ZIMSEC O Level · Biology · Form 3
Biological molecules
- Carbohydrates (C, H, O): sugars and starch for energy; cellulose in cell walls.
- Proteins: made of amino acids; used for growth, repair, enzymes and hormones.
- Fats and oils: made of fatty acids and glycerol; energy store and insulation.
Food tests: iodine (starch, blue-black), Benedict's (reducing sugar, brick-red on heating), biuret (protein, purple), ethanol emulsion (fat, cloudy white).
Enzymes
Enzymes are proteins that act as biological catalysts: they speed up reactions without being used up.
- Each enzyme is specific: its active site fits only one substrate (lock and key).
- Temperature: activity rises to an optimum (about 37 °C in humans); above it the enzyme is denatured – the active site changes shape permanently.
- pH: each enzyme has an optimum pH (pepsin in the stomach works best in acid; amylase near neutral).
Uses
Enzymes in biological washing powders, in brewing, and in making fruit juice.
Exam tip: Low temperatures make enzymes inactive but do NOT denature them – activity returns when warmed.
Check yourself
- Name the building blocks of proteins.
- What happens to an enzyme at 70 °C?
- Why can amylase not digest proteins?
Show answers
- Amino acids.
- It is denatured – the active site changes shape so the substrate no longer fits.
- Enzymes are specific; protein molecules do not fit amylase's active site.
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