Biological molecules and enzymes for O-Level Biology (SEAB 6093): the elements and building blocks of carbohydrates, fats and proteins, how enzymes work as biological catalysts, the effect of temperature and pH, and the standard food tests

What this module covers

Biological molecules and enzymes is the chemistry foundation of O-Level Biology (SEAB 6093) and sits in the Cells and Chemistry of Life theme. You need to know what the main food molecules are made of, how the enzymes that build and break them work, what changes their rate, and how to test a food sample in the laboratory. The same ideas return throughout the syllabus: enzymes appear again in digestion, photosynthesis and respiration, and the food tests are a favourite Paper 3 practical task.

This overview links every dot point in the module. Work through them in order, then use the questions at the end to check yourself: see the full set at /sg-o-level/biology/syllabus.

The three main food molecules

Start with the molecules themselves. The page on carbohydrates, fats and proteins sets out, for each, the chemical elements it contains, its building block, and its role in the body. Carbohydrates (carbon, hydrogen, oxygen) are built from simple sugars and provide energy. Fats (carbon, hydrogen, oxygen) are built from fatty acids and glycerol and store energy and insulate. Proteins (carbon, hydrogen, oxygen and nitrogen) are built from amino acids and are used for growth, repair and making enzymes. The presence of nitrogen is what sets proteins apart.

How enzymes work

Enzymes are the workhorses that build and break these molecules. The page on enzymes and how they work defines an enzyme as a biological catalyst, a protein that speeds up a reaction without being used up, and explains the lock and key model. The substrate fits the active site like a key in a lock, which is why each enzyme is specific to one reaction: only its own substrate has the complementary shape. Because the enzyme is not consumed, a small amount can catalyse many substrate molecules in turn.

What changes the rate of an enzyme reaction

Two conditions matter most. The page on factors affecting enzyme activity explains how temperature and pH change the rate. Raising temperature speeds the reaction up to an optimum (usually about 37 degrees Celsius in humans), then a higher temperature denatures the enzyme so the active site changes shape and the substrate no longer fits. pH behaves the same way around an optimum: too acidic or too alkaline denatures the enzyme. The crucial exam idea is that denaturing is a permanent change of shape, not a temporary slowing.

Testing for food molecules

The practical pay-off is the standard food tests. The page on food tests gives the reagent, method and colour change for each, and how to write up a result for full marks. These four tests are examined in both the written and practical papers, so learn the reagent and the colour change together as a pair.

How this module is examined

• Compare with structure. Comparison questions on the three food molecules reward a clear three-part answer (elements, building block, role) rather than a vague description.
• Explain, do not just describe. For temperature and pH, examiners want the mechanism: link the change in shape of the active site to whether the substrate still fits.
• Practical precision. In Paper 3 the food tests reward the exact reagent, the right condition (heat for Benedict's), and the correct colour change, written as a clear result and conclusion.

Check your knowledge

A mix of recall and explanation questions covering the molecules, the enzymes that act on them, and the food tests. Attempt them, then check against the solutions.

1. State the chemical elements, building block and one role of a protein. (3 marks)
2. Using the lock and key model, explain why an enzyme is specific to one substrate. (2 marks)
3. Explain why the rate of an enzyme-controlled reaction falls to zero above a certain temperature. (3 marks)
4. State the reagent and colour change for the test for starch. (2 marks)
5. A student tests a food sample and observes a brick-red colour after heating with Benedict's solution. State what this shows. (1 mark)