This topic explains how the human body turns food into usable molecules: the parts of the gut, the enzymes that digest each food group, how the small intestine absorbs the products, and how cells use them. Questions usually ask you to follow one substance along the whole route. Check the current Cambridge syllabus for your exam year to confirm the exact content.
It sits inside Cambridge IGCSE Biology and depends on biological molecules and enzymes, where you meet enzymes, substrates and the effect of temperature and pH.
What should you know first?
You should know that enzymes are biological catalysts, that each one has a specific substrate, and that heat or extreme pH can stop them working. You should also recognise carbohydrates, proteins and fats as the three large food molecules.
Simple arithmetic helps with the data questions: multiplying a value per 100 g up to a portion, and working out a rate as 1 divided by time.
One orienting example
The data here are invented. A student chews a cracker and keeps it in the mouth for a minute. The cracker starts to taste slightly sweet.
Ask three questions: the food component, the enzyme and the product.
The component is starch, the enzyme is amylase in saliva, and the product is a sugar called maltose, which tastes sweet.
The same three questions, component, enzyme, product, solve most questions in this topic. Only the names change: protein with protease gives amino acids, and fat with lipase gives fatty acids and glycerol.
In what order should you study it?
- Trace a food component through digestion: start here, because it gives you the route that every other lesson zooms into.
- Link an enzyme with substrate and products: this fixes the three-part pattern that exam questions test repeatedly.
- Explain absorption using villus structure: the route ends at the small intestine wall, so the next skill is explaining why it is built that way.
- Distinguish assimilation from absorption: this separates two words that are easy to swap.
- Interpret a dietary dataset without personal diet prescriptions: this puts the topic into tables and numbers, and keeps conclusions inside the data.
- Human nutrition and digestion practice set: mixed questions that do not tell you which idea to use.
What are the common traps?
- Writing “breaks down food” with no enzyme, substrate or product. Name all three.
- Placing digestion in the wrong organ, such as starch digestion in the stomach.
- Saying digestion and absorption are the same. Digestion changes the molecules; absorption moves them.
- Confusing egestion with excretion. Egestion removes undigested food as faeces.
- Giving a feature without its function, such as “villi have a lot of capillaries” with no link to the concentration gradient.
- Turning a data table into personal advice. The data describe a sample or a food, not you.
How should you use the practice set?
Attempt the practice set after the first three lessons, then again after the whole topic. Write full answers on paper first and read the worked solutions to compare your reasoning, not only your final answer.
The mistake log and retest queue helps you record slips by type and retest them a few days later. If you would like a teacher to go through your answers, online one-to-one Biology tuition starts with a paid one-hour trial.