Photosynthesis

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

Equations

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

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

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.

Carbohydrate	Primary use/function
Glucose	Used immediately in respiration to release energy for plant growth and active transport.
Starch	Used as an energy store. It is insoluble, meaning it does not affect the water potential of the cell (prevents unwanted osmosis).
Cellulose	Used to build cell walls, providing structural strength and support to the plant cells.
Sucrose	Used for transport through the phloem. It is less reactive than glucose and dissolves easily.
Nectar	Used to attract insects to the flowers for pollination.

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

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

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.

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 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.

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.

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

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.

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.

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