IGCSE Biology: Human Nutrition — Revision Notes

Topic 7 spans five sub-topics (7.1-7.5) and is examined on every tier — get the Core statements completely secure first, since Supplement content builds directly on top of them rather than replacing them.

7.1 Diet — all Core, no Supplement content

A balanced diet contains the right foods in the right amounts. Know the principal dietary source and importance of each nutrient:

Nutrient	Importance
Carbohydrates	Energy
Fats and oils	Energy store, insulation
Proteins	Growth and repair
Vitamin C	Deficiency causes scurvy
Vitamin D	Deficiency causes rickets
Calcium	Bones and teeth
Iron	Haemoglobin production
Fibre (roughage)	Aids movement of food through the gut
Water	Solvent, transport medium

Only vitamins C and D and mineral ions calcium and iron are syllabus-specified — do not spend revision time on other vitamins or minerals for this topic; questions naming scurvy or rickets are asking specifically for a vitamin deficiency link, not a general “eat healthily” answer.

7.2 Digestive system — naming and function

Alimentary canal (Core): mouth, oesophagus, stomach, small intestine (duodenum and ileum), large intestine (colon, rectum, anus).

Associated organs (Core): salivary glands, pancreas, liver, gall bladder.

Five processes to define precisely, not just name:

• Ingestion — taking food/drink into the body.
• Digestion — breakdown of food.
• Absorption — movement of nutrients from intestines into blood.
• Assimilation — uptake and use of nutrients by cells.
• Egestion — removal of undigested food as faeces.

A common exam-trap confusion: egestion is not excretion. Egestion removes food that was never absorbed into the body; excretion (a different topic) removes waste made by the body’s own metabolism.

7.3 Physical digestion — mechanical breakdown

Physical digestion (Core) breaks food into smaller pieces without chemical change to the molecules — this increases surface area for enzymes to act on in chemical digestion.

• Teeth types (Core): incisors, canines, premolars, molars — know their structure (enamel, dentine, pulp, nerves, blood vessels, cement) and function in physical digestion.
• Stomach’s role in physical digestion (Core): churning food.
• Supplement only: bile’s role in emulsifying fats and oils — breaking large fat droplets into smaller ones to increase surface area for lipase, without any chemical change to the fat itself (bile is not an enzyme).

7.4 Chemical digestion — enzymes and their exact roles

Chemical digestion (Core) breaks large insoluble molecules into small soluble molecules that can be absorbed.

Enzyme	Substrate → Product
Amylase	Starch → simple reducing sugars
Proteases	Protein → amino acids
Lipase	Fats and oils → fatty acids and glycerol

Hydrochloric acid in gastric juice (Core) has two distinct functions — kills harmful microorganisms in food, AND provides optimum acidic pH for enzyme activity. Both functions are examinable separately; naming only one loses marks.

Supplement adds precision:

• Amylase breaks starch to maltose specifically (not glucose directly); maltase then breaks maltose to glucose at the small intestine’s epithelium membranes.
• Pepsin breaks down protein in the stomach’s acidic conditions; trypsin breaks down protein in the small intestine’s alkaline conditions — same job (protease), opposite pH environments.
• Bile is alkaline and neutralises acidic stomach contents entering the duodenum, creating a suitable pH for the small intestine’s own enzymes to work.

7.5 Absorption — where and how nutrients enter the blood

Core: the small intestine is where nutrients are absorbed; most water is absorbed from the small intestine, with some also absorbed from the colon.

Supplement: the small intestine’s absorbing surface area is increased by villi and microvilli. Know the structure of a villus and the distinct roles of its two internal transport routes:

• Capillaries absorb sugars and amino acids.
• Lacteals (part of the lymphatic system) absorb fatty acids and glycerol (fat digestion products) specifically — not sugars or amino acids.

Exam traps

• Confusing egestion (undigested food leaving the body) with excretion (metabolic waste removal) — these are different processes in different topics.
• Naming only one function of hydrochloric acid in the stomach when the syllabus requires both (killing microorganisms and providing optimum pH).
• Stating physical digestion involves a chemical change to food molecules — it does not; only chemical digestion does.
• Mixing up pepsin and trypsin’s pH environments (pepsin = acidic stomach; trypsin = alkaline small intestine).
• Assuming capillaries and lacteals absorb the same nutrients — they specialise in different digestion products.

Self-test

1. Which two vitamins and which two mineral ions does this topic specify, and what deficiency disease is linked to each vitamin?
2. Define egestion and explain how it differs from excretion.
3. What does bile do to fats, and why is this not chemical digestion?
4. Name the two functions of hydrochloric acid in gastric juice.
5. What is the key difference between what capillaries and lacteals absorb in a villus?

Answers: 1. Vitamin C (scurvy) and Vitamin D (rickets); calcium and iron. 2. Egestion is the removal of undigested food from the body as faeces; excretion removes waste substances made by the body’s own metabolism, which is a different process entirely. 3. Bile emulsifies fats and oils, breaking large droplets into smaller ones to increase surface area for lipase — this is a physical change, not a chemical one, since the fat molecules themselves are unchanged. 4. Killing harmful microorganisms in food, and providing an acidic pH for optimum enzyme activity. 5. Capillaries absorb sugars and amino acids; lacteals absorb fatty acids and glycerol.