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).
| Large molecule | Chemical elements present | Smaller subunits |
|---|---|---|
| Carbohydrates | C, H, O | Glucose (forms starch, glycogen, cellulose) |
| Proteins | C, H, O, N, S | Amino acids |
| Fats and oils | C, H, O | Fatty acids and glycerol |
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
- Dissolve the sample by shaking it thoroughly with ethanol.
- 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