IGCSE Biology Human nutrition notes

A balanced diet provides all the nutrients the body needs, in the correct proportions for energy, growth and health. The seven components the syllabus expects: carbohydrates, fats and oils, proteins, vitamins C and D, mineral ions (calcium and iron), fibre (roughage) and water.

Two deficiency diseases are named in the syllabus: scurvy — bleeding gums, poor wound healing, caused by vitamin C deficiency — and rickets — soft, deformed bones in children, caused by vitamin D deficiency (with calcium shortage), which stops calcium being absorbed for bone mineral.

A labelled diagram to picture: a long tube from mouth to anus with pouches and glands. In order: mouth → oesophagus → stomach → small intestine (duodenum then ileum) → large intestine (colon, rectum, anus). Hanging off it: salivary glands near the mouth, the liver and gall bladder feeding bile into the duodenum, and the pancreas sending enzymes below the stomach.

Physical digestion breaks food into smaller pieces without any chemical change to the food molecules. Its purpose: increasing the surface area of food so enzymes in chemical digestion can work faster on it. Teeth do it by cutting and grinding; the stomach wall does it by churning food with muscular waves, mixing it into a liquid.

Tooth structure, from the outside in: hard enamel coating, dentine beneath it, and the pulp cavity containing nerves and blood vessels. The root is covered with cement and held in the bone and gums. Enamel is the hardest substance in the body but cannot regrow — which is why sugar damage is permanent.

Supplement: bile, made in the liver and stored in the gall bladder, does two jobs. First, emulsification: bile breaks large fat droplets into tiny droplets, multiplying the surface area for lipase — physical digestion by chemistry-adjacent means, since no fat molecules are chemically changed. Second, bile is alkaline: it neutralises the acidic food and gastric juice arriving in the duodenum, giving the intestinal enzymes the slightly alkaline pH they need.

Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules — the only form that can be absorbed through the gut wall into the blood. Three enzyme families do the work:

Supplement: starch digestion is a two-step relay — amylase breaks starch into maltose, then maltase, sitting on the membranes of the epithelium lining the small intestine, breaks maltose into glucose, the absorbable end-product. Protein digestion splits by location: pepsin works in the acidic stomach, trypsin in the alkaline small intestine — same substrate, different pH optima (Topic 4). The stomach’s hydrochloric acid kills most harmful microorganisms in food and provides pepsin’s acidic optimum. Bile then raises the pH as the food enters the duodenum, so trypsin and the other intestinal enzymes can work.

A labelled diagram to picture: the digestive tube as a pH map — mouth (near neutral), stomach (strongly acidic, pH ~2), duodenum onward (slightly alkaline, buffered by bile and pancreatic juice). Enzyme position matches enzyme optimum, every time.

The small intestine — especially the ileum — is where nutrients are absorbed. Most water is absorbed there too, with the remainder absorbed by the colon as the faeces form. The absorbing surface is multiplied at three scales (Supplement): the inner wall is folded; the folds carry microscopic finger-like projections called villi; and each villus’s epithelial cells bristle with microvilli. Together they increase the internal surface area enormously, so absorption keeps pace with digestion.

Supplement: each villus is a finger a fraction of a millimetre long, with: an outer layer of epithelial cells (with microvilli on their free surface); a network of capillaries inside, receiving glucose and amino acids; and a single blind-ended lacteal, a lymph vessel that receives the reassembled fats. Thin walls (one cell thick) mean short diffusion distances; the rich blood supply maintains a steep concentration gradient by carrying absorbed nutrients away.