Nutrition.

Nutrition is how living things obtain and use food — for energy, for growth and repair, and to keep the body working. Plants make their own; animals must take theirs in and break it down.

1 · Two ways to feed

• Autotrophic nutrition — organisms make their own food. Green plants do this by photosynthesis (see Bioenergetics).
• Heterotrophic nutrition — organisms take in ready-made food from others. Three styles: holozoic (ingest solid food and digest it inside — animals), saprotrophic (feed on dead matter by secreting enzymes — many fungi & bacteria), and parasitic (feed on a living host).

2 · A balanced diet

A healthy human diet supplies energy and raw materials in the right proportions:

3 · The human digestive system

Food passes along the alimentary canal through five stages:

1. Ingestion — taking food in.
2. Digestion — breaking large insoluble molecules into small soluble ones (mechanically and by enzymes).
3. Absorption — soluble products pass into the blood.
4. Assimilation — cells use the absorbed nutrients.
5. Egestion — undigested waste leaves as faeces.

Along the canal

• Mouth — teeth chew (mechanical digestion); salivary amylase begins starch digestion.
• Oesophagus — carries food to the stomach by peristalsis (waves of muscle).
• Stomach — churns food; pepsin + hydrochloric acid (≈ pH 2) digest protein and kill microbes.
• Small intestine — bile from the liver emulsifies fat; pancreatic and intestinal enzymes finish digestion; the ileum absorbs nutrients through its villi.
• Large intestine — absorbs water; forms faeces, egested via the rectum and anus.

4 · Absorption — why villi matter

The ileum's inner surface is folded into millions of finger-like villi. They give a huge surface area, a thin (one-cell) wall, a dense blood capillary network (for sugars & amino acids) and a lacteal (for fats) — making absorption fast and efficient.