Topic 2 of 18Pearson EdExcel

Nutrition & Digestion

Food groups, the digestive system, and nutrient absorption

Food groups and their functions:

Carbohydrates — energy source. Starch (complex) and glucose (simple). Tested with iodine (starch turns blue-black).

Proteins — growth and repair. Made of amino acids. Tested with Biuret reagent (turns purple).

Fats (lipids) — energy storage, insulation, cell membranes. Tested with ethanol emulsion (turns milky white).

Vitamins and minerals — regulatory functions. Vitamin C prevents scurvy; iron prevents anaemia.

Fibre — aids peristalsis in the gut (prevents constipation).

Water — transport, reactions, temperature regulation.

The digestive system:

Mouth — chewing + amylase (breaks starch → maltose)

Stomach — pepsin (breaks proteins), HCl (kills bacteria, pH 2)

Small intestine — lipase (fat → fatty acids + glycerol), protease, amylase from pancreas. Villi absorb nutrients (large surface area, thin walls, good blood supply)

Large intestine — absorbs water; faeces formed

Enzymes are substrate-specific. Amylase digests starch but NOT protein.

Key Points to Remember

1Carbohydrates for energy, proteins for growth, fats for storage
2Iodine → starch, Biuret → protein, ethanol emulsion → fat
3Digestion: mouth → stomach → small intestine
4Villi maximise absorption in small intestine

Pakistan Example

Daal Chawal and Nutrition — Pakistan's Everyday Science

A Pakistani student's daal chawal lunch covers carbohydrates (rice = starch), protein (lentils = protein), and fat (cooking oil). Iodine would turn the rice blue-black (starch confirmed). The daal's protein is broken down by pepsin in the stomach, then absorbed as amino acids through intestinal villi — exactly as EdExcel Biology describes.

Quick Revision Infographic

Biology — Quick Revision