at which the thermometers heat up depends on how well their bulbs absorb this radiant energy. The two main factors that affect absorption are color (dark = good absorber, light = good reflector) and texture (dull = good absorber, shiny = good reflector).
    • Chose B (dull white).
      • Why it’s WRONG: Even though white is a good reflector of heat, it has a dull texture which is good at absorbing heat. Therefore, it does not have the slowest temperature rise.

14. An object is placed 30cm in front of a plane mirror. Which statement describes the image of the object?

    • Correct: B (The image is the same size and 60cm from the object.)
      • Why it’s CORRECT: Plane mirrors create images with specific properties:
        • The object distance is the same as the image distance. The virtual image appears just as far behind the mirror as the object is in front of it.
        • The magnification is 1. The image is always the exact same size as the original object.
      • The question asks for the distance between the OBJECT and its IMAGE, NOT the distance from the MIRROR.
        • Total dist. = Dist. from obj. to mirror + Dist. from mirror to image
        • Total dist. = 30cm + 30cm = 60cm.
    • Chose A (The image is the same size and 30cm from the object.)
      • Why it’s WRONG: The fundamental rule of plane mirrors is that the image is formed the exact same distance behind the mirror as the object is in front of it. You have to add the two distances of the object from the mirror and the image from the mirror together.

15. A wheelbarrow has a weight W of 140N. Which vertical force F is needed to support the wheelbarrow in the position shown?

    • Correct: A (60N)
      • Why it’s CORRECT: To find the correct vertical force F, you take moment about the wheel (the pivot). For the wheelbarrow to be supported in equilibrium, the clockwise moment must equal the anticlockwise moment.
      • First, find the clockwise moment. Force = 140N, Perpendicular distance from pivot = 1.4m - 0.8m = 0.6m. Clockwise moment = 140N x 0.6m = 84Nm
      • Second, find the anticlockwise moment. Force = F, Perpendicular distance from pivot = 1.4m, Anticlockwise moment = F x 1.4m
      • Third, equate and solve. Anticlockwise = Clockwise. **F x 1.4m = 84Nm. F = 84/1.4 = 60N.
    • Chose B (80N).
      • Why it’s WRONG: When finding the clockwise moment, you used the wrong distance, 0.8m. You were supposed to use 1.4m, which yielded 80N.

16. Which property of an object is a consequence of the effect of a gravitational field acting on it?

    • Correct: D (weight)
      • Why it’s CORRECT: Weight is the gravitational force acting on an object’s mass. It is calculated using the formula, W = mg. Since weight directly depends on the strength of the gravitational field, g, an object’s weight will change depending on where it is in the universe.
    • Chose A (density)
      • Why it’s WRONG: Density is the mass per unit volume, ρ = m/V. Since both mass and volume are independent of gravity, density is as well.

17. A spring, which obeys Hooke’s Law, has an unstretched length of 10cm. A load of 20N is suspended from the spring. The new length of the spring is 36cm. What is the spring constant, k, of the spring?

    • Correct: B (0.77N/cm)
      • Why it’s CORRECT: Hooke’s Law states that the force F applied to a spring is directly proportional to its extension x. The formula is F = kx. where:
        • F is the load/force applied (in this case 20N)
        • k is the spring constant
        • x is the extension (how much the spring stretched and NOT its total length)
      • First, find the extension, x. The extension is final length - initial length. x = 36cm - 10cm = 26cm.
      • Second, rearrange the formula to solve for k. k = F/x.
      • Third, plug in the values into the formula. k = 20N/26cm = 0.77N/cm.
    • Chose A (0.56N/cm).
      • Why it’s WRONG: You have to calculate the extension, NOT the total length. Error is found when calculating k: k = 20N/36cm.

18. A cricket ball has a mass of 0.16kg. The ball travels at 30m/s. The ball is hit by a bat with a force of 10,800N. After being hit, the ball moves off at 30m/s in the opposite direction. For how long was the ball in contact with the bat?

    • Correct: B (0.00089s)
      • Why it’s CORRECT: The problem uses the relationship between force, impulse, and momentum. The formula relating these quantities is F = Δp/t. Rearrange this to t = Δp/F to solve for time.
      • First, calculate the change in momentum, Δp. Because momentum is a vector quantity, direction matters immensely. Let the ball’s initial direction be positive (+30m/s) and the opposite return direction be negative (-30m/s). Δp is calculated as Δp = m(v - u).
        • Δp = 0.16kg x (-30m/s - 30m/s)
        • Δp = 0.16kg x (-60m/s)
        • Δp = -9.6kg • m/s (Negative sign indicates direction of the force; the magnitude 9.6kg • m/s can be used for the time calculation)
      • Second, solve for contact time, t.
        • t = Δp/F
        • t = 9.6kg • m/s / 10,800N
        • t = 0.00089s
    • Guessed A.

Physics Structured Questions (Internal) Exam

Topics to work on

• Volumetric measurements and drop calculations
  • Calculating net volume change ($\Delta V=V_{\text{final}}-V_{\text{initial}}$) prior to finding single-unit averages (e.g. volume per drop).
• Speed-time graphs and distance calculations
  • Determining total distance via complete area under $v-t$ graphs: ensuring inclusion of both triangular ($\frac{1}{2}bh$) and rectangular ($bh$) areas within specified time bounds ($t_1$ to $t_2$).
• Energy transfers and exam description terminology
  • Explicitly describing energy transfer pathways (source energy $\to$ target energy + physical mechanism) rather than merely listing types of stored energy.
  • Explaining non-useful energy dissipation: detailing mechanical friction leading to thermal and sound energy loss.
• Hooke’s law and spring constant calculations
  • Limit of proportionality: Defining the precise point up to which extension remains directly proportional to force ($F\propto x$), distinguishing it from the elastic limit.
  • Graph axis interpretation ($L$ vs. $F$): Calculating spring constant $k$ when length is on the y-axis ($k=\frac{1}{\text{gradient}}$) and subtracting initial unextended length ($L_0$) to determine true extension.

Structured questions

1. A student adds 20 drops of water to the water that is in the measuring cylinder in Fig 1.1 (for reference the volume of water inside is 21cm³). The new volume of water in the measuring cylinder is 25cm³. Calculate the average volume of one drop of water.

   • Correct working:
     • Added vol. = 25cm³ - 21cm³ = 4cm³
     • Avg. = 4 / 20 = 0.2cm³ for one drop
   • Feedback: You wrote 20cm³ when you were trying to find the volume added when the drops were added.

2. Fig 2.3 shows the speed-time graph (view image in 2026/sq) for another car. Calculate the distance travelled by this car between time = 2.0s and time = 6.0s.

   • Correct working:
     • In the graph, draw two lines that go down to 2.0s and 6.0s. That is your area to calculate.
     • In order to calculate the area of the part between 2.0s and 6.0s, you draw a triangle and a rectangle. You can label either of those as A₁ and A₂. A₁ will be the triangle, and A₂ will be the rectangle for this working out.
       • With this, calculate A₁’s area. The formula is (b * h)/2. To apply the formula for speed, s = d/t, you will replace “b” and “h” with “speed” and “time”, forming (s * t)/2.
       • (12 * 4)/2 = 24m
       • Now, calculate A₂’s area. The formula for calculating this area is simply bh, but replace those with the variables inside the formula for calculating distance to make it st.
       • 6 x 4 = 24m
     • With our values for A₁ and A₂, we can simply add those together to find the total distance travelled within 2.0s and 6.0s:
       • 24m + 24m = 48m
   • Feedback: In your previous working, you only found the area for the triangle, leaving out the rectangle, which caused it to be wrong.

3. i) Describe two useful energy transfers when the sailor uses the winch to raise the sail.

   • Correct answers:
     • Chemical energy to kinetic energy. The chemical energy stored in the sailor’s muscles are transferred into kinetic energy of the winch handle and the internal gears.
     • Kinetic energy to gravitational potential energy. The kinetic energy of the winch as it rotates is transferred into kinetic energy of the rope attached to the sail and the sail which lifts the sail upward.
   • Feedback:
     • You simply listed “kinetic energy” and “gravitational potential energy”. This is wrong because the questions asks you to DESCRIBE. The question also asks you for ENERGY TRANSFERS, NOT only the types of energy involved.

4. ii) Describe one non-useful energy transfer when the sailor uses the winch to raise the sail.

   • Correct answer:
     • Kinetic energy to thermal and sound energy. As the sailor turns the winch and the rope tightly wraps around the winch drum, moving parts rub against each other. This causes friction and it also causes heat. Some energy is also transferred as sound waves from the mechanical components and from the rope.
   • Feedback:
     • Same mistakes as before, did NOT describe the energy transfers and only listed one energy type.

5. State what is meant by the limit of proportionality.

   • Correct answer:
     • The limit of proportionality is the point up to which the extension of an elastic object is directly proportional to the force applied to it.
   • Feedback:
     • You wrote “The limit of proportionality is when the elasticity of an elastic object reaches the limit of the elastic limit”. This no longer obeys Hooke’s law.

6. Using Fig. 6.2 (refer to the image in the 2026/sq folder), determine the spring constant of this spring.

   • Correct working:
     • On the graph, weight (F) is on the x-axis and length (L) is on the y-axis. We will calculate the gradient of the line on the graph. The formula will be Δy/Δx.
     • The change in length is basically exstension. Therefore, the gradient formula would be extension / weight, x/F.
     • We need to find the spring constant, so the formula for Hooke’s law becomes k = F/x.
       • The gradient is the upside-down version of the spring constant. Therefore, k effectively becomes k = 1/gradient.
     • First, pick two points on the straight-line section of the graph.
       • Point 1 will be (0N, 0.12m).
       • Point 2 will be (10.8N, 0.58m).
     • Second, calculate Δy and Δx.
       • Δy = 0.58 - 0.12 = 0.46m
       • Δx = 10.8 - 0 = 10.8m
     • Third, find the gradient.
       • Δy/Δx = 0.46/10.8 = 0.043N/m
     • Fourth, invert the gradient to find the spring constant k.
       • k = 1/gradient = 1 / 0.043N/m = approx. 23.25N/m.

Chemistry Multiple Choice Questions (Internal Exam)

Topics to work on

• Relative atomic mass and isotope calculation
  • Calculating weighted averages using isotopic mass and percentage abundance
  • Applying significant figure rules strictly to calculated atomic masses
• Acid-base theory and dissociation equations
  • Differentiating strong acids from weak acids

Multiple choice questions

10. A sample of element Q has two isotopes. Their relative masses and abundances are shown. What is the relative atomic mass of this sample of Q to three significant figures?
    • Correct: A (240)
      • Why it’s CORRECT: In order to calculate the relative atomic mass of the sample, you calculate the weighted average of the isotope masses based on their percentage abundances: (mass₁ x abundance₁) + (mass₂ x abundance₂) / total abundance.
        • Multiply each mass by its abundance. The first isotope would be 15,708 (238 x 66) and the second isotope would be 8,296 (244 x 34).
        • Sum the values and divide by 100%. The total sum would be 24,004 (15,708 + 8,296). After dividing by the total abundance, 24,004 / 100, you would get 240.04.
        • Round to three significant figures (in instructions). 240.04 rounded to 3 significant figures gives 240. Therefore, the correct choice in the question is A.
    • Chose B.
      • Why it’s WRONG: Most likely a guess or wrong calculation.

24. Which row shows the equations for the dissociation of hydrochloric acid and of ethanoic acid in aqueous solution?
    • Correct: A
      • Why it’s CORRECT:
        • Hydrochloric acid is a strong acid. Strong acids dissociate completely in aqueous solution, and complete dissociation is represented by a single forward arrow.
        • Ethanoic acid is a weak acid. Weak acids only partially dissociate in aqueous solution, setting up a dynamic equilibrium. Partial/reversible dissociation is represented by a reversible reaction arrow.
    • Chose C.
      • Why it’s WRONG: In C, the equation for the dissociation of ethanoic acid has a single forward arrow. This is wrong because ethanoic acid partially dissociates.

Chemistry Structured Questions (Internal) Exam

Topics to work on

• Moles and quantitave chemistry
  • Converting mass to particles via Avogadro’s constant
  • Multi-step titration calculations and volumetric unit conversions
• Practical titrations and salt preparation
  • Distinguishing chemical reaction steps (neutralization) from physical separation steps (crystallization)
  • Purification techniques: Using activated charcoal/carbon to absorb indicator dye before crystallization.
• Enthalpy and energetics
  • Counting total individual bonds broken/formed
• Collision theory and exam terminology
  • Explicitly including activation energy, Eₐ, and collision frequency (collisions per unit time) when explaining temperature effects on reaction rate.
• Experimental design and thermometric titrations
  • Describing specific trends *(e.g. “higher temperature increase”) rather than vague impact terms like “affects the temperature”.
  • Explaining post-endpoint temperature drops (reaction completion + excess cold reactant/heat loss).
  • Equipment trade-offs: Burette vs. measuring cylinder

Structured questions

1. (c) Complete Table 1.2.
   • Correct answers (format: x(atomn) y(protn) El(elem) e+/-(electrons))
   • Feedback: You wrote negative for the electrons, which is wrong because there are more protons than electrons. Protons give a positive charge.

2. (e) Calculate the number of atoms in 2.00g of argon. Give your answer in standard form.

   • Correct working:
     • First, find the molar mass (Mr) of Argon. It is approximately 39.95 g/mol.
     • Second, calculate the number of moles. Use the formula:
       • mol = mass / Mr
       • mol = 2.00g / 39.95g/mol = 0.0501 mol
     • Third, calculate the number of atoms. You must multiply the number of moles by Avogadro’s constant, 6.02 x 10²³ atoms/mol.
       • no. of atoms = no. of mol x 6.02 x 10²³
       • no. of atoms = 0.050065 mol x (6.02 x 10²³) = 3.0149 x 10²² atoms
     • Fourth, express in standard form.
       • 3.01 x 10²² atoms
   • Feedback: You went straight into calculating the number of atoms using 2.00g instead of calculating the number of moles. You can’t calculate the number of atoms using the value 2.00g because Avogadro’s constant uses atoms/mol, not atoms/g.

3. (a) Complete the symbol equation for the reaction. Include state symbols.

   • Correct answer:
     • H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) +2H₂O(l)
   • Feedback: You were missing the 2 in 2H₂O.Your symbol equation would be unbalanced without the 2 in front of the H₂O.

4. (b) State the type of exothermic reaction taking place.

   • Correct answer: Neutralization.
     • Neutralization is an acid reacting with a base to form a salt and water is specifically called a neutralization reaction.
     • This fits the symbol equation of sulfuric acid reacting with sodium hydroxide as there is an acid reacting with a base to form sodium sulfate, the salt, and the water (2H₂O).
   • Feedback: Initially, you wrote “crystallization” due to the phrase “dry crystals” in step 4. This would be right IF the question specifically asked for the process in step 4. The question asked for the type of REACTION. The actual chemical reaction occurs in step 3, where the H₂SO₄ is mixed with NaOH to form the salt and water.

5. (e) Calculate the volume of H₂SO₄(aq), in cm³, added in step 3.

   • Correct answer:
     • First, find the moles of H₂SO₄ reacted:
       • From the balanced equation, the molar ratio of H₂SO₄ : NaOH is 1 : 2.
       • mol of H₂SO₄ = mol of NaOH / 2
     • Second, calculate the volume in dm³.
       • Given that the concentration of H₂SO₄ is 0.200 mol/dm³, we can use the volume formula to find the answer.
       • vol = mol / conc = 0.00500 mol / 0.200 mol/dm³ = 0.0250 dm³
     • Third, convert the volume to cm³.
       • vol in cm³ = 0.0250 x 1000 = 25.0 cm³
     • The answer is 25.0 cm³.

6. (g) The dry crystals formed in step 4 are colored and not white. This is because the student should do an additional step between step 3 and step 4. Suggest what the student should do in this additional step to produce white crystals.

   • Correct answer: Add activated charcoal (carbon) to the solution to absorb the indicator, and filter the mixture to remove the charcoal before evaporating the solution.
     • In order to obtain pure white crystals, the student needs to remove the indicator before evaporating.
   • Feedback: You wrote “heat the crystals so it becomes anhydrous.” Both hydrated sodium sulfate (Na₂SO₄ • 10H₂O) and anhydrous sodium sulfate (Na₂SO₄) are NATURALLY white. The crystals turned colored due to the methyl orange indicator added in step 2, which remained in the solution when it was evaporated. Therefore, heating the crystals will NOT remove or decolorize the indicator dye.

7. (b) The equation for the reaction can be represented as shown in Fig 4.2. Table 4.1 shows some bond energies. Use the bond energies in Table 4.1 and the value of ΔH of the reaction to calculate the S=O bond energy in kJ/mol.

   • Correct answers:
     • Calculate the energy needed to break the bonds in the reactants.
       • S-F bonds: 4 x 330 = 1320 kJ
       • O-H bonds: 4 x 460 = 1840 kJ
       • Total reactants = 1320 + 1840 = 3160 kJ
     • Calculate the energy released when the bonds in the products form.
       • Let x equal S=O bond energy.
       • S=O bonds: 2 x x = 2x
       • H-F bonds: 4 x 570 = 2280 kJ
       • Total products = 2280 + 2x kJ
     • Calculate the S=O bond energy.
       • Using the fundamental bond energy enthalpy formula, ΔH = Energy to break bonds (reactants) - Energy released forming bonds (products)
       • Substitute all known values into the equation. -54 = 3160 - (2280 + 2x)
       • Simplify and solve for x.
       • -54 = 3160 - 2280 - 2x
       • -54 = 880 - 2x
       • 2x = 880 + 54
       • 2x = 934
       • x = 934 / 2 = 467 kJ/mol.
   • Feedback: When counting the O-H bonds in water, you wrote that there are 2 total bonds to break instead of 4 O-H bonds, which resulted in a wrong calculation. The product SO₂ contains 2 S=O bonds. You need to account for the S=O bonds in products to prevent miscalculations.

4. (c) The equation for the reaction is shown (see photos directory). State the effect, if any, on the position of equilibrium when the following changes are made. Give a reason for each of your answers.

   • Correct answers:
     • The temperature is increased: Equilibrium shifts to the left because the reverse reaction is endothermic (absorbs heat).
   • Feedback: Since ΔH = -54 kJ/mol, the forward reaction is EXOTHERMIC (gives off heat). Le Chatelier’s principle states that when you increase temperature, the system tries to cool down by shifting in the endothermic direction to absorb heat.

5. (d) Explain, in terms of collision theory, why reducing the temperature decreases the rate of the reverse reaction.

   • Correct answer:
     • Lower temperature means particles have less kinetic energy (and move slower).
     • There are fewer collisions per second (less frequent collisions).
     • Fewer particles have greater energy than or equal to the activation energy (Eₐ).
   • Feedback: Exam mark schemes usually demand the term activation energy (Eₐ) for temperature-rate questions.

6. (d) Explain why the temperature change decreases towards the end of each experiment.

   • Correct answer: The reaction is complete (all the alkali has reacted), so adding excess cold acid cools down the solution (or heat is lost to the surroundings).
   • Feedback: You wrote “temperature decreases because the OH- ions in the sodium hydroxide and the H+ ions in hydrochloric acid have dissociated.” Saying ions have dissociated does NOT explain why cooling occurs once the peak is passed.

7. (f) Explain how the results would be different if a polystyrene cup is used instead of the beaker.

   • Correct answer:
     • The temperature changes would be higher/greater.
     • Polystyrene is a better insulator than glass (or reduces heat less to the surroundings).
   • Feedback: You wrote “the polystyrene cup would affect the temperature changes of solution G and H.” Exam schemes require you to state the direction of change rather than using vague words like “affect,” followed by the scientific reason.

8. (g) Give an advantage and a disadvantage of using a burette rather than a measuring cylinder to add the dilute hydrochloric acid to solution G and solution H.

   • Correct answer:
     • Disadvantage: It is harder to read or set up.
   • Feedback: You left the disadvantage blank.

Biological Molecules

Chemical elements and large molecules

• All living organisms are made of organic molecules containing Carbon (C). These large molecules (polymers) are built from smaller, simpler subunits (monomers).

Food tests

Iodine test for starch

• Add a few drops of iodine solution directly to the sample.
• Negative: Remains yellow-brown.
• Positive: Turns blue-black.

Benedict’s test for reducing sugars

• Add an equal volume of Benedict’s solution to the sample. Must heat the mixture in a water batth for about 5 minutes.
• Negative: Remains blue.
• Positive:
  • Turns green/yellow (low concentration)
  • Turns orange (medium)
  • Turns brick-red (high concentration)

Biuret test for proteins

• Add an equal volume of Biuret solution to the sample. Shake gently.
• Negative: Remains blue.
• Positive: Turns purple/violet.

Ethanol emulsion test for fats and oils

1. Dissolve the sample by shaking it thoroughly with ethanol.
2. Filter or pour the liquid into a clean test tube containing distilled water.

• Negative: Clear, colorless liquid.
• Positive: A milky-white emulsion forms.

DCPIP test for Vitamin C

• Add the sample drop by drop into a fixed volume of blue DCPIP solution.
• Negative: Solution stays blue.
• Positive: The blue solution turns colorless. The fewer drops required to make it colorless, the higher the concentration of Vitamin C.

DNA (deoxyribonucleic acid) structure

• Double helix: A DNA molecules consists fo two strands coiled together to form a double helix (like a twisted ladder).
• Bases: Each strand contains smaller chemical subunits along its length called bases.
• Cross-bonds: The two strands are held firmly together by bonds forming between the pairs of bases.
• Complementary base pairing:
  • Adenine to Thymine
  • Cytosine to Guanine

Enzymes

• A catalyst as a substance that increases the rate of a chemical reaction and is not changed by the reaction.
• Enzymes as proteins that are involved in all metabolic reactions, where they function as biological catalysts

Why enzymes are important

• In living organisms, metabolism must happen very quickly to maintain life processes like respiration, digestion, and excretion. At normal body temps., these reactions would occur far too slowly to sustain life.
• Enzymes lower the activation energy required for these reactions, allowing them to happen fast enough to keep organisms alive.

Enzyme action

Key words

• A substrate is the molecule(s) entering the reaction (the “key”).
• An active site a specifically shaped region on the surface of the enzyme where the substrate binds (the “lock”).
• Enzyme-substrate complex: The temporary structure formed when the substrate binds tightly to the enzyme’s active site.
• Product: The molecule(s) released from the active site at the end of the reaction.

Process

1. The enzyme and substrate collide.
2. The substrate binds to the active site because their shapes are complementary.
3. An enzyme-substrate complex is formed, and the reaction takes place.
4. The products are released. The enzyme emerges completely unchanged and is free to catalyze another reaction.

Enzyme specificity

• Enzymes are highly specific. This means one enzyme can only catalyze one specific reaction.
• The reason for this is entirely due to the complementary shape of the active site. If the substrate’s three-dimensional shape does not perfectly match the fit of the active site, no enzyme-substrate complex can form, and no reaction will occur.

Factors affecting enzyme activity

Temperature

• When an enzyme denatures, the shape of its active site is permanently altered. The substrate can no longer fit into the active site, meaning no effective collisions can happen and the reaction stops.

pH

• Every enzyme has an optimum pH where it functions best. For most cellular enzymes, this is around pH 7 (neutral), but there are exceptions (e.g. pepsin in the acidic stomach works best at a low pH, like pH 2).
  • Going too far or below the optimum pH alters the chemical bonds within the enzyme protein, resulting in the change of the shape and fit of the active site. This is denaturation.

Leaf Structure

Leaf adaptations

• Large surface area maximizes the absorption of sunlight and provides a larger area for carbon dioxide to diffuse out the leaf.
• Its thinness creates a very short diffusion distance for carbon dioxide to travel from the air spaces to the photosynthesizing cells, and allows light to penetrate completely through the leaf.

Dicotyledonous plant

Diagram

Structural adaptations for photosynthesis

Photosynthesis

• Photosynthesis is the process by which plants synthesize carbohydrates from raw materials using energy from light.

Equations

Word equation

$\text{carbon dioxide}+\text{water}\stackrel{\text{light \& chlorophyll}}{\to }\text{glucose}+\text{oxygen}$

Balanced chemical equation

$6{\text{CO}}_2+6{\text{H}}_2\text{O}\stackrel{\text{light \& chlorophyll}}{\to }{\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2$

Chlorophyll

• Chlorophyll is the green pigment found inside chloroplasts within plant cells.
• It absorbs light energy and transfers that light energy into chemical energy for the synthesis of carbohydrates.

Use and storage of carbohydrates

Nitrate and magnesium ions

Nitrate ions

• Essential for amino acid production, which link together to build proteins.
• A deficiency of this is stunted growth and yellowing of older leaves.

Magnesium ions

• Required for chlorophyll production.
• A deficiency of this is chlorosis, where leaves turn yellow between the veins because they cannot produce enough green pigment.

Limiting factors

• A limiting factor is something present in the environment in such short supply that it restricts life processes.
  • In this case, limiting the rate of photosynthesis.
• The three main limiting factors are:
  • light intensity
  • carbon dioxide concentration
  • temperature.

Light intensity

• As light intensity increases, the rate of photosynthesis increases proportionally. Light energy is providing the power for the reaction.
• At a certain point, the graph plateaus. Further increases in light intensity do not increase the rate. This means light is no longer the limiting factory; either temperature or carbon dioxide concentration is in short supply.

Carbon dioxide concentration

• Carbon dioxide is a raw material for the reaction. As carbon dioxide concentration increases, the rate of photosynthesis increases.
• The graph plateaus when the plantt’s chloroplasts are working at maximum capacity or another factor (like light/temp.) becomes limtiing.

Temperature

• Photosynthesis is controlled by enzymes.
• As temperature increases, kinetic energy increases, leading to more frequent successful collisions between enzymes and substrates, boosting the rate.
• Beyond the optimum temperature, the enzymes denature. The active sites lose their shape, the reaction slows down rapidly, and eventually stops.

Practical investigations

Starch test

1. Boil leaf in water for 30 secs. to break down cell membrans and stop all chemical reactions.
2. Boil leaf in ethanol using a water bath to remove the chlorophyll so color changes can be seen clearly.
3. Rinse in cold water to soften the leaf.
4. Add iodine solution.
   • Positive result: Blue-black
   • Negative result: Brown/orange

Investigating requirements for photosynthesis

• Before running these tests, the plant must be destarched by placing it in a dark cupboard for 48 hrs. so it uses up its stored starch.
  • Destarching is the process of removing stored starch from the leaves of a plant before running a photosynthesis experiment.
• Chlorophyll: Use a variegated leaf (a leaf that has more than one color, usually appearing with green parts and white or pale yellow parts, occurs because chlorophyll is not distributed evenly across the leaf). Only the green parts will turn blue-black witth iodine.
• Light: Cover part of a destarched leaf with aluminium foil and expose it to light. Only the uncovered areas will test positive for starch.
• Carbon dioxide: Place a plant inside a sealed bag with soda lime (to absorb carbon dioxide). The leaves will test negative for starch because no carbon dioxide was available for the plant to photosynthesize.

Investigating the effects of environmental factors on rate

• Usually done using an aquatic plant like pondweed.
• Measuring the rate: Count the number of oxygen bubbles released per minute, or collect the gas volume over time using a gas syringe.
• Varying factors:
  • Light intensity: Move a lamp to different distances from the plant.
  • Carbon dioxide concentration: Add varying amounts of sodium hydrogencarbonate to the water.
  • Temperature: Place the boiling tube in water baths set to different temperatures.

Investigating gas exchange with hydrogencarbonate indicator

• Hydrogencarbonate incdicator is sensitive to pH changes caused by changes in carbon dioxide concentration (dissolved carbon dioxide forms a weak acid).
• Atmospheric carbon dioxide levels (equilibrium): Red
• High carbon dioxide levels: Yellow (indicates respiration is happening more than photosynthesis, or it’s dark).
• Low carbon dioxide levels: Purple (indicates rapid photosynthesis is consuming carbon dioxide faster than respiration produces it).
• Experimental setup with an aquatic plant:
  • Tube in light: Purple (photosynthesis > respiration > carbon dioxide decreases)
  • Tube in dark (wrapped in foil): Yellow (respiration only > carbon dioxide increases)
  • Control tube (no plant): Stays red

Absorption

• Absorption is the movement of small, digested food molecules and ions through the walls of the intestine into the blood or lymphatic system.
• The small intestine is where nutrients are absorbed.
  • A common misconception is that water is only absorbed by the large intestine. Most water is actually absorbed in the small intestine alongside dissolved nutrients. The colon (large intestine) also absorbs some of the remaining water to compact undigested material into solid feces.

Villi and microvilli

• To absorb massive quantities of nutrients efficiently before food passes out of the body, the small intestine requires a huge internal surface area. This is achieved through thousands of finger-like projections called villi (sg. villus).
• The villi are folds in the inner lining (mucosa) of the small intestine. They multiply the inner surface area by a factor of many hundreds.
• The microvilli are microscopic foldings on the cell surface membranes of the individual epithelial cells that cover each villus.
• By having both villi and microvilli, the small intestine dramatically increases its surface area : volume ratio. This structural adaption ensures a significantly faster rate of diffusion and active transport, allowing nutrients to move out of the gut cavity and into the bloodstream as quickly as possible.

Structure and adaptaions of villi

Single-cell thick epithelium

• The outer layer of the villus is only one cell later thick, providing a short diffusion distance for food molecules to pass through, accelerating the rate of absorption.

Dense capillary network

• A rich network of microscopic blood vessels runs directly beneath the epithelium.
• They absorb water, soluble carbohydrates (glucose), amino acids, mineral ions, and water-soluble vitamins.
• The continuous flow of blood rapidly carries these absorbed nutrients toward the liver (via the hepatic portal vein), maintaining a steep concentration gradient so diffusion can continue uninterrupted.

Central lacteal

• A small tube located right in the center of the villus, which is a part of the lymphatic system.
• Lipids (fats and oils) are too large and hydrophobic to safely enter standard blood capillaries directly. Instead, digested fatty acids and glycerol are recombined into small fat droplets inside the epithelial cells and absorbed straight into the lacteal. The lymphatic system later empties these lipids into the main bloodstream.

Chemical Digestion

• Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules.

Role of chemical digestion in absorption

• Large food molecules (e.g. starch, proteins, lipids) are polymers that are physically too big to cross the cell membranes of the epithelium lining the alimentary canal.
• Chemical digestion breaks the covalent bonds within these polymers, turning them into tiny, soluble monomers (e.g. glucose, amino acids).
• Once they are small and soluble, they can dissolve in the watery fluids of the digestive tract, pass through the walls of the small intestine, and enter the blood capillaries or lacteals to be transported around the body.

Functions of digestive enzymes

Role of hydrochloric acid

1. Killing harmful microorganisms. The extreme acidity denatures the enzymes and structural proteins of bacteria and other pathogens present in swallowed food, protecting the body from infection.
2. Providing an optimum acidic pH. The main protease enzyme in the stomach requires a highly acidic environment to change into its active shape and catalyze the breakdown of proteins efficiently.

Digestion of starch

• Starch cannot be broken down into glucose all at once. It requires a precise two-step biochemical pathway.

1. Amylase breaks down starch into maltose. This occurs via salivary amylase in the mouth and pancreatic amylase in the duodenum.
2. Maltase breaks down maltose into glucose. This enzyme is located on the membranes of the epithelium lining the small intestine, ensuring glucose is produced directly at the surface where it can be immediately be absorbed.
   • $\text{Starch}\stackrel{\text{Amylase}}{\to }\text{Maltose}\stackrel{\text{Maltase (on epithelium)}}{\to }\text{Glucose}$

Digestion of protein

• Proteins are digested by two different types of protease enzymes that are structurally adapted to function at opposite ends of the pH scale.
• Pepsin, site of action is the stomach.
  • Requires acidic conditions (provided by HCl) to function safely without denaturing. It breaks long protein chains down into smaller polypeptide fragments.
• Trypsin, site of action is the duodenum (small intestine).
  • Secreted by pancreas as part of pancreatic juice. It breaks down the polypeptides into individual, fully soluble amino acids, but it requires alkaline conditions to function.

Neutralizing role of bile

• Bile is an alkaline mixture that neutralizes the acidic mixture of food and gastric juices entering the duodenum from the stomach, to provide a suitable pH for enzyme action.

Diet

• A balanced diet is a diet that contains all the essential nutrients in the correct proportions and right quantities needed to maintain health, growth, and development.

• A balanced diet must include:

  • enough energy (provided by carbohydrates, fats, and proteins)
  • essential amino acids from proteins
  • essential fatty acids from fats
  • micronutrients (vitamins and minerals)
  • water to replace water lost in urine, sweat, breath and feces
  • fibre for moving food from the mouth to the anus along the gut.
  • If eaten in the correct proportions, these components make up a balanced diet which is a healthy diet.

Principal dietary sources and importance

Carbohydrates

• From rice, pasta, bread, potatoes, cereals.
• Primary source of energy. Broken down during respiration to release energy for cellular processes.

Fats and oils

• From butter, cheese, vegetable oils, nuts, fatty fish, meat.
• Long-term energy storage, thermal insulation and making cell membranes.

Proteins

• From meat, fish, eggs, beans, pulses, tofu, dairy.
• Growth and tisue repair. Essential for manufacturing enzymes, anttibodies, and certain hormones.

Vitamin C

• From citrus fruits (e.g. oranges, lemons), strawberries, green vegetables.
• Essential for maintaining healthy skin and gums, critical for collagen production and wound healing.

Vitamin D

• From oily fish, eggs, liver, dairy products, sunlight.
• Needed for the absorption of calcium in the small intestine to build strong bones and teeth.

Calcium and iron

• Calcium comes from milk, cheese, yogurt, green leafy vegetables (e.g. broccoli).
• Calcium is essential for structural strength of bones and teeth, muscle contraction, and blood clotting.
• Iron comes from red meat, liver, spinach, egg yolks, dark chocolate.
• Iron is an essential component of hemoglobin in red blood cells, which binds and transports oxygen around the body.

Fibre (roughage)

• From wholemeal bread, brown rice, oats, fruit, vegetable skins.
• Adds bulk to food in the alimentary canal, providing muscles with something to push against to stimulate peristalsis and prevent constipation.

Water

• From drinking water, juices, milk, water-rich fruits (e.g. melon, tomatoes).
• Acts as a solvent for metabolic/chemical reactions, serves as a transport medium (plasma, urine) and aids in temperature regulation (sweating).

Nutrient deficiency diseases

Scurvy

• Scurvy is caused by a severe deficiency of Vitamin C.
• Key symptoms of scurvy are:
  • severe joint pain
  • bleeding gums
  • loose teeth
  • wounds that fail to heal properly because of the breakdown of collagen fibers.

Rickets

• Rickets is caused by a severe deficiency of Vitamin D.
• Key symptoms of rickets are:
  • soft, weak bones that bend or bow outward under the body’s weight that lead to skeletal deformities
  • dental problems

Portion sizes

• Fruit and vegetables: 5 portions per day
• Starchy carbs: 3-4 portions per day
• Beans, pulses, fish, eggs, meat, and other proteins: 2-3 portions per day
• Dairy and alternatives: 2-3 portions per day

Digestive System

Diagram

The five stages of food processing

1. Ingestion, the taking of substances (e.g. food and drink, into the body).
2. Digestion, the breakdown of food. This involves both mechanical breakdown and chemical breakdown by enzymes).
3. Absorption, the movement of nutrients from the intestines into the blood.
4. Assimilation, the uptake and use of nutrients by cells.
5. Egestion, the removal of undigested food from the body as feces.

Structures and functions of the digestive system

Alimentary canal

• Mouth
  • Function: Site of ingestion.
  • Mechanical digestion: Teeth chew and break food into smaller pieces (increasing surface area)
  • Chemical digestion: Saliva containing the enzyme amylase begins breaking down starch into maltose.
• Oesophagus
  • Function: A muscular tube connecting the mouth to the stomach. It moves the bolus downward via peristalsis.
• Stomach
  • Function: A muscular bag where about 1L of food (but can stretch to accommodate more) is held for a few hours.
  • Mechanical digestion: Churns and squeezes food into a liquid called chyme.
  • Chemical digestion: Secretes gastric juice containing protease (pepsin) to break down proteins into peptides. It also secrets hydrochloric acid which kills harmful bacteria and provides the optimum acidic pH (pH 1.5-2.0) for pepsin to work.
• Small intestine, divided into two main parts: duodenum and ileum.
  • Function of duodenum (first part): Main site of chemical digestion. Enzymes from the pancrease and bile from the liver are released here to complete the breakdown of large molecules.
  • Function of ileum (second part): Main site of absorption. Highly adapted for moving digested nutrients (glucose, amino acids, fatty acids) out of the intestine and into the bloodstream. It has a massive surface area covered in microscopic, finger-like projections called villi.
• Large intestine, divided into the colon, rectum, and anus.
  • The colon is responsible for the absorption of remaining water from undigested material, turning liquid waste into semi-solid feces.
  • The rectum stores the feces temporarily until they are ready to be expelled.
  • The anus is a sphincter muscle controlling egestion; it relaxes to remove undigested food from the body as feces.

Associated organs

• Salivary glands secrete saliva into the mouth to lubricate food and release amylase.
• The pancreas secretes pancreatic juice into the duodenum. This juice contains a cocktail of digestive enzymes:
  • amylase for carb. digestion
  • protease/trypsin for protein digestion
  • lipase for fat digestion.
  • NaHCO to neutralize stomach acid.
• The liver produces bile, which neutralizes stomach acid and emulsifies fats. The liver is also heavily involved in assimilation, where it sorts, processes, and stores nutrients arriving from the blood via the hepatic portal vein (e.g. converting excess glucose to glycogen, or deaminating excess amino acids).
• The gall bladder is a small storage sac that stores bile produced by the liver before releasing it into the duodenum via the bile duct.

Physical Digestion

• Physical digestion is the breakdown of food into smaller pieces without chemical change to the food molecules.
• Physical digestion increases the surface area of food for the action of enzymes in chemical digestion.
  • When large lumps of food are broken into many smaller pieces, the total surface area exposed to the outside increases significantly.
  • This provides a much larger area for digestive enzymes to collide with the food molecules, speeding up the rate of chemical digestion.

Teeth

Diagram of types of human teeth

Diagram of tooth structure

Types of human teeth

• Incisors
  • Shape: Chisel-shaped with a sharp, flat edge. Located at the front of the mouth.
  • Function: Used for biting, cutting, and slicing pieces off chunks of food.
• Canines
  • Shape: Pointed, dagger-like teeth situated next to the incisors.
  • Function: Used for tearing and gripping tough food.
• Premolars
  • Shape: Broad, flat top surface with two pointed ridges (cusps)
  • Function: Used for crushing and chewing food down into manageable pieces.
• Molars
  • Shape: Larger, broader flat top surface with four or five ridges (cusps). Located at the back of the mouth.
  • Function: Used for grinding and chewing food into a smooth paste before swallowing.

Tooth structure

• The enamel is the extremely hard, outer layer covering the crown (visible part) of the tooth. It contains calcium salts and is the hardest substance made by the human body. It protects the tooth from the mechanical wear and tear of chewing.
• The dentine (labeled as dentin) is the layer directly beneath the enamel. It forms the bulk of the tooth structure and is similar in texture to bone but slightly softer than enamel.
• The pulp cavity is the central living region of the tooth. It contains blood vessels (to deliver nutrients and oxygen to the tooth cells) and nerves (to detect pressure and temp. changes).
• The cement is a specialized bone-like substance covering the root of the tooth. Itt helps anchor the root firmly to the surrounding fibers within the jawbone.

Function of stomach in physical digestion

• When the mouth starts physical digestion, the stomach continues it mechanically through churning.
• The stomach wall contains three strong layers of smooth muscle oriented in different directions. These muscles periodically contract and relax, rhythmically squeezing, pounding, and mixing the food with gastric juices.
  • This mechanical action breaks the solid bolus down into a thick, semi-liquid mixture called chyme, maximizing its exposure to stomach acid and pepsin.

Role of bile in emulsification

• Fats and oils are hydrophobic and naturally clump together into large, dense globules when mixed with water in the watery environment of the duodenum.
  • This presents a major obstacle for the water-soluble enzyme lipase, only attacking the outer edges of these giant droplets.

The mechanical action of bile

• Bile is produced by the liver and stored in the gall bladder. It does not contain enzymes, so its action is entirely physical.
• When bile enters the duodenum, it breaks down these big, greasy fat globules into millions of tiny microscopic droplets.
• This physical process is called emulsification.
  • Emulsification does not break any chemical bonds within the lipids. Instead, it drastically increases the total surface area of the fats and oils, giving the enzyme lipase vastly more physical space to bind and rapidly digest lipids into fatty acid and glycerol.

Translocation

• Translocation is the movement of sucrose and amino acids (in phloem) from the leaves (sources) to regions of respiration, growth and storage (sinks).
  • Unlike transpiration which is a passive process driven by the evaporation of water, translocation is an active process. It requires metabolic energy from the plant’s living phloem cells to move dissolved nutrientts (sap) throughout the organism.
  • Transport in translocation is bidirectional, where it can move up or down the plant depending on where the materials are needed.

Sources and sinks

Sources

• The parts of a plant that release sucrose or amino acids into the phloem.
• Examples include:
  • fully grown, photosynthesizing green leaves (producing sucrose)
  • germinating seeds or storage organs (like tubers) when they break down stored food reserves.

Sinks

• The parts of a plant that use or store sucrose or amino acids.
• Examples include:
  • for growth/usage, growing shoot and root tips, developing buds, flowers and expanding leaves (requires materials to build new cells + respire)
  • for storage, developing fruits, seeds, or specialized underground storage organs (e.g. potato tubers, carrots, flower bulbs)

Why some parts of a plant may act as a source and a sink at different times

During the growing season (summer/autumn)

• The leaves are the SOURCE. The leaves are fully developed and photosynthesizing rapidly. They produce far more glucose (converted to sucrose) than they need.
• The roots/tubers are the SINK. The excess sucrose is sent downward to the roots or underground storage organs (e.g. potato tuber) where it is converted into insoluble starch and stored for the winter.

During the early spring (after winter)

• The roots/tubers become the SOURCE. The plant has lost its leaves over winter and cannot photosynthesize. To grow new leaves, it must break down the insoluble starch stored in its roots/tubers back into soluble sucrose. The roots release this sucrose into the phloem.
• The new buds/leaves become the SINK. The newly forming buds and young leaves absorb this sucrose from the phloem, using it for cellular respiration and rapid growth until they are mature enough to photosynthesize on their own.

Transpiration

• Transpiration is the evaporation of water at the surfaces of the mesophyll cells, followed by the diffusion of water vapour through stomata to the atmosphere.

Factors affecting transpiration rate

• A potometer is used to measure the rate of water uptake (which is closely equal to the rate of transpiration).
  • Function: As the leafy shoot transpires, it draws water from the capillary tube. This moves an air bubble along a scale. By measuring the distance the bubble travels in a set amount of time, you can calculate the rate of transpiration.
  • Varying conditions in an experiment:
    • Testing temp.: Place the apparatus under a heat lamp or in rooms of different temps.
    • Testing wind speed: Place a fan at varying distances or speed settings next to a plant.

Structural factors affecting water vapour loss

• Large internal surface area: The spongy mesophyll layer contains a vast network of interconnecting air sppaces. This creates a highly maximized internal surface area for water to evaporate into, accelerating the rate of water vapour accumulation.
• Size and number of stomata: Leaves with more stomata or wider stomatal openings provide more exit pathways, increasing the rate of diffusion out of the leaf.

Transpiration pull

• Transpiration pull is when water moves upwards in the xylem and this pull draws up a column of water molecules, held together by forces of attraction between water molecules.

Mechanism

• Cohesion: Water molecules tend to attract each other, sticking together and moving upwards together.
• Adhesion: The water molecules tend to stick to the inside of the xylem vessel so reducing the chance that the columns of water will be broken.
• Continuous column: Since water molecules cohere strongly, they form an unbroken, continuous column of water stretching all the way from the roots, up through the xylem in the stem, and into the leaves.

Wilting

• If water loss > water uptake, plants wilt because of lack of turgor in cells in stems and leaves.

Detailed explanation

• The water loss (or transpiration) > water uptake causes a water deficit which then causes wilting.
• Water moves out of vacuoles of plant cells. These cells lose their internal fluid pressure and change from being turgid to flaccid.
• Without this turgor pressure pushing against the rigid cell walls, the cells can no longer mechanically support the plant’s weight. The leaves and stem droop downward.

How it helps

• Wilting is a survival mechanism.
  • When the leaves droop downward, their exposed surface area decreases and the stomata often close, which cuts down further water loss.

Water Uptake

Root hair cells

Structure

Functions and adaptations

• A root hair cell’s function is to absorb water via osmosis and mineral ions via active transport from the soil, anchoring the plant firmly.
• It has a large surface area. Each cell has a long, narrow extension that projects out into the soil spaces.
  • This significantly increases the surface area available for absorption, maxmizing the rate of water and mineral uptake.
• It also has a thin cell wall that provides a short diffusion distance for faster movement of water into the cell.

Pathway of water through the plant

• Once water enters the root hair cell, it travels down a water potential gradient across the plant tissues. The exact sequence of structures it passes through is:
  • **Water in soil ** $\to$ Root hair cells $\to$ Cells of cortex $\to$ Xylem $\to$ Mesophyll cells

Process

1. Root hair cells: Water enters from the soil by osmosis because the cytoplasm of the root hair cell has a lower water potential than the surrounding soil water.
2. Root cortex cells: Water moves from cell to cell across the root cortex via osmosis until it reaches the central vasuclar cylinder.
3. Xylem: Water enters the hollow xylem vessels, where it is pulled upwards throught he stem to the leaves by the transpiration stream.
4. Mesophyll cells: Once in the leaves, water leaves the xylem and moves into the surrounding mesophyll cells, where it is used for photosynthesis or evaporates into the air spaces before transpiring out through the stomata.

Investigation of the pathway of water (staining experiment)

• Apparatus needed:
  • a fresh stalk of celery with leaves
  • beaker
  • water
  • water-soluble stain

1. Place the base of the plant stem or celery stalk into the beaker filled with the colored dye.
   • This is to allow the plant to take up the stained water via the transpiration stream.
2. Leave the plant in a warm, well-lit, and well-ventilated area for a few hrs.
   • Warmth and light maximize the rate of transpiration, speeding up water movement.
3. Cut thin cross-sections (transverse cuts) of the steam at various heights using a scalpel.
   • Allows you to see exactly which internal structures have carried the colored water.

Observations

• In the stem, there are distinct colored dots arranged in a ring. They are the xylem vessels within the vascular bundles, demonstrating that water travels up the stem specifically through the xylem.
• In the leaves, the colored dye will eventually spread into the veins, highlighting the continuation of the vascular system from the stem into the leaves.

Xylem and Phloem

Functions of xylem and phloem

• Vascular tissues are responsible for mass flow transport over long distances within a plant. They are split into two distinct systems:
  • the xylem, which transports water and mineral ions from the roots up to the leaves and provides structural support to the plant due to its rigid walls
    • transportation is in one direction only. from roots via stem to leaves
  • the phloem, which transports sucrose and amino acids from the source (where they are made or stored, like leaves) to the sink (where they are used or stored, like roots, flowers, and growing tips).
    • transportation is throughout the whole plant

Diagram of positions of xylem and phloem

Structure of xylem vessels

• Xylem vessels are highly specialized dead cells that form a continuous piping system. Their structures directly enable efficient water transport.

Blood Vessels

Diagram

Structure of arteries, veins and capillaries

How the structure relates to blood pressure

• For arteries:
  • the heart pumps blood into arteries in high-pressure surges. To withstand this without bursting, arteries have thick, reinforced walls.
  • To maintain this presure between heartbeats, their walls contain elastic fibers that stretch when blood surges and recoil to push the blood along.
• For veins:
  • by the time blood reaches the veins, it has lost most of its pressure. Since the pressure is low, thick walls are unnecessary; instead, veins have a wide lumen to minimize resistance and help blood flow easily back to the heart.
  • Because blood moves slowly under low pressure, veins require valves to prevent the backflow of blood due to gravity.

How the structure of capillaries is related to their function

• Diagram:
• Capillaries do not only transport blood, but they are the sites of material exchange between the blood and body tissues.
• They allow the exchange of oxygen, glucose, carbon dioxide, nutrients, and waste products between blood plasma and the surrounding tissue cells.
• Their walls are made of a single layer of endothelial cells (flat cells that form the inner lining of blood vessels). This feature provides an extremely short diffusion distance, allowing substances to move into and out of tissues rapidly.
• Capillaries form vast, intricate networks through every tissue in the body, providing a massive total surface area for efficient diffusion.
• The lumen is so tiny that red blood cells must squeeze through in a single-file line. This slows down the blood flow for maximum time for gas and nutrient exchange to occur.

Main blood vessels of the body.

To and from the heart

• Vena cava: Main vein that collects deoxygenated blood from the body organs and drops it into the right atrium.
• Aorta: Largest artery in the body. Leaves the left ventricle to deliver high-pressure, oxygenated blood to the rest of the circulatory system.

To and from the lungs

• Pulmonary artery: Carries deoxygenated blood away from the right ventricle to the lungs.
• Pulmonary vein: Returns oxygenated blood from the lungs back to the left atrium.

To and from the kidney

• Renal artery: Branches off the aorta to bring oxygenated blood containing metabolic wastes (e.g. urea) into the kidneys for filtration.
• Renal vein: Carries filtered, deoxygenated blood away from the kidneys and joins the vena cava.

To and from the liver

• Hepatic artery: Supplies oxygenated blood directly from the aorta to the liver tissue to fulfill its high oxygen demands.
• Hepatic vein: Carries deoxygenated blood from the liver back to vena cava.
• Hepatic portal vein: An important vessel. It carries nutrient-ricj blood directly from the digestive tract (stomach and intestines) straight to the liver.
  • This allows the liver to sort, store, or detoxify absorbed nutrients before they are distributed to the rest of the body.

Blood

Diagram

Components as viewed in diagrams

• Red blood cells: Very numerous. Circular discs with a lighter center (biconcave shape). Contains no nucleus.
• White blood cells: Larger than red blood cells, fewer in numbers. Has a dark-staining nucleus.
  • Phagocytes: Easily identified by their lobed (segmented) nucleus and an irregular, flexible outer cell membrane.
  • Lymphocytes: Identified by a large, round nucleus that occupies nearly the entire volume of the cell, leaving only a thin rim of cytoplasm inside.
• Platelets: Irregular cell fragments. Appear as small dots scattered between the red blood cells.
• Plasma: Empty space surrounding the blood cells.

Functions of blood components

Roles of blood clotting

1. Prevents excessive blood loss by sealing the broken vessel.
2. Prevents the entry of pathogens into the bloodstream, which could otherwise cause infections.

Mechanism

1. Activating platelets: Damage to the blood vessel lining exposes collagen fibers, causing platelets to adhere to the site and release clotting factors.
2. Protein conversion: These factors trigger a process that converts a soluble plasma protein called fibrinogen into an insoluble fibrous protein called fibrin.
3. Mesh formation: The fibrin strands stretch across the damaged area, forming a sticky, microscopic mesh.
4. Trapping cells: Red blood cells and additional platelets flowing past become trapped within the fibrin mesh, forming a solid plug (clot) that later hardens into a scab.

Circulatory Systems

• The circulatory system is a system of blood vessels with a pump and valves to ensure one-way flow of blood.

Key components

• The heart acts as the central pump, providing the necessary pressure to push blood through the network.
  • The heart and valves ensure a one-way flow of blood around the circulatory system.
• Blood vessels (arteries, veins, capillaries) form a continuous closed loop.
• Valves are seen in the heart and veins. Their crucial function is to prevent the backflow of blood, ensuring a strictly one-way flow.

Single circulation of a fish

• In a fish, blood passes through the heart only once for each complete circuit of the body.

Diagram

Pathway

1. Deoxygenated blood from the body enters the heart.
2. Heart pumps it to the gills (where it picks up oxygen).
3. Oxygenated blood travels directly from the gills to the rest of the body organs.
4. Deoxygenated blood returns to the heart.

Anatomy

• Fish have a single two-chambered heart consisting of one atrium and one ventricle.

Double circulation of a mammal

Diagram

Pathway

• In mammals, blood passes through the heart twice for each complete circuit of the body. It is split into two distinct loops.

1. Pulmonary circuit, heart to lungs to heart. The right side of the heart pumps dexoygenated blood to the lungs to pick up oxygen and release carbon dioxide. The oxygenated blood then returns to the left side of the heart.
   • When the heart pumps blood to the lungs, there is low pressure required, resulting in little resistance. The reasons why are because:
     • the distance from heart to lungs is short
     • the low pressure is also important to protect the delicate lung tissue, and the capillaries are only one cell thick for efficient diffusion, which can be ruptured if the heart pumped at high pressures.
2. Systemic circuit, heart to body to heart. The left side of the heart pumps this oxygenated blood at high pressure to the rest of the body organs. Deoxygenated blood then returns to the right side of the heart.

Advantages of double circulation

• Oxygenated and deoxygenated blood are kept separate.
• Blood is pumped to lungs at low pressure so they are not damaged.
• Blood is pumped to the rest of the body at high pressure to give fast supply of oxygen and nutrients to respiring tissues.

The Heart

Diagram

Structures of the heart

• Atria (sg. atrium): The two upper chambers. Thin walls and receive blood entering the heart from the veins.
• Ventricles: Two lower chambers. They have thick muscular walls and pump blood out of the heart into the arteries
• Septum: A solid wall of muscle that separates the left and right sides of the leart.
  • It prevents oxygenated blood (left side) from mixing with deoxygenated blood (right side), ensuring the body receives the maximum possible concentration of oxygen.
• Valves: Flaps of tissue that keep blood flowing in one direction.
  • Atrioventricular (AV) valves: Situated between the atria and ventricles. They stop blood from flowing back into the atria when the ventricles contract.
  • Semilunar (SL) valves: Found at the entrances to the pulmonary artery and aorta. They prevent blood from flowing back into the ventricles when the heart relaxes.
• Coronary arteries: A network of blood vessels running across the outside of the heart. They branch off the aorta to supply the heart muscle tissue itself with its own constant supply of oxygenated blood and glucose.
• Blood is pumped AWAY FROM the heart by the ARTERIES.
• Blood is returned TO the heart by the VEINS.

Relative thickness of the ventricles and atria

Monitoring heart activity

• The activity of the heart may be monitored by:
  • ECG (electrocardiogram): Traces the electrical impulses passing through the heart muscle during the cardiac cycle.
  • Pulse rate: Measuring the expansion and recoil of an artery (commonly at the wrist or neck) per minute as the left ventricle pumps blood.
  • Listening to sounds of valves closing: Listening directly to the lub-dub sounds of the internal heart valves closing and opening

Effect of exercise

• When you exercise, your muscles contract much more frequently and powerfully.
• Active muscles require a significantly higher amount of energy, which they release via aerobic respiration.
• To sustain this, the muscles need an increased supply of oxygen and glucose. They also produce carbon dioxide waste much faster, which must be removed.
• The brain detects the increase of carbon dioxide in the blood and signals the heart to beat faster and contract harder (increasing stroke volume). This speeds up blood circulation to fulfill the metabolic demands of the working tissues.

Coronary heart disease

• Coronary heart disease is caused by blockage of the coronary arteries that supply the heart with glucose and oxygen.

Risk factors

• Diet: Consuming high amounts of saturated fats and cholesterol increases plaque buildup.
• Lack of exercise: Weakens the heart muscle and contributes to high blood pressure and obesity.
• Smoking: Nictonie constricts blood vessels and damages their linings, making plaque buildup more likely. Carbon monoxide reduces the blood’s oxygen-carrying capacity.
• Stress: Releases hormones that chronically elevate blood pressure, putting strain on the arteries.
• Genetic predeposition: A family history of cardiovascular disease can mean naturally higher blood pressure or cholesterol levels.
• Age: The risk increases naturally as blood vessels lose their elasticity over time.
• Sex: Males statistically face a higher risk than pre-menopausal females due to the protective effects of oestrogen.

Prevention (roles of diet and exercise)

• Diet adjustments: Replacing saturated fats with unsaturated fats, reducing salt intake (lowers blood pressure), and increasing dietary fiber helps lower blood cholesterol levels and reduces the rate of plaque buildup.
• Regular exercise: Strengthens the heart muscle, increases cardian efficiency (lower resting heart rate), lowers systemic blood pressure, and helps metabolize excess lipids/cholesterol.

Mechanics of a heartbeat

1. Contraction of atria: The muscles of the left and right atria contract simutaneously. This squeezes blood through the open atrioventricular valves into the ventricles. The semilunar valves remain shut.
2. Contraction of ventricles: The ventricles contract from the bottom up. This rise in pressure forces the atrioventricular valves shut to prevent backflow to the atria and also to create the first “lub” sound of the heartbeat. The pressure forces the semilunar valves open, shooting blood into the pulmonary artery and the aorta.
3. Heart relaxes: The heart muscles relax. To prevent the high-pressure blood in the arteries from slipping back down into the ventricles, the semilunar valves snap shut to create the second “dub” sound. Blood from the vena cava and pulmonary vein trickles back into the passive atria, and the cycle repeats.

Drugs

• A drug is any substance taken into the body that modifies or affects chemical reactions in the body.

Use of antibiotics

• Antibiotics are a distinct class of drugs used to treat infections.
  • They are used for the treatment of bacterial infecttions.
• They work by **disrupting the cellular processes of bacteria (e.g. preventing them from building cell walls/reproducing), killing them or stopping their growth.
• Antibiotics do NOT affect viruses.

Why antibiotics can’t kill viruses

• Bacteria are living cells with their own metabolic pathways and structures (such as cell walls).
• Viruses are non-living particles that do not have their own cell walls, cell membranes, or independent transport systems. Instead, they hijack your body’s own cells to reproduce.
  • Antibiotics target bacterial structures specifically, rendering it useless against viral infections (e.g. common cold, flu, chickenpox).

Antibiotic resistance

Issue

• Some bacteria possess genes that make them resistant to specific antibiotics. When an antibiotic is used, these resistant bacteria are not killed, which significantly reduces the effectiveness of the drug.

Development of antibiotic resistance (natural selection)

1. Random mutation. Within a population of bacteria, some individuals undergo random genetic mutations. A mutation may accidentally give a bacterium a gene that makes it resistant to a specific antibiotic.
2. Selection pressure. When a person takes an antibiotic, the drug acts as a selection pressure. It kills off all the normal, vulnerable bacteria.
3. Survival. The mutant, resistant bacteria survive the antibiotic treatment.
4. Reproduction. With their competitors wiped out, the surviving resistant bacteria have more resources. They multiply rapidly, passing on the gene for antibiotic resistance to their offspring.
5. Spread. Over time, the entire population becomes resistant, creating a strain of superbugs.

MRSA (Methicillin-Resistant Staphylococcus aureus)

• MRSA is an example of a dangerous bacterium that has developed resistance to many commonly used antibiotics. It is problematic in hospitals where open wounds and weakened immune systems are common.

Limiting the development of resistant bacteria

• No prescriptions for viral infections: Doctors shoulld never prescribe antibiotics for colds or sore throats caused by viruses.
• Complete the full course: Patients must finish their entire prescribed course of antibiotics, even if they feel better. Stopping early leaves the most resillient bacteria alive to mutate and replicate.
• Restrict agricultural use: Avoid using antibiotics as growth promoters in livestock farming, as this introduces low levels of drugs into the environment, accelerating resistance.
• Hygeine controls: Maintaining strict hygeine in medical settings prevents the spread of existing resistant strains like MRSA.

Aerobic Respiration

• Aerobic respiration is the series of chemical reactions occurring inside living cells that use oxygen to completely break down nutrient molecules (primarily glucose) to release energy for metabolic processes.
  • Since it completely oxidizes glucose, aerobic respiration releases a relatively large amount of energy compared to anaerobic respiration. Most of these reactions take place inside specialized cellular structures called mitochondria.

Equations of aerobic respiration

Word equation

glucose} + oxygen -> carbon dioxide + water

Balanced chemical equation

C6H12O6 + 6O2 -> 6CO2 + 6H2O

Key components

• The reactants are glucose from digested food and oxygen which is absorbed by gas exchange surfaces.
• The products are carbon dioxide and water (both are metabolic waste products excreted by the body).
• Useful energy is transferred to the cell in the form of ATP.

Anaerobic Respiration

• Anaerobic respiration is the series of chemical reactions in cells that break down nutrient molecules to release energy without using oxygen.
  • Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration.

Equations

In yeast

• Word equation:
  • glucose -> alcohol + carbon dioxide
• Balanced chemical equation:
  • C6H12O6 -> 2C2H5OH + 2CO2

In human muscles during vigorous exercise

• Word equation:
  • glucose -> lactic acid

Lactic acid

• Lactic acid builds up in muscles and blood during vigorous exercise causing an oxygen debt.
  • High concentrations of lactic acid can cause muscle fatigue and cramps.
• This lactic acid build up creates an oxygen debt. This is the extra volume of oxygen required by the body after exercise to completely break down the accumulated lactic acid.

How oxygen debt is removed after exercise

1. Continuation of a fast heart rate: Your heart continues to beat quickly after exercise to rapidly transport lactic acid via the bloodstream away from your muscles and carry ti to the liver.
2. Continuation of deeper and faster breathing: You continue to breathe heavily to take in extra oxygen. This oxygen is absorbed into the blood and transported directly to the liver.
3. Aerobic respiration of lactic acid in the liver: Once inside the liver, the extra oxygen is used to aerobically respire the lactic acid, breaking it down safely into carbon dioxide and water.

Summary

Gas Exchange

Diagram

Parts of the respiratory system

• Larynx: Located at the top of the trachea. Contains vocal cords used to produce sound.
• Trachea: Main airway leading from throat to lungs.
  • Function of cartilage: Trachea is lined with C-shaped rings of cartilage. These rings keep the trachea open at all times, preventing it from collapsing inward whent eh pressure drops during inhalation.
• Bronchi: Trachea splits into 2 tubes, the left and right bronchi, each leading into a lung.
• Bronchioles: Smaller, highly branched tubes branching off the bronchi. Distributes air throughout all parts of the lungs.
• Alveoli: Tiny air sacs at the end of the bronchioles where gas exchange actually takes place. Surrounded by a network of associated capillaries.
• Diaphragm: A dome-shapped sheet of muscle separating the thorax (chest cavity) from the abdomen.

Features of gas exchange surfaces

1. Large surface area: Millions of alveoli collectively create a massive surface area, allowing large amounts of gas to diffuse simultaneously.
2. Thin surface: The walls of both the alveoli and the capillaries are only one cell thick. This ensures a very short diffusion distance for oxygen and carbon dioxide.
3. Good blood suppply: A dense network of capillaries constantly moves blood past the alveoli. This brings carbon dioxide-rich blood to the lungs and carries oxygenated blood away, maintaing a steep concentration gradient.
4. Good ventilation with air: Breathing continuously replaces old, low-oxygen air with fresh, high-oxygen air, preserving the steep concentration gradient required for rapid diffusion.

Investigating the differences (limewater test)

• To demonstrate that expired air contains significantly more carbon dioxide than inspired air, you can set up a simple boiling tube apparatus containing limewater.
  • Carbon dioxide turns clear limewater milky or cloudy.
  • Air is drawn through one flask when you inhale, and your exhaled breath is bubbled through a second flask when you exhale.
  • The result is that the limewater in the exhalation tube turns cloudy almost instantly, whereas the inhalation tube remains clear for a long time, proving that expired air has a much higher concentration of carbon dioxide.

Inhalation vs. exhalation actions

Composition of inspired vs. expired air

Effects of physical activity on breathing

• When you exercise, your muscles contract harder each time. This triggers a specific physiological loop.

The feedback loop

1. Physical activity begins. Musccle cells increase their rate of aerobic respiration to produce more ATP for muscle contraction.
2. Carbon dioxide accumulates. As a waste product of this respiration, the carbon dioxide concentration in the blood increases.
3. Brain detects deoxygenated blood. The blood passes throught the brain, where specialized receptors detect this high carbon dioxide level.
4. Signal is sent to the diaphragm and intercostal muscles.
5. The response is an increase rate (more breaths per minute) and a greater depth (larger volume per breath of breathing.
6. The purpose of the rapid gas exchange accelerates the removal of toxic carbon dioxide from the blood and supplies the extra oxygen needed by the muscles.

Protection of the breathing system

• The air we breathe contains dust, smoke particles, and pathogens. The respiratory tract uses a two-part defense mechanism to keep the lungs clean.
• Goblet cells are specialized cells scattered along the lining of the trachea and bronchi. They synthesize and secrete sticky mucus.
• Mucus traps dust, dirt, and pathogens before they can reach the alveoli.
• Ciliated cells are cells that have tiny, hair-like extensions called cilia.
  • The cilia beat in a synchronized, wave-like motion, pushing the trapped mucus up and out of the airway toward the larynx/throat, where it can be safely swallowed or coughed out.

Respiration

• Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.
  • Unlike breathing, which is the mechanical movement of air into and out of the lungs, respiration is a microscopic process that occurs continuously inside every living cell.

Uses of energy in living organisms

• Living organisms do NOT produce energy. They RELEASE it from food and use it to drive metabolic processes. There are seven specific uses of energy.
• Muscle contraction: To allow movement of the whole body, as well as internal movements (e.g. churning of stomach, heart beating).
• Protein synthesis: To link amino acids together into long chains to create proteins (e.g. enzymes, hormones, antibodies).
• Cell division: To power processes like mitosis and meiosis for growth and tissue repair.
• Active transport: To move substances across cell membranes against their concentration gradient (e.g. root hair cells absorbing mineral ions, or small intestine absorbing glucose).
  • Reminder that active transport is the movement of ions or molecules across the cell membrane, against a concentration gradient, using energy from respiration.
• Growth: To make new cells and build complex biological molecules.
• Passage of nerve impulses: To pump ions across axon membranes, allowing electrical signals to travel through the nervous system.
• Maintenance of a constant body temperature: Used by mammals and birds to release heat energy to keep their internal core temperature steady, even when the environment is cold.

Investigating the effect of temperature on respiration in yeast

• Yeast is a single-celled fungus that respires to break down sugar. Because respiration is an enzyme-controlled reaction, it is highly sensitive to changes in temperature.

Method

1. Mix a fixed volume of yeast suspension with a fixed volume of glucose solution in a test tube.
2. Place the test tube into a water bath set at a specific temperature.
3. Leave the tube for 5 mins. to allow the mixture to acclimatize (to adapt or adjust to a new environment) to the temperature.
4. Attach a delivery tube leading from the test tube into a gas syringe, or invert a measuring cylinder over water to collect gas. Alternatively, count the number of bubbles produced per minute.
5. Record the volume of carbon dioxide gas produced in a set time frame.
6. Repeat the entire process using water baths set at different temps.

Identifying variables

• Independent variable: Water bath temp.
• Dependent variable: Rate of respiration (measured by the volume of carbon dioxide gas produced per minute or number of bubble per minute)
• Controlled variables: Concentration of glucose, volume of yeast suspension, volume of glucose solution, pH

Explaining results on a graph

• At low temperatures (e.g. 10°C – 20°C), the rate of respiration is low. Kinetic energy is low, meaning enzymes and glucose molecules move slowly and rarely collide.
• At optimum temperatures (~35°C – 40°C), the rate of respiration increases. Molecules gain more kinetic energy, leading to more frequent successful collisions between active sites and substrates.
• Past optimum temperatures (e.g. above 45°C – 50°C), the rate of respiration drops to zero. The high temperature disrupts the bonds holding the respiratory enzymes together which alters the shape of their active sites. The enzymes are denatured, and the reaction stops.

Excretion in Humans

• Excretion is the removal from the body of the waste products of metabolism, toxic materials, and substances in excess of requirements.
• The waste substances are removed from the body by the excretory organs, which are the lungs, liver and kidneys.

Excretory products

• Carbon dioxide, $C{O}_2$, is excreted through the lungs. Carbon dioxide is made in body tissues during respiration, and here, it diffuses out of the blood into the air in the alveoli, then exhaled out.
• Urea, excess water and ions are excreted through the kidneys. Urea is carried to the kidneys in the plasma where it is filtered out and leaves the body dissolved in the form of urine. Urea is formed in the liver from excess amino acids.

Diagram of the human excretory system

Diagram of kidney structure

The nephron

• The nephron is a functional, microscoping filtering unit of the kidney. Thousands of them span across the cortex and medulla.

Process of urine production

1. Filtration in the glomerulus
   • Blood enters the kidney under high pressure. It reaches the glomerulus, a tightly coiled knot of capillaries.
   • Water, glucose, and urea are filtered out.
   • Large molecules like blood cells and plasma proteins are too big to pass through, so they remain in the blood.
2. Selective reabsorption
   • As the filtered fluid flows along the nephron tubule, the body reclaims the substances it still needs. The blood vessels wrapped around the nephron pull these substances back into the bloodstream.
   • All of the glucose is reabsorbed back into the blood for respiration.
   • Some of the ions are reabsorbed depending on body requirements.
   • Most of the water is reabsorbed to prevent dehydration.
3. Urine formation
   • Whatever is not reabsorbed passes to the end of the nephron and collects as urine. Urine flows into the ureter, then down to the bladder.
   • Urine contains: urea, excess water, and excess ions.

Role of liver

Importance of excretion

• Excretion is important because metabolic wastes like urea are highly toxic. If urea builds up in the blood, it damages cells, disrupts cellular chemistry, and can lead to organ failure.
• Amino acid metabolism in the liver:
  • Assimilation is when the food molecules that have been absorbed now become part of the cells or are used by the cells. The liver converts absorbed amino acids into useful plasma proteins, such as fibrinogen for blood clotting.
  • Deamination is the removal of the nitrogen-containing part of amino acids to form urea. This process takes place in the liver to safely break down excess amino acids that the body cannot store.

Coordination and Response

• Electrical impulses travel along neurones.
• The role of the nervous system is the coordination and regulation of body functions.

Mammalian nervous system

• The mammalian nervous system consists of the CNS and PNS.
  • The central nervous system consists of the brain and spinal cord. It acts as the body’s primary center for processing info and directing body functions. It is connected to different parts that make up the peripheral nervous system.
  • The peripheral nervous system consists of the nerves outside the brain and spinal cord. The cranial nerves link the brain to all organs in the head and some in the thorax and abdomen. The spinal nerves leave the spinal cord in pairs through gaps in between the vertebrae.

Reflex actions

• A reflex action is automatically and rapidly integrating and coordinating stimuli with the responses of effectors (muscles and glands).
• The sequence of events is:
  • stimulus $\to$ receptor $\to$ coordinator $\to$ effector $\to$ response
  • A stimulus is a change in the environment that is detected by a sense organ.
  • A receptor is a cell that detects stimuli.
  • A coordinator is a part of the body connects info about the stimulus to the effector.
  • An effector is an organ that brings about responses.
  • A response is the reaction of an organism to a particular stimulus.

Neurones and reflex arcs

• Neurones are nerve cells. They are highly specialized. Their structure allows them to transmit information as nerve impulses over long distances.

• Sensory neurones transmit impulses from sense organs to the brain and spinal cord.
• Motor neurones transmit impulses away from the brain and spinal cord to effector organs (muscles and glands)
• Relay neurones connect the sensory neurone to the motor neurone inside the brain and spinal cord to facilitate communication.

Structure of neurones

• Sensory and motor neurones are surrounded by myelin.
  • Myelin is formed by separate cells that grow around the neurones to form a layer rich in fat. This insulation makes impulses travel very quickly.

Reflex arcs

• A reflex arc is the arrangement of neurones that control a reflex action.

Synapses

• A synapse is a junction between two neurones.
• Synapses ensure that impulses travel in one direction only.

Structure

• The gap is called the synaptic gap.
• The presynaptic neurone is the neurone that carries the impulse to the synapse. The postsynaptic neurone is the neurone that carries the impulse away from the synapse.
• The synaptic bulbs are swellings at the axons of neurones.
• The presynaptic membrane is the surface of the synaptic bulb. It is separated by the synaptic gap from the postsynaptic membrane of the cell body or axon of the next neurone.
  • The postsynaptic membrane has a large number of protein receptors for the neurotransmitter substance.
• There are vesicles in the synaptic bulb that contain many mitochondria and neurotransmitter molecules.

Synaptic transmission

1. When an impulse arrives at the synaptic bulb it causes vesicles containing the neurotransmitter to move towards the presynaptic membrane.
2. The vesicles fuse with the presynaptic membrane, releasing the neurotransmitter into the synaptic gap.
3. The neurotransmitter diffuses across the synaptic gap and attaches to specific receptor sites on the postsynaptic membrane. These receptor sites ahve a complementary shape to the neurotransmitter, but the binding is only temporary.
4. The binding of the neurotransmitter triggers an impulse in the postsynaptic neurone. Once this has happened the neurotransmitter is broken down by an enzyme in the synaptic gap.
5. The mitochondria provide energy to reform the transmitter.

Homeostasis

• Homeostasis is the maintenance of a constant internal environment.
  • An example is how the hormone insulin decreases blood glucose concentration when it gets too high.

Negative feedback control

• Homeostasis relies on a mechanism called negative feedback.
  • The set point is the normal, ideal level for a condition in the body (e.g. a core body temp. of 37°C or a specific blood glucose level).
  • How it works: If a factor deviates from the set point, the body detects this change and triggers a response that reverses the change to bring it right back to the set point.

Control of blood glucose concentration

High blood glucose levels

1. The pancreas detects the rise and secretes insulin into the blood.
2. Insulin travels to the liver.
3. The liver converts excess glucose into stored glycogen.
4. This causes the blood glucose concentration to decrease back to the set point.

Low blood glucose levels

1. The pancreas detects the drop and secretes glucagon into the blood.
2. Glucagon travels to the liver.
3. The liver breaks down stored glycogen into glucose.
4. The glucose is released into the blood, causing the blood glucose concentration to increase back to the set point.

Treatment of Type 1 diabetes

• Type 1 diabetes is a condition where the pancreas fails to secrete enough insulin, leading to dangerously high blood glucose levels. It is managed and treated using a combination of methods:
  • Insulin injections/insulin pump therapy: Regular doses of insulin are injected directtly into the subcutaneous fat layer to artificially lower blood glucose levels.
  • Blood glucose monitoring: Testing blood drop samples throughout the day to calculate exactly how much insulin is required.
  • Controlled diet: Carefully managing and balancing carbohydrate intake to prevent massive blood sugar spikes.

Skin

Diagram

Structures

• Hairs: Extend above the skin surface. Can be raised or lowered.
• Hair erector muscles: Tiny muscles attached to the base of each hair follicle.
• Sweat glands: Coiled tubes in the dermis that produce sweat.
• Receptors: Specialized nerve endings that detect temp. and pressure changes.
• Sensory neurones: Send electrical impulses from the skin receptors to the CNS.
• Blood vessels: Arterioles and capillaries that supply nutrients and play a massive role in temp. control.
• Fatty tissue (subcutaneous layer): Located beneath the dermis. Acts as a layer of insulation.

Maintaining a constant body temperature

• The hypothalamus detects changes in the temp. of the blood and sends electrical impulses along nerves to trigger a corrective response *(like shivering or sweating)

High body temperature

• Sweating: Sweat glands secrete sweat onto the skin surface. As the water in sweat evaporates, it absorbs and removes heat energy from the skin, cooling the body down.
• Vasodilation: Arterioles supplying the skin surface capillaries widen (dilate). This allows more blood to flow close to the skin surface, meaning more heat energy is lost to the environment via radiation.

Low body temperature

• Shivering: Muscles contract and relax rapidly. This muscle activity requires a high rate of respiration, which releases heat energy to warm the blood.
• Insulation:
  • The fatty tissue layer reduces heat loss from the core.
  • Hair erector muscles contract, pulling the hairs upright. This traps a layer of still, warm air next to the skin, acting as insulation. More effective in furry mammals than humans.
• Vasoconstriction: Arterioles supplying the skin surface capillaries narrow (constrict). This restricts blood flow to the skin surface, redirecting it to vital core organs and minimizing heat loss via radiation.

Hormones

• A hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

Endocrine glands and hormones

• An endocrine gland is a ductless organ that produces and secretes hormones directly into the bloodstream.

Adrenaline and “fight or flight”

• Adrenaline is released by the adrenal glands during stressful, scary or exciting situations to prepare your body to either fight the danger or flee away from it.

Effects of adrenaline

• Increased breathing rate. Delivers more oxygen to lungs so it can diffuse into the blood.
• Increased heart rate. Pumps blood faster to deliver oxygen and glucose to muscles rapidly.
• Increased pupil diameter. Allows more light into eyes to improve visuall claritty and awareness.
• Increased blood glucose concentration. Adrenaline stimulates the liver to break down stored glycogen into glucose. This extra glucose is released into the blood to provide energy for increased cellular respiration in working muscles.

Nervous and hormonal control

Sense Organs

• Sense organs are groups of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals.

Eye structure

• The cornea refracts light.
• The lens changes its shape to focus light onto the retina.
• The suspensory ligaments hold the lens in place.
• The pupil is a small opening in the center of the eye. It regulates the amount of light entering the eye to enable vision.
• The iris controls the intensity of light entering through the pupil.
• The ciliary muscle changes the thickness of the lens when focusing.
• The retina is the inner light-sensitive layer that contains:
  • rod cells for dim light
  • cone cells for deteting color and details.
• The yellow spot or fovea is the most sensitive part of the retina that is responsible for sharp, detailed and color-rich central vision.
• The blind spot is the point where the optic nerve attaches to the eye. No light-sensitive cells here.
• The optic nerve carries nerve impulses away to the brain.

Pupil reflex

• The pupil reflex is an involuntary, automatic contraction of the iris muscle causing the pupil to constrict in response to bright light, and dilate in dim light.
  • In bright light, eye receptors detect high light intensity and send signals via the optic nerve to the brain. The circular muscles contract and the radial muscles relax. The pupil constricts (becomes smaller).
  • IN dim light, eye receptors detect low light intensity and send signals to the brain. The circular muscles relax and the radial muscles contract. The pupil dilates (becomes larger).
  • These muscles never contract at the same time. This makes them antagonistic.

Tropic Responses

• Plants don’t have a nervous system, so they rely on chemical growth responses to interact with their environment. These growth responses are called tropisms.
  • Gravitropism is a response in which parts of a plant grow towards or away from gravity.
  • Phototropism is a response in which parts of a plant grow towards or away from the direction of the light source.
• Positive tropism is growth towards a stimulus.
• Negative tropism is growth away from a stimulus.

Investigating tropic responses

In the lab

• Phototrophism experiment:
  • First, grow two sets of seedlings on wet cotton wool in Petri dishes in the dark.
  • Second, cover the seedlings with a cardboard box with a slit in the side that allows the light to enter from only one direction so the seedlings are exposed to unilateral light.
  • Third, place the seedlings on a clinostat arranged in a vertical position. The clinostat rotates four times every hour, exposing each side of the seedlings to the light.
  • Fourth, after two days you can see that the seedlings exposed to unilateral light grew towards the light source.
  • In conclusion, they were positively phototrophic.
• Gravitrophism experiment:
  • Place a seedling horizontally in a dark room. The shoot will grow upwards and the root will grow downwards.
  • To eliminate the effect of gravity as a control, a clinostat is used.

Auxin

• A plant hormone called auxin manages the growth of a plant.

The mechanism

1. Production: Auxin is made continuously in the shoot tip.
2. Diffusion: Auxin diffuses down through the plant from the shoot tip.
3. Unequal distribution:
   • In response to unidirectional light, auxin moves away from the light and accumulates on the shaded side of the shoot.
   • In response to gravity (horizontal shoot), auxin settles due to gravity and accumulates on the lower side of the shoot.
4. Cell elongation: Auxin stimulates cell elongation.
5. The result is that since there is more auxin on the shaded/lower side, the cells on that size elongate much faster than the cells on the lit/upper side. This unequal growth causes the shoot to bend toward the light or upward away from gravity.

Asexual and Sexual Reproduction

Asexual reproduction

• Asexual reproduction is a process resulting in the production of genetically identical offspring from one parent.
  • Since they are genetically identical, there is little variation among offspring. Any variation is due to the effect of the environment (e.g. availability of nutrients and water determine growth of organism).
• Binary fission is a form of asexual reproduction that is used by prokaryotes (e.g. bacteria) where a single cell divides into two identical daughter cells.
  • EXAMPLE:Bacteria are microscopic organisms made up of one cell. THey do not have a nucleus, but a loop of DNA. When bacteria reproduce asexually, the DNA loop is duplicated so that there is some for each new cell. The bacterial cell divides into two by making a new cell wall.
  • 

Advantages and disadvantages of asexual reproduction (to a population of species in the wild)

Advantages and disadvantages of asexual reproduction (crop production)

Sexual reproduction

• Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote and the production of offspring that are genetically different from each other.
• In sexual reproduction, it requires two parents that have sex organs. The sex organs make gametes.
  • Male gametes:
    • In animals, the male gametes are sperm cells.
    • In flowering plants, the male gametes are nuclei inside pollen grains.
  • Female gametes:
    • In animals, the female gametes are egg cells.
    • In floweing plants, the female gametes are inside structures called ovules.
• During sexual reproduction, the nuclei of the gametes fuse together at fertilization. The fertilized egg or zygote divides to form an embryo, which may grow into a new individual plant.
  • Fertilization is the fusion of nuclei of gametes.
• The nuclei of gametes are haploid and the nucleus of a zygote is diploid.
  • A haploid nucleus contains two full sets of chromosomes.
  • A diploid nucleus contains only one set of unpaired chromosomes.

Advantages and disadvantages of sexual reproduction (to a populaton of species in the wild)

Sex Hormones

Puberty and adolescence

• Puberty is the age at which secondary sexual characteristics appear in boys and girls.
  • The changes that take place are all controlled by hormones.
• The pituitary gland at the base of the brain starts to make hormones that stimulate the testes and the ovaries.

Puberty in boys

• The testes produce testosterone, which stimulates:
  • the growth of the male sex organs
  • the testes to make sperm cells
  • growth of hair on face
  • deepened voice
  • development of muscles in the body.

Puberty in girls

• The ovaries start making oestrogen, which stimulates:
  • the growth of female sex organs
  • the start of the first menstrual cycle and the first period
  • growth of hair on parts of body
  • growth and development of breasts
  • widening of hips.

Menstrual cycle

1. Changes in the lining of the uterus:
   • The uterus lining (endometrium) changes to ensure that if an egg is fertilized, it has a blood-rich place to implant.
   • Days 1-5 is menstruation. If fertilization did not occur in the previous cycle, the thick uterus lining breaks down and is lost through the vagina, a period.
   • Days 6-14 is repair and thickening. Stimulated by oestrogen, the lining starts to thicken again and its blood vessels regrow to prepare for a possible embryo.
   • Days 15-28 is maintenance. Stimulated by progesterone, the lining is maintained at its full thickness. It becomes more filled with blood vessels to provide nutrients for an implanted embryo.
2. Changes in the ovaries:
   • The ovaries focus on the maturation and release of the female gamete (egg).
   • Days 1-13 is the follicular phase. Inside the ovary, a follicle, which is a sac containing an immature egg, begins to develop and mature. This is triggered by Follicle Stimulating Hormone (FSH).
   • Day 14 is ovulation. A surge in Luteinizing Hormone (LH) causes the mature follicle to burst, releasing the egg from the ovary into the oviduct.
   • Days 15-28 is the luteal phase. The empty follicle transforms into a structure called the yellow body.
     • If no pregnancy occurs, the yellow body degenerates, progesterone levels drop, and the cycle restarts with menstruation.

Sites of production of oestrogen and progesterone in the menstrual cycle

• The production sites change as the follicle develops and then transforms after ovulation.
  • Oestrogen is produced by the ovaries, specifically by the maturing follicles. Its role is to stimulate the repair and thickening of the uterus during the first half of the cycle.
  • Progesterone is produced by the yellow body (left behind in the ovary after egg is released). Its role is to maintain the thickness and blood supply of the uterus during the second half of the cycle.

Sites of production of oestrogen and progesterone during pregnancy

• If an embryo implants, the body must prevent menstruation, which requires keeping progresterone and oestrogen levels high.
  • Early pregnancy, first trimester:
    • The yellow body continues to produce oestrogen and progesterone. It is “saved” from breaking down by hormones released by the developing embryo.
  • Late pregnancy, four months onward:
    • The placenta takes over the role of hormone production. The placenta secretes high levels of both oestrogen and progesterone for the remainder of the pregnancy.

Role of hormones in controlling the menstrual cycle and pregnancy

In the menstrual cycle

• FSH is produced in the pituitary gland and it is secreted at the start of the cycle. It stimulates the maturation of a follicle in the ovary and triggers the ovaries to start secreting oestrogen.
• Oestrogen is produced in the ovaries (developing follicle). It stimulates the repair and thickening of the uterus lining. High levels of oestrogen inhibit the further release of FSH to prevent multiple eggs from maturing, while stimulating a surge in LH.
• LH is produced in the pituitary gland. A sudden surge in LH around Day 14 triggers ovulation and also stimulates the remains of the follicle to develop into the yellow body.
• Progesterone is produced in the ovaries (yellow body). It maintains the thickness and blood supply of the uterus lining. It also inhibits the release of both FSH and LH to ensure no new follicles develop during the second half of the cycle.

During pregnancy

• If fertilization and implantation occur, the hormonal balance shifts to protect the developing embryo.
  • Lining is maintained. Progesterone levels remain high instead of dropping, preventing the uterus lining from breaking down and also prevents mestruation and losing the embryo.
  • Uterus and mammary gland development. Oestrogen levels remain high.
  • Inhibition of FSH and LH. Since progesterone and oestrogen levels remain high, FSH and LH are suppressed to ensure no ovulation occurs during pregnancy.
  • Placenta handles hormones. The placenta takes over the production of progesterone and oestrogen from the yellow body to maintain the pregnancy until birth.

Sexual Reproduction in Humans

Male reproductive system

Structures

• Testes: Produces sperm cells/male gametes. Produces testosterone.
  • Testosterone is a hormone that stimulates changes in a boy’s body as he develops into an adult during puberty.
• Scrotum: Pouch of skin containing the testes. Hangs outside body. Keeps testes cooled because sperm cells need less than 37°C.
• Sperm duct: Connects tubules to urethra.
  • Urethra is a duct that brings urine out of the body from the bladder.
• Prostate gland: Secretes fluid that mixes with sperm to form semen, which nourishes the sperm and helps them move.Sugars are used as an energy source for respiration.
• Seminal vesicles: Glands that produce a nutrient-rich, sugary fluid that provides energy for the sperm to swim. This fluid is a major component of semen.
• Penis: An organ that transfers semen into the female reproductive system during sexual intercourse.

Adaptive features of sperm

1. The flagellum (tail) is a long, whip-like structure that extends the back of the sperm cell.
   • Provides movement for sperm cell.
   • By lashing back and forth, the flagellum propels through the cervix and uterus toward the oviduct, where fertilization occurs.
2. Sperm cells have a high concentration of mitochondria packed into the section between the head and the tail.
   • Mitochondria are the site of aerobic respiration.
   • Provides energy for the flagellum to beat. Without this energy source, the sperm would not be able to sustain the long journey to the egg.
3. The acrosome is a specialized vesicle (a small sac) located at the very front of the sperm’s head.
   • It contains digestive enzymes.
   • When the sperm contacts an egg, these enzymes are released to digest a pathway through the jelly coat of the egg cell. This allows the sperm nucleus to enter the egg and fuse with the nucleus.

Female reproductive system

Structures

• Ovaries: Female sex organs that produce the female gametes (ova or eggs).
• Oviducts: Narrow tubes that connect the ovaries to the uterus and contains cilia to push the egg along the oviduct. Also uses peristalsis. Site of fertilization.
  • Fertilization is the fusion of nuclei from a male gamete (sperm) and a female gamete (egg cell).
• Uterus: Organ in the center of the pelvic cavity. Site of implantation of the embryo. Supports fetus by providing a thick, blood-rich lining (endometrium) and muscular walls that contract during birth.
• Cervix: Ring of muscle at the lower end of the uterus. Holds developing fetus in place during pregnancy. Dilates during labor to allow baby to pass into the vagina.
• Vagina: Muscular tube leading from cervix to outside of body. Site where sperm is deposited during sexual intercourse. Acts as birth canal during childbirth.

Adaptive features of egg cells

1. The egg cell contains a large volume of cytoplasm which acts as an energy store.
   • Rich in nutrients. Contains the yolk.
   • Provides necessary energy and raw materials for the early development of the embryo. After fertilization, the zygote must divide many times as it travels down the oviduct toward the uterus; the energy store sustains it until it can implant in the uterus lining and begin receiving nutrients from the mother.
2. The egg is surrounded by a specialized outer layer known as the jelly coat. This layer has two critical roles during fertilization:
   • First, selection. It contains receptor sites that ensures only sperm from the same species can bind to and penetrate the egg.
   • Second, the fertilization membrane. Immediately after the first sperm nucleus enters the egg, the jelly coat undergoes a chemical change that makes it harden or become impenetrable.
   • The function of the jelly coat prevents more than one sperm cell penetrating the egg. If multiple sperm fertilized one egg, there would be too many sets of chromosomes and would not survive.

Fertilization and implantation

• In early development, the zygote forms an embryo which is a ball of cells that implants into the lining of the uterus.
• Implantation is the embedding of an embryo into the soft lining of the uterus.
  • It may take a number of days for the embryo to reach the uterus.

Pregnancy

Structures

• Umbilical cord: Tube-like structure connecting fetus to placenta. Contains blood vessels that contains blood vessels that transport substances between placenta and fetus.
  • The umbilical vein carries oxygenated blood and nutrients (e.g. glucose/amino acids, oxygen, mineral ions/vitamins, and water) to baby.
  • The umbilical artery carries deoxygenated blood and waste products (e.g. $C{O}_2$, urea, water) away from baby.
• Placenta: Disc-shaped organ embedded in the wall of uterus, connected to fetus by umbilical cord. Exchanges materials, removes waste, barrier from pathogens, and produces hormone (progesterone, maintain lining of uterus during pregnancy).
  • Some pathogens and toxins can pass across the placenta and affect the fetus.
• Amniotic sac: Tough, thin, transparent membrane that encloses the fetus and amniotic fluid. Contains amniotic fluid and prevents infections from reaching the fetus from the vagina.
• Amniotic fluid: Liquid that fills the amniotic sac, surrounds the developing fetus. Acts as a shock absorber, preventing the fetus from physical impact or injury. Maintains constant temperature. Allows fetus to move freely for bone and muscle development.

Sexual Reproduction in Plants

Flower diagram

Functions of the structures in flowers

• Sepals: Leaf like structures that protect the flower when it is a bud.
• Petals: Brightly colored and scented to attract insects to feed on nectar. Many have a nectary at the base that produce sugary nectar.
• Stamens: Male sex organs. Each one is made up of two parts:
  • the anther, pollen grain production
  • the filament, a stalk that holds the anther.
• Carpels: Female sex organs. Made up of:
  • a stigma, where pollen grains attach during pollination
  • a style, to connect the stigma to ovary, pollen tube grows down the style
  • an ovary, swollen base that contains the ovules, usually becomes the fruit after fertilization
  • an ovule, found in the ovary, containing female gametes (egg cells), after fertilization the ovules become seeds.

Wind-pollinated flowers

Diagram

Anther and stigma

• In wind-pollinated flowers, the anthers are different from those in insect-pollinated flowers:
  • They are positioned like a pendulum, they are attached to long, thin filaments that hang outside the flower.
  • The function of this is to allow the anthers to swing freely in the air, easily releasing light, smooth pollen grains into the air.
• The stigmas are made to catch pollen grains in the air.
  • They are feathery or branched in appearance.
  • They hang outside the flower.
  • The feathery structure allows a large surface area to catch wind-borne pollen grains.

Pollen grains

• In terms of quantity, they are mass produced since most are lost in the air.
• They are very light in weight so they can be easily carried by the wind.
• They are smooth to prevent clumping together and to drift easily.
• They might have air sacs to stay in the wind longer.

Insect-pollinated flowers

Diagram

Pollen grains

• In terms of quantity, they are produced in smaller amounts for efficient transport.
• They are relatively heavy and larger.
• They are sticky and spiky to hook onto insect bodies.
• There are no special features for aerodynamics.

Comparison of pollen grains

Pollination

• Pollination is the transfer of pollen grains from an anther to a stigma.
• Self-pollination is the transfer of pollen grains from the anther of a flower to the stigma of the same flower or a different flower on the same plant.
• Cross-pollination is the transfer of pollen grains from the anther of a flower to the stigma of a flower on a different plant of the same species.

Potential effects of self- and cross-pollination on a population

1. Genetic variation:
   • Since self-pollination is the transfer of pollen grains of the same flower, this leads to very low genetic variation in the offspring since they are very similar to the parent.
   • Since cross-pollination is the transfer of pollen grains to a flower on a different plant of the same species, this leads to high genetic variation in the offspring because it combines two different genetic material from two individuals.
2. Capacity to respond to environmental change:
   • Self-pollinated flowers result in not being able to adapt easily to changes in the environment due to the lack of genetic variation .
   • Cross-pollinated flowers provides the “raw material” for natural selection. If there is an afvantageous trait in a plant that allows them to survive or adapt to a change in the environment, this trait can be passed on to offspring.
3. Reliance on pollinators:
   • For self-pollinated flowers, there is little to no reliance on external pollinators (e.g. bees, wind), which is an advantage in habitats where pollinators are scarce or where plants are wildly scattered. This ensures there is still seed production even in isolation.
   • For cross-pollinated flowers, there is a high reliance on pollinators or wind. If the specific pollinator for a species declines (e.g. caused by habitat loss or pesticides), the plant population may fail to reproduce, leading to a decline in numbers.

Sexually Transmitted Infections

• A sexually transmitted infection is an infection that is transmitted through sexual contact.

Human immunodeficiency virus (HIV)

• Human immunodeficiency virus (HIV) is a pathogen that causes an STI.
• HIV enters human cells and uses each cell to make more viruses, which then enters even more cells. HIV mainly affects a type of white blood cell.

Transmission

• HIV is transmitted from the blood and semen. The virus can pass from one person to another during unprotected sexual intercourse.
• HIV can also be transmitted via hypodermic needles contaminated with infected blood.
  • HIV can be spread very quickly among drug addicts who share needles.
• Babies are also at risk of HIV.
  • The virus can pass from the mother’s blood to the baby’s blood at birth when the two bloodstreams come into close contact.
  • HIV can also be transmitted in breast milk.
• HIV can be transmitted through blood used for transfusions.

AIDS

• If left untreated, HIV may lead into the development of AIDS, acquired immunodeficiency syndrome.

Controlling the spread of STIs

• Methods to reduce the spread of STIs:
  • using condoms to provide a physical barrier to the transmission of STIs during sexual intercourse
  • setting up free needle exchange schemes for those who inject drugs; this reduces the risk of transmission from the user of shared needles and syringes
  • screening donated blood for HIV antibodies and eliminating contaminated blood being used for transfusion
  • education programmes to spread awareness of the methods of the spread of STIs and how it can be prevented.

Chromosomes, genes, and DNA

Key definitions

• Chromosomes are made of DNA, which contains genetic information in the form of genes.
• A gene is a length of DNA that codes for a protein.
• An allele is an alternative form of a gene.

Inheritance

• Inheritance is the transfer of genetic information from generation to generation.
  • These characteristics are controlled by genes, similar to coded instructions.
  • People who belong to the same family often resemble each other. Characteristics or traits (e.g. physical appearance, personality, talents) are often shown by people in different generations of a family. These people have inherited the feature from their parents.
• The study of inheritance is called genetics.

Production of generations from flowering plants and humans

• Genes from one generation are transmitted to the next in the gametes.
• At fertilization, the gametes fuse to form a zygote, which contains the genetic information from both parents.
  • A new individual grows from the zygote. Half of its genetic information derives from its male parent and half from its female parent.

Chromosomes

• Chromosomes are made of DNA, which contains genetic information in the form of genes.
  • Textbook definition: “A chromosome is a thread of DNA carrying genetic information in the form of genes.”
• These chromosomes are visible through a microscope during cell division.
• Most cells have nuclei containing chromosomes.
• The diploid number is the total count of chromosomes in a standard body cell of an organism, consisting of two complete sets (one set from each parent).
  • There are 46 chromosomes in the nucleus of a human cell. This is the diploid number.
• Chromosomes can be sorted into 23 pairs based on their size and shape. In each pair, one of the chromosomes have been inherited from the male parent and one from the female.
• When unraveling a chromosome, it would form a very long thread. This thread is made up of DNA (deoxyribonucleic acid).
  • The DNA is wound around molecules of protein. During cell division, the DNA and protein are packed very tightly together and the chromosomes are coiled up.
  • At other times, they are uncoiled so the cell can use the information in the DNA.

Inheritance of sex in humans (in terms of X and Y chromosomes)

• As mentioned before, the human body’s cells contain 23 pairs of chromosomes. One pair is the sex chromosomes.
• A female is XX, so all her eggs carry an X chromosome.
• A male is XY, so approximately half of his sperm carry an X chromosome and half carry a Y chromosome.

• This table shows that the father’s sperm determines the sex of the child:
  • an X-carrying sperm produces an XX female
  • a Y-carrying sperm produces an XY male.

Genes

• A gene is a length of DNA that codes for a protein.
• Each chromosome has many genes along its length, although the real number depends on its length and the length of the genes.
  • Some of our chromosomes are much longer than others, therefore containing lots of genes.
• Each gene is a unit of inheritance in that it codes for a specific protein. It is a chemical code that the cell interprets as an instruction to make a protein molecule.
  • There is a gene for each protein, such as haemoglobin, amylase, lipase, etc.
• Humans have between 20,000 and 25,000 different genes.
• The proteins produced by cells influence how the body works and what it looks like.
  • Some features that genes control are influenced by the environment.
    • For example, your height is determined by both genes and the environment (the environment being your diet and how much you exercise).
  • Other genes are not affected by the environment.
    • For example, your blood group is determined by a gene and the environment has no effect on this.

Protein Synthesis I

• Remember that:
  • each chromosome is made up of a long, super-coiled molecule of DNA
  • each molecule is divided into thousands of shorter sections called genes
  • a gene is a small part of the DNA strand and codes for a protein.
  • the length of DNA making up a particular gene carries the information needed to make a particular protein
  • inside every cell, there are thousands of different chemical reactions taking place.

DNA

• Enzymes control all the chemical reactions that occur inside our body.
• It is important to remember that all enzymes are proteins.
  • Since DNA codes for proteins, it determines:
    • which enzymes are produced in each cell,
    • and therefore which chemical reactions take place inside cells.
• The structure of DNA contains bases. The information is found in the sequence of bases along the length of DNA.
  • The sequence of bases in a gene determines the sequence of amino acids used to make a specific protein.
• DNA also controls cell function by controlling the production of proteins, including enzymes, membrane carriers, and receptors for neurotransmitters.
• DNA carries information to build proteins from amino acids. The information moves from the DNA to the site of protein synthesis in the ribosomes via a kind of messenger molecule.
  • This messenger molecule is called the RNA. The name given is messenger RNA (mRNA).
  • The role of mRNA is to carry a copy of the base sequence on DNA out of the nucleus to the ribosomes in the cytoplasm where protein synthesis occurs.

The genetic code

• A, T, C and G are the four different bases found in DNA.
• Each amino acid is coded for, by a sequence of three of these bases on DNA.
  • There are about 20 different amino acids and each is coded for by a different base triplet on DNA.
  • Different sequences of amino acids give different shapes to protein molecules.
• When mRNA copies the base sequence on DNA, it does so by the rule of base pairing.
  • The rule of base pairing states that specific nitrogenous bases in DNA and RNA always pair up with each other through hydrogen bonds.
    • In terms of DNA base pairing rules, A is always paired with T and C is always paired with G.
    • mRNA does not have the base T, therefore it is replaced with base U (uracil). A on DNA will code for U on mRNA.
• The coded information on mRNA is used to assemble amino acids in the correct sequence for each protein.
  • Scroll down for what the syllabus says for how a protein is made.
  • The mRNA arrives at the ribosomes from the nucleus.
  • The mRNA strand contains the base triplets for each particular amino acid in the protein.
  • The mRNA then passes through the ribosomes and each ribosome assembles amino acids into protein molecules.
  • The amino acids bond together forming a long chain, the protein.
  • The specific order of amino acids is determined by the sequence of bases in the mRNA and each amino acid is joined to the next one by a peptide bond.
• All body cells in an organism contain the same genes. However, in a particular cell, many genes may not be expressed because the cell makes only the specific proteins it needs.

Syllabus definition for protein creation

• Explain how a protein is made, limited to:
  • the gene coding for the protein remains in the nucleus
  • messenger RNA (mRNA) is a copy of a gene
  • mRNA molecules are made in the nucleus and move to the cytoplasm
  • the mRNA passes through ribosomes
  • the ribosome assembles amino acids into protein molecules
  • the specific sequence of amino acids is determined by the sequence of bases in the mRNA.

Protein Synthesis II

Discussed in protein synthesis I

• A gene is a sequence of DNA nucleotides.
• Each nucleotide has one of four bases (A, T, C, G). So, a gene will contain a particular sequence of bases.
  • This sequence instructs the cell to make a particular protein.
• Proteins are made up of amino acids linked together by peptide bonds.
• What determines the type of protein formed is the type and sequence of the amino acids that are joined together.
  • As mentioned before, the sequence of bases in a gene determines the sequence of amino acids used to make a specific protein.
• Each base triplet codes for one particular amino acid.

Protein structure

• The basic structure of a protein is the sequence of amino acids in the chain.
  • It determines the eventual shape of the protein and hence its function.
• There are 20 different amino acids and all their different combinations can build the 100,000 known proteins.
  • It is important to be aware that proteins are three dimensional structures instead of amino acid chains lying flat. They form of helices, which twist and fold around themselves.
• The result is a complex 3D structure held together by a number of different chemical bonds. This structure gives the protein its function.
• The only genes that are expressed are those that code for the proteins that each cell needs to function correctly.
  • Most body cells in an organism contain the same genes, but many genes in a particular cell are not expressed because the cell only makes the specific protein it needs.

Important proteins

• DNA controls cell function by controlling the production of proteins, including enzymes, membrane carriers, and receptors for neurotransmitters.

Enzymes

• Enzymes control the chemical reactions that take place in a cell. All enzymes are proteins. So:
  • the genetic code of DNA, by determining which proteins (particularly enzymes) are made in the cell, will determine the cell’s structure and function.
• The 3D shape of an enzyme is critical to its function.
• Remember that the active site on the surface of the enzyme has a complementary shape to the enzyme’s substrate. This is important because it gives the enzyme its specificity.

Antibodies

• Antibodies are proteins that are made by white blood cells called lymphocytes.
• Each antibody has a binding site that can attach to pathogens like bacteria.
  • This either directly destroys the pathogen or labels it in a way that can be detected by other white blood cells called phagocytes.
• Each pathogen has antigens on its surface that have a particular shape, so only specific antibodies, with a complementary shape can attach to a particular antigen.

Membrane carriers

• Membrane carriers are proteins that control the movement of water-soluble ions and certain molecules across the cell membrane.
• They are able to maintain different concentrations of ions on either side of the cell membrane.

Receptors for neurotransmitters

• Receptors for neurotransmitters are proteins found in the membrane at the synapses between neurones.
• They have a complementary structure to the transmitter molecules, which attach to them and enable an impulse to pass across the synapse.

Mitosis

Key definition

• Mitosis is nuclear division giving rise to genetically identical cells.
  • Textbook definition: Mitosis is the type of nuclear division that occurs during growth and asexual reproduction. The daughter cells are genetically identical.

Process

• Before mitosis, an exact replication of chromosomes occur. This is a very reliable process in which exact copies are made.
• While this copying process goes on, the DNA in the chromosomes is uncoiled and arranged very loosely in the nucleus.
  • You won’t be able to see chromosomes in the nuclei as this happens.
• The new copy of DNA of each chromosome is attached to the original copy.
• As mitosis begins, the DNA coils up so that each chromosome becomes thicker.
  • They become visible with the microscope when a suitable staining technique is used.
• During mitosis, the copies of chromosomes separate, maintaining the chromosome number in each daughter cell.
  • As the chromosomes have been copied by a reliable system, they are genetically identical to each other and to the parent cell. If they were genetically different, the body’s immune system would reject it.

Significance of mitosis

Growth

• In animals, this happens all over the body.
• In plants, it happens in special growing areas such as the tips of stems and roots.
  • In plants, group is confined to certain areas called meristems. They are plant regions made of unspecialised (not yet developed the particular structure and job of a mature plant cell), actively dividing cells.
• As multicellular organisms grow, the number of cells making up their tissues increases.
  • The new cells must be identical to the existing ones.

Repair (of wounds)

• Your skin cells divide by mitosis to repair damaged tissues and wounds.

Replacement

• Mitosis is crucial for replacement of cells that wear out and die.
  • For example, red blood cells, which only live for a short time.
  • Skin cells and the cells in the gut are constantly dying and being replaced by identical cells.

Asexual reproduction

• This occurs in fungi and in plants, but is rare in the animal kingdom.
• Asexual reproduction results in offspring that are identical to the parent. Mitosis occurs when unicellular organisms reproduce.
  • For example, yeast and potato tubers.

Stem cells

• Stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions.
• Some of the cells that form an embryo remain as unspecialised cells. They do not develop into any type of unspecialised cell.
  • These cells keep dividing by mitosis to provide the body with a constant supply of new cells. These cells are needed to replace cells that get worn out and die.
• There is a layer of stem cells just under the outer layer of skin cells.
  • These stem cells continually get rubbed off and need to be constantly replaced.
  • When skin is wounded, these stem cells produce more cells to repair the damage.
• There are many stem cells in the bone marrow to produce new red blood cells and phagocytes as replacements.

Meiosis

Key definition

• Meiosis is a reduction division in which the chromosome number is halved from diploid to haploid resulting in genetically different cells.
  • Remember that diploid is the total count of chromosomes and haploid is only half the diploid number. It is also the number of chromosomes in a single complete set, or also the number of chromosomes in the gametes.

Meiosis in sexual reproduction

• If human sperm cells and egg cells both had 46 chromosomes, the zygote formed at fertilization would have 92 chromosomes.
  • If this continued generation after generation, the number of chromosomes in the nuclei would double every generation. But, the number remains at 46 chromosomes.
• The number remains at 46 because there is a different sort of nuclear division in the life-cycle in the production of gametes. This type of nuclear division is a reduction division called meiosis.
  • Meiosis is involved in the production of gametes.
• Meiosis halves the number of chromosomes so egg cells and sperm cells have only 23 chromosomes each.
  • The zygote has 46 chromosomes. 23 from the mother (in the egg) and 23 from the father (in the sperm).
• In sexual reproduction, the number of chromosomes stays constant from generation to generation.
• Meiosis occurs in sex organs in humans, the ovaries and testes.
• In flowering plants, meiosis occurs in the anthers and the ovules.
• Since the daughter cells are genetically different, this contributes to the genetic variation that appears in the next generation following sexual reproduction.
  • Genetic variation is important in natural selection.

Chromosomes and meiosis

• This diagram shows what happens to one pair of chromosomes during meiosis.
  • There are two divisions of the cell, resulting in four cells. Each of these four cells has one chromosome, so the number has been halved to give a haploid gamete.
  • The pair of chromosomes shown in the first cell are different from each other. One is paternal in origin as it was inherited from the father, and one is maternal as it was inherited from the mother.
• When the 23 pairs of chromosomes separate in the first division of meiosis, the daughter cells gain different combinations of paternal and maternal chromosomes.
  • This is one way in which the haploid cells produced in meiosis are genetically different from one another.

Differences between mitosis and meiosis

• This diagram visually compares mitosis and meiosis.

Inheritance and genes

Key definitions

• Inheritance is the transmission of genetic information from generation to generation.
• Genotype is the genetic make-up of an organism.
  • It is also the combination of alleles an organism possesses for a particular characteristic.
• Phenotype is the observable features of an organism.
• Homozygous is having two identical alleles of a particular gene.
• Heterozygous is having two different alleles of a particular gene.
  • A heterozygous individual will not be pure-breeding.
• A dominant allele is an allele that is expressed if it is present in the genotype.
• A recessive allele is an allele that is only expressed when there is no dominant allele of the gene present in the genotype.
• Codominance is a situation in which both alleles in heterozygous organisms contribute to the phenotype.
• A sex-linked characteristic is a feature in which the gene responsible is located on a sex chromosome and that this makes the characteristic more common in one sex than in the other.
  • Red-green colorblindness is an example of sex-linkage.

Genes and alleles

• Remember that a gene is a length of DNA that codes for a protein.
• Each gene is always located in the same place on one of the chromosomes.
• Each species has its own genes, and every individual from the same species has the same genes.
  • However, genes vary between individuals within each species.
• The different versions of each gene are known as alleles.
  • There may be two alleles of a gene, but usually there are many more.
• In Figure 16.6.2, you can see three genes on one pair of chromosomes.
  • The alleles are A/a, B/B and d/d.
    • These two alleles, A/a, are different, therefore the individual is heterozygous for this gene. Heteozygous means having two different alleles of a particular gene. The dominant allele (A) (an allele expressed if present in the genotype, the genetic make-up of an organism) will be expressed in the phenotype, the observable features of an organism.
    • B/B are the same, therefore the individual is homozygous dominant. Homozygous means that the two alleles of a particular gene are identical. It is dominant because it is present in the genotype.
    • d/d are the same, therefore the individual is homozygous recessive. It is recessive because there is no dominant allele present in the genotype. The recessive allele (d) will be expressed in the phenotype.
  • It is important to notice that a dominant allele uses CAPITAL LETTERS and a recessive allele uses lowercase letters.

Note

A gene controls a particular feature in the phenotype. A gene may have two or more alleles (different versions of the same gene). The dominant allele produces a different effect on the phenotype than the recessive allele (e.g., hair length in cats). It is important to never write that dominant alleles are ‘strong’ or that recessive alleles are ‘weak’.

Monohybrid Inheritance

Key definitions

• Monohybrid inheritance concerns the inheritance of a single characteristic, such as plant height or flower color.
  • It involves the inheritance of the alleles of one gene.

Example (Mendel’s peas)

• There is one gene for height in pea plants. The gene for plant height has two alleles: tall, T, and dwarf, t.
  • There are three possible genotypes for plant height:
    • TT = homozygous tall
    • Tt = heterozygous tall
    • tt homozygous dwarf

The Punnett square

• Using our previous example, we could produce a Punnett square to show the combinations of gametes.

The parental cross

• In Figure 16.7.2,, it shows a cross between a homozygous tall plant, TT, and a homozygous dwarf plant, tt.
  • These represent the pure breeding plants that Mendel used in his crosses. This is called the parental cross.
  • The parental cross is the mating of two initial organisms to study how specific genes, traits, and alleles are passed down to their offspring.
• As a result of meiosis, all the gametes from the tall plant contain the dominant allele T, and all the gametes from the dwarf plant contain the recessive allele t.
• When fertilization occurs, the new plants receive one dominant allele and one recessive allele, making it heterozygous. They will also all have the genotype, Tt.
  • Their phenotype will be tall, because the allele T is dominant to the recessive allele t.
• The first generation is known as the $F_1$ generation, and in this particular genetic cross, all the plants in the $F_1$ generation are heterozygous tall (Tt).

The Punnett square for this scenario

Crossing the $F_1$ generation

• The $F_1$ plants are allowed to self-pollinate (review “The Punnett square” at the start of this page).
  • When meiosis occurs, half of the gametes of each plant will have the dominant allele T, and half will have the recessive allele t.
• When fertilization occurs, there are three possible combinations of alleles in the $F_2$, or second generation:
  • TT, homozygous tall
  • Tt, heterozygous tall
  • tt, homozygous dwarf.
• Among this generation:
  • $\frac{1}{4}$ (or 25%) will be homozygous tall.
  • $\frac{1}{2}$ (or 50%) will be heterozygous tall.
  • $\frac{1}{4}$ will be homozygous dwarf.
• Since the phenotypes of TT and Tt are the same, $\frac{3}{4}$ of the $F_2$ plants will be tall and $\frac{1}{4}$ will be dwarf.
  • This can also be written as 3 tall : 1 dwarf, $3:1$.

A test cross (supplement)

• The last two crosses mentioned show that the recessive allele is not expressed in the $F_1$ generation, but is expressed in the $F_2$ generation.
  • This means that we can’t immediately know the genotype of a tall plant only by looking at it, because plants with genotypes TT and Tt have the same phenotype.
  • We already know the genotype of a dwarf plant, because it can only be tt.
• A test cross is used to determine the genotype of an individual to find out whether it is homozygous or heterozygous.

If unknown tall plant is homozygous

• If our tall plant is homozygous (TT), then crossing it with a dwarf plant (tt) gives it all tall plants (Tt).

If unknown tall plant is heterozygous

• If our tall plant is heterozygous (Tt), then crossing it with a dwarf plant (tt) will give half tall plants (Tt) and half dwarf plants (tt) in a ratio of $1:1$.

Codominance

Key definition

• Codominance is a situation in which both alleles in heterozygous organisms contribute to the phenotype.

Overview

• With the previous examples discussed, the involved alleles are either dominant or recessive.
  • Remember that the recessive allele is not expressed in the phenotype of individuals with the heterozygous genotype.
• Sometimes, both alleles are expressed and neither is dominant. This means the phenotype would be a mixture of the effects of each allele. This is called codominance.
  • The alleles are called codominant alleles.

Example (the four o’clock plant)

• The four o’clock plant can have red flowers or white flowers. Flower color is determined by one gene.
• If you cross homozygous red-flowered plants with homozygous white-flowered plants, the offspring are all pink.
  • The two parents produce heterozygous offspring (the $F_1$ generation) in which both alleles are expressed to give pink, an intermediate color.
• A Punnett square can be used to show these characteristics.

• There are three possible combinations for this plant:
  • 25% homozygous $C^R{C}^R$
  • 50% heterozygous $C^R{C}^W$
  • 25% homozygous $C^W{C}^W$
• The phenotypes are:
  • 25% red
  • 50% pink
  • 25% white
• These ratios can be given as $1:2:1$.
• Notice also that each allele is represented as a CAPITAL LETTER.
  • a red flower would be represented as $C^R$
  • a white flower would be represented as $C^W$.
  • This shows that neither allele is recessive and that they both exert an equal effect on the phenotype.

Multiple alleles

• With the previous examples discussed, it only shows two alternative alleles for a gene. There are examples where a gene may have three or more alleles.

The human ABO blood group system

• The human ABO blood group system is determined by one gene, $I$, with three different alleles:
  • $I^A$
  • $I^B$
  • $I^O$
• The alleles $I^A$ and $I^B$ are codominant and code for slightly different molecules on the surface of red blood cells.
• The allele $I^O$ is recessive to both $I^A$ and $I^B$ and does not code for one of these molecules.
• The four different blood groups are:
  • A
  • B
  • AB
  • O

• This is an example of two parents that have blood groups AB and O:
  • The children will have blood groups A or B. None of them will have the same blood group as their parents.

• This is an example of two heterozygous parents that have blood groups A and B.
  • Their children can have any of the four blood groups: A, B, AB, and O.

Sex Linkage

Key definitions

• A sex-linked characteristic is a feature in which the gene responsible is located on a sex chromosome and that this makes the characteristic more common in one sex than in the other.
• Red-green colour blindness is an example of sex linkage.

Sex determination

• Your chromosomes determine which sex you are.
• In humans, there are 46 chromosomes and these occur in 23 pairs.
  • In females, all 46 chromosomes can be matched together into pairs. This is done by taking images of the chromosomes and matching them for size and shape.
  • In males, there are two chromosomes that are not alike. These are known as the X and Y chromosomes.
• Females have two X chromosomes and males have an X chromosome and a Y chromosome.

• In this Punnett square, you can see how sex chromosomes in the gametes determine the sex of individuals.
  • Remember that the number of chromosomes is halved in meiosis so each gamete can have only one of the two sex chromosomes.
• All egg cells have an X chromosome. Half of the sperm contain an X chromosome and half of the sperm contain a Y chromosome.
• At fertilization, the egg could fuse with either a sperm with an X chromosome or a sperm with a Y chromosome.
  • Since there are equal numbers of sperm with the X chromosome and sperm with the Y chromosome, there is an equal chance of the zygote being XX or XY and the child being female or male.

Sex inheritance

• In this image with an X chromosome (left) and a Y chromosome (right), the Y chromosome is much smaller than the X chromosome.
  • Therefore, the Y chromosome has far fewer genes than the X chromosome or any other chromosome.
• One gene on the Y chromosome stimulates the development of testes in the embryo.
  • If it is not present (or if it has mutated) then the body that develops is female.

Supplement

Sex linkage

• The genes that are located on the X chromosome are described as sex-linked even though they have no relation to determining gender or controlling sexual characteristics.
• Among the genes on the X chromosome are genes involved with:
  • controlling vision
  • blood clotting.
• Males only have one copy of the genes that are on the X chromosomes. This means that if any of them are recessive, the effect will be seen.
• Since women have two X chromosomes, they are less affected by sex-linked recessive alleles and this is why sex-linked conditions are more common in boys than in girls.

Colour blindness

• One of the genes on the X chromosome controls the ability to see red and green colours.
• The gene works in the receptors, known as cones, in the retina of the eye.
  • There is an allele of this gene that does not produce a protein necessary for colour vision. It is a recessive allele, so any girl or woman who is heterozygous, Rr, has normal colour vision.
• Males only have one X chromosome so if they have inherited the allele r, they will have red-green colour blindness.

• This Punnett square shows what to expect if the mother is homozygous dominant and the father is colour blind.
• None of the children can expect to be colour blind, but the girls will inherit the allele for colour blindness on the X chromosome from their father.
  • They have this allele, but it does not affect their phenotype. Any females who are heterozygous are carriers of red-green colour blindness.

• This Punnett square shows what to expect if the mother is a carrier and the father has normal colour vision.
• In this case, there is a 1 in 4 chance that one of the children will be colour blind.
  • Note that there is a 1 in 2 chance that any boy will be colour blind.

Isotopes

Key definition

• Isotopes are different atoms of the same element that have the same number of protons but different number of neutrons.
  • Isotopes of the same element have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration.

Writing symbols for atoms and ions

Atoms

• For atoms, the top number is represented as the nucleon number. The nucleon number is calculated as the number of protons and neutrons.
• The bottom number is represented as the proton number (also called the atomic number, and the electron number).

Ions

• For ions, the same concept for atoms is applied here, but the valid charge is written at the top right of the element.

Relative atomic mass

• In order to calculate the relative atomic mass of an element, you need these values:
  • the relative masses
  • the relative abundances

Formula

$$A_r=\frac{({\text{mass}}_1\times {\text{abundance}}_1)+({\text{mass}}_2\times {\text{abundance}}_2)}{100}$$

The Mole and the Avogadro Constant

Key definitions

• The mole, mol, is the unit of amount of substance and that one mole contains $6.02\times 10^{23}$ particles, (e.g. atoms, ions, molecules). This is the Avogadro’s constant.
• The molar mass of a substance is the mass of 1 mole of its particles.

Main formula

$$\text{amount of substance (mol)}=\frac{\text{mass (g)}}{\text{molar mass (g / mol)}}$$

• Using this relationship, you are able to calculate:
  • the amount of substance (mol)
  • mass
  • molar mass ${\text{M}}_{\text{r}}$
  • relative atomic mass ${\text{A}}_{\text{r}}$ or relative molecular/formula mass ${\text{M}}_{\text{r}}$
  • number of particles, using the value of Avogadro’s constant.

Finding the molar mass

• The molar mass of a substance is the mass of 1 mole of its particles.

1. Write down the symbol or formula of the substance.
2. Find its ${\text{A}}_r$ or ${\text{M}}_r$.
3. Express that mass in grams (g).

Reactions involving gases

• The molar gas volume is taken as $24{\text{dm}}^3$ at room temperature and pressure, r.t.p., in calculations involving gases.

Formula for calculating the number of moles

$$\text{no. of moles}=\frac{\text{vol at r.t.p.}}{24{\text{dm}}^3}$$

• Using this relationship, you are able to calculate:
  • the number of moles
  • the volume at room temperature and pressure, r.t.p.

The concentration of a solution

• The concentration of a solution is the amount of solute that is dissolved in $1{\text{dm}}^3$ of solution. This can be expressed in grams, or moles.
• Remember that:
  • the solute is the substance you dissolve in a liquid to make a solution
  • the solvent is the liquid you use (usually water).

Formula for calculating the concentration of a solution

$${\text{conc (mol/dm}}^3{\text{ or g/dm}}^3\text{)}=\frac{\text{amount (mol)}}{{\text{volume (dm}}^3\text{)}}$$

• Using this relationship, you are able to calculate:
  • the concentration of a solution
  • the amount of solute present
  • the volume

Finding the concentration in moles

• This is how to find the concentration of a solution in moles per ${\text{dm}}^3$.

1. Find the ${\text{M}}_r$ of the solute.
2. Use the equation above to calculate the concentration of a solution.

Finding the empirical formula

• The empirical formula shows the simplest ratio in which atoms combine to form a compound.

1. Find the masses that combine (in grams) by experiment.
2. Change grams to moles of atoms.
3. You are able to find the ratio in which atoms combine.
4. With the ratio, you can write the formula.

Template

Finding % yield and % purity

• The yield is the amount of product you obtain from a reaction.
• To have something pure means that only one substance is in it. You might need to minimize the amount of impurities.

Finding the % yield

$$\text{\% yield}=\frac{\text{actual mass of product obtained}}{\text{theoretical mass of product}}\times 100\%$$

Finding the % purity

$$\text{\% purity of a product}=\frac{\text{mass of the pure product}}{\text{mass of the impure product obtained}}\times 100\%$$

Electrolysis

• Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.

Simple electrolytic cell

• A simple electrolyic cell consist of:
  • an anode as a positive (+) electrode
  • a cathode as a negative (-) electrode
  • the electrolyte as the molten or aqueous solution that undergoes electrolysis.

Molten lead(II) bromide

• When the power is turned on and electricity is passed through the molten substance, the positive metal lead ions will move to the cathode.
  • Here, the lead ions will gain electrons to become lead atoms, and lead will form.
  • The lead will be deposited on the surface of the electrode.
• The negative bromide ions will move to the anode
  • Here, each bromide ion will lose one electron, forming bromine ions
  • Two of the bromine atoms formed will combine to form bromine molecules, and you will see bubbles of bromine gas given off
• Equation: PbBr2 -> Pb(l) + Br2 (g)

Concentrated aqueous sodium chloride

• The products formed in the electrolysis of NaCl (aq) are:
  • Hydrogen gas at the cathode
  • Chlorine gas at the anode
  • Leaves behind sodium hydroxide solution
• The observations at:
  • the cathode: efferversence/bubbles of a colorless gas
  • the anode: bubbles of a pale yellow-green gas formed by chlorine
• NOTE: Metals or hydrogen are formed at the cathode. Non-metals (except hydrogen) are formed at the anode.

Molten binary compounds

• A binary compound consists of only two elemets. When molten, the ions are free to move and are discharged at the electrodes.
• RULE In a molten binary compound, there are only two ions present, so predicting the products is straightforward.
• In order to predict the products, you do these steps:
  1. Identify compound and its ions A binary compound splits into a cation and an anion.
  2. Apply electrode rules

Electroplating

• Metal objects are electroplated to improve their appearance and resistance to corrosion.
• Electroplating is using electricity to coat one metal with another.
• To electroplate an object with metal x, the set-up is:
  • the cathode: object that is undergoing electroplating
  • the anode: metal x
  • electrolyte: a solution of a soluble compound of x.

Transfer of charge during electrolysis

Movement of electrons in the external circuit

• The power supply drives electrons through the external circuit
• Electrons flow from the negative terminal of the battery to the cathode
• At the other end, electrons flow from the anode to the positive terminal of the battery
• So in the external circuit, electrons travel from anode to cathode through the wire.

LOSS or GAIN of electrons at the electrodes

• NOTE: Remember this mnemonic: OIL RIG, Oxidation is Loss, Reduction is Gain.

Movement of ions in the electrolyte

• Ions in the molten/dissolved electrolyte carry the charge through the liquid
• Cations move toward the cathode
• Anions move toward the anode
• This ion movement completes the circuit through the electrolyte. Notice how opposite charges attract?

When electrodes are NOT inert

• Inert means chemically inactive. This example uses a solution of copper(II) sulfate, which has blue copper ions, sulfate ions, and hydrogen and hydroxide ions.
• Using carbon/graphite electrodes (inert)
  • At the cathode: Copper ions are discharged. The copper coats the electrode. 2Cu 2+ (aq) + 4e - -> 2Cu (s)
  • At the anode: Oxygen bubbles off. 4OH - (aq) -> 2H2O (l) + O2 (g) + 4e -
  • So copper and oxygen are produced. The blue color of the solution fades because the concentration of copper ions in it decreases as the ions are discharged.
• Using copper electrodes (not inert)
  • At the cathode: Copper is formed and coats the electrode. Cu 2+ (aq) + 2e - -> Cu (s)
  • At the anode: The anode dissolves, giving copper ions in solution. Cu (s) -> Cu 2+ (aq) + 2e -
  • As the anode dissolves, the copper ions move to the cathode to form copper. So, copper moves from the anode to the cathode. The solution stays blue because the concentration of copper stays the same.

Rules for electrolysis of a solution

• At the cathode, either a METAL or HYDROGEN forms.
  • The more reactive an element is, the longer it stays as an ion. Therefore, if a metal is more reactive than hydrogen, its ions stays in solution and hydrogen bubbles off.
  • If a metal is less reactive than hydrogen, the metal forms.
• At the anode, a NON-METAL OTHER THAN HYDROGEN forms.
  • If it is a concentrated solution of a halide (a compound containing Cl-, Br-, or I- ions), then chlorine, bromine, or iodine form.
  • If the halide solution is dilute, or there is no halide, oxygen forms.

Exothermic and Endothermic Reactions

• An exothermic reaction transfers thermal energy to the surroundings leading to an increase in the temperature of the surroundings.
• An endothermic reaction takes in thermal energy from the surroundings leading to a decrease in the temperature of the surroundings.

Describing exothermic reactions

• The equation for exo. reactions is described as:
  • $\text{reactants}\to \text{products}+\text{thermal energy}$
  • The reactants and products have energy. It is stored as chemical energy in their bonds. The total energy does not change, during a reaction. It is the same on each side of the arrow. The products must have less energy than the reactants do.
• In an exothermic reaction, the products have less energy than the reactants do. Energy has been given out.

Describing endothermic reactions

• The equation for endo. reactions is described as:
  • $\text{reactants}+\text{thermal energy}\to \text{products}$
  • The total energy does not change, during the reaction. It is the sam eon each side of the arrow. So it follows that the products must have more energy than the reactants do.
• In an endothermic reaction, the products have more energy than the reactants do. Energy has been taken in.

Activation energy and enthalpy

• Activation energy is the minimum energy that colliding particles must have to react.
• The transfer of thermal energy during a reaction is called the enthalpy change, delta H of a reaction.
  • Delta H is negative for exo. reactions and positive for endo. reactions.

Making and breaking bonds

• Bond breaking is an endo. process.
  • Enthalpy change: Positive. The bonds in the reactant particles must be broken, taking in energy.
• Bond making is an exo. process.
  • Enthalpy change: Negative. New bonds are made, giving the product particles, which releases energy.

Calculating enthalpy changes

• The overall change in enthalpy of the reaction is found like this:
  • enthalpy change for the reaction = energy in for bond breaking - energy out from bond making
• Energy changes in reactions are given as $\text{kJ/mol}$.
• The bond energy is the energy needed to break bonds, or released when the same bonds form. It is given in kJ/mol.

The exothermic reaction between hydrogen and chlorine

• Equation: ${\text{H}}_2+{\text{Cl}}_2\to 2\text{HCl}$
• Bond equation: H-H + Cl-Cl -> 2 H-Cl
• Energy in to break each mole of bonds in the reactants (given in exam):
  • 1 x H-H, 436 kJ
  • 1 x Cl-Cl, 242 kJ
  • Total energy in: 436 + 242 = 678 kJ
• Energy out from the two moles of bonds that form to give the product:
  • 2 x H-Cl which is 2 x 431 = 862 kJ
• Enthalpy change $\delta H$ = energy in - energy out
  • 678 kJ - 862 kJ = -184 kJ
• It is negative, therefore it is an exo. reaction. 184 kJ of energy is given out when 1 mole of hydrogen and 1 mole of chlorine react.

The endothermic decomposition of ammonia

• Equation: $2{\text{NH}}_3\to {\text{N}}_2+3{\text{H}}_2$
• Energy in to break the two moles of bonds:
  • 6 x N-H which is 6 x 391 = 2346 kJ
• Energy out from the four moles of bonds forming:
  • 1 x N≡N, 946 kJ
  • 3 x H-H, 3 x 436 = 1308 kJ
  • Total energy out: 947 + 1308 = 2254 kJ
• Enthalpy change delta H = energy in - energy out
  • 2346 kJ - 2254 kJ = +92 kJ
• It is positive, indicating that this is an endo. reaction. So 92 kJ of energy is taken in to decompose 2 moles of ammonia.

Hydrogen-Oxygen Fuel Cells

• A hydrogen-oxygen fuel cell uses hydrogen and oxygen to produce electricity with water as the only chemical product.

Advantages & disadvantages of using hydrogen-oxygen fuel cells in comparison with gasoline engines in vehicles

The hydrogen-oxygen fuel cell

• Advantages:
  • Only chemical produced is water, which is harmless
  • Hydrogen can be made by the electrolysis of water, which is plentiful, on adding a little acid.
• Disadvantages:
  • Hydrogen is a gas. To reduce its volume and make it practical to use, it must be stored as a liquid under high pressure. This can be expensive or hard to maintain.
  • Hydrogen filling stations are not yet widely available.
  • Most hydrogen is obtained by fossil fuels, and where it is obtained by electrolysis, the electricity is mostly from power stations that burn fossil fuels. These are linked to pollution and climate change.

The gasoline engine

• Advantages:
  • Gas stations are widely available for drivers.
  • Gas is relatively cheap (varies depending on world supply)
• Disadvantages:
  • The burning of gas in engines produce harmful substances:
    • $C{O}_2$, linked to climate change
    • Oxides of nitrogen
    • $CO$ if gas burns in limited oxygen
    • Unburnt volatile gases from the gas
  • Petrol is obtained from oil, a limited and non-renewable resource.

Chemical Reactions

Physical and chemical changes

Properties

• The properties of physical changes are:
  • same internal structure
  • reversible
  • involves lower amount of energy exchange comapred to chemical reactions
  • involves change in texture, shape, temp. or a change in the state of matter.
• Examples of physical changes are an ice cube melting, boiling water, or shredding paper.
• The properties of chemical changes are:
  • new molecular bonds formed/broken, creating a new chemical formula
  • irreversible
  • involves significant energy changes
  • involves color changes, releasing scent, formation of a precipitate, or fizzing.
• Examples of chemical changes are iron rusting, baking a cake, or digesting food.

Differences

Rate of reaction

• Rate is a measure of the change that happens in a single unit of time.
• In general, to find the rate of a reaction, you should measure:
  • the amt. of a reactant used up per unit of time, or
  • the amt. of a product produced per unit of time.

Ways to change the rate of a reaction

1. Changing the concentration of solutions
   • A reaction goes faster when the concentration of a reactant is increased. Reduce the concentration to slow down the reaction.
   • EXPLANATION: There are more particles per unit volume, ∴ higher frequency of collisions ∴ higher frequency of SUCCESSFUL collisions.
2. Changing the pressure of gases
   • A reaction involving gases goes faster when the pressure is increased. Decrease the presure to slow down the reaction.
   • EXPLANATION: Increasing gas pressure means more gas particles per unit volume, ∴ successful collisions are more frequent.
3. Changing the surface area of solids
   • A reaction goes faster when the surface area of a solid reactant is increased. Decrease the surface area of a solid reactant to slow down the reaction.
   • EXPLANATION: More particles of the solid are exposed, ∴ successful collisions are more frequent.
4. Changing the temperature
   • A reaction goes faster when the temperature is raised. A rate generally doubles for an increase of 10°C. Decrease the temperature to slow down a reaction.
   • EXPLANATTION: There are two reasons: there are more collisions, and more have sufficient energy to be successful.
5. Adding or removing a catalyst, including enzymes
   • A catalyst is a substance that increases the rate of a reaction, and is unchanged at the end of the reaction.
   • Enzymes are proteins made by cells, to act as biological catalysts.
   • Adding a catalyst increases the rate of a reaction.
   • EXPLANATION: A catalyst lowers the activation energy of a reaction, now the particles need less energy to react, ∴ more successful collisions and the reaction is faster.

Collision theory

• For a chemical reaction to occur, reacting particles must collide with one another. Not every collision results in a reaction. A successful collision must meet these conditions:
  • The particles must collide with sufficient energy. This is the activation energy.
  • The particles must collide with the correct orientation.

Reversible Reactions and Equilibrium

• In a reversible reaction, you can change the direection of the reaction by changing the reaction conditions.
• The symbol ⇄ is used to indicate that a reaction is reversible.
• A reversible reaction in a closed system is at equilibrium when:
  • the rate of the forward reaction is equal to the rate of the reverse reaction
  • the concentrations of reactants and products are no longer changing

Changing conditions to change direction

• The general reversible reaction for hydrated/anhydrous compounds is:
  • hydrated salt ⇄ anhydrous salt + water

Effect of heat on hydrated compounds

• The forward reaction, hydrated → anhydrous + water, is endothermic because it requires heat to break the bonds between the salt and water moleucules.
  • When heat is applied, the equilibrium shifts to the right (forward reaction)
  • The hydrated compount loses its water of crystallization, forming the anhydrous compound.
  • This is essentially thermal decomposition.
• Example: Blue hydrated copper(II) sulfate, $CuS{O}_4\cdot 5H_{2}O$, is heated, then it turns white as it becomes $CuS{O}_4$, releasing water vapor.
  • Equation: $CuS{O}_4\cdot 5H_{2}O\to CuS{O}_4+5H_{2}O$

Addition of water to anhydrous compounds

• The reverse reaction, anhydrous + water → hydrated, is exothermic because energy is released when water bonds to the salt.
  • When water is added to the anhydrous salt, the equilibrium shifts to the left (reverse direction).
  • The anhydrous compound absorbs the water, reforming the hydrated compound.
  • This is the reverse of dehydration.
• Example: White anhydrous $CuS{O}_{4}haswateradded\to itturns\ast \ast blue\ast \ast againas$CuSO_{4}·5H_{2}O$reforms,andthemixturegetswarm\ast (exothermic)\ast .-\ast \ast Equation\ast \ast :$CuSO_{4} + 5H_{2}O → CuSO_{4}·5H_{2}O$

Shifting the equilibrium

• When a reversible reaction is in equilibrium and you make a change, the system acts to oppose the change, and restore equilibrium. A new equilibrium mixture forms.

Change in temperature

• A high temperature favors the endothermic direction.
  • TIP: If ΔH is given, it is endo. if it is +Δ.
• A low temperature favors the exothermic direction.
  • TIP: If ΔH is given, it is exo. if it is -Δ.

Change in pressure (GASES ONLY)

• A high pressure shifts to the side with fewer moles of gas.
• A low pressure shifts to the side with more moles of gas.
• There is no effect is the gas moles are equal on both sides.

Change in concentration

• A high concentration of a reactant shifts the equilibrium to use up a reactant.
• A low concentration of a reactant shifts the equilibrium to replace it.

Using a catalyst

• There is NO CHANGE in position of equilibrium. Rate of both forward and reverse reactions increase equally.
  • Remember that a catalyst speeds up the rate of a reaction without it being used up. It does not affect the equilibrium.

The Haber process

• The equation for the Haber process:
  • $N_{2}(g) + 3H_{2}(g) ⇄ 2NH_{3}(g)$
  • nitrogen + hydrogen ⇄ ammonia

How ammonia is made

1. Nitrogen is obtained by:
   • reacting natural gas, methane with steam
   • cracking hydrocarbons from petroleum. The two gases are mixed and scrubbed to remove impurities.
2. The mixture is compressed. More gas is pumped in until pressure reaches 200 atm.
3. The compressed gas flows into the converter, a round tank with beds of iron at 450°C, where the key reaction occurs. Iron is the catalyst.
4. The mixture is cooled until the ammonia condenses to a liquid. The nitrogen and hydrogen are recycled to the converter for another chance to react. Steps 3 and 4 are continually repeated.
5. The ammonia is run into tanks and stored as a liquid under pressure.

• The typical conditions in the Haber process:
  • 450°C to give a faster rate
  • 200 atm / 20,000 kPa is safer and saves money
  • an iron catalyst to speed up the reaction but does not affect yield.

The contact process

• The equation for the Contact process:
  • $2SO_{2}(g) + O_{2}(g) ⇄ 2SO_{3}(g)$-sulfurdioxide+oxygen\rightleftarrows sulfurtrioxide1.Sulfuris\ast \ast burned\ast \ast in\ast \ast air\ast \ast .2.\ast \ast Theequationtomakesulfurdioxide\ast \ast is$S(s) + O_{2}(g) → SO_{2}(g)$.Thesulfurdioxideismixedwithmoreair.-Sulfurdioxideisobtainedby\ast \ast burningsulfur\ast \ast or\ast \ast roastingsulfurores\ast \ast and\ast \ast oxygen\ast \ast \ast (air)\ast .3.Itisthenpassedoverfourbedsofcatalyst,\ast \ast pelletsofvanadium(V)oxide\ast \ast at\ast \ast 450°C\ast \ast .4.Thesulfurtrioxidegasis\ast \ast dissolved\ast \ast in\ast \ast concentratedsulfuricacid\ast \ast .Anyunreactedsulfurdioxideisrecycled.-\ast \ast Concentratedsulfuricacid,$H_{2}SO_{4}$\ast \ast :-$H_{2}O(l) + SO_{3}(g) → H_{2}SO_{4}$