Digestion — a journey
Biology · Key Stage 3 · The School
The route
Mouth → oesophagus → stomach → small intestine → large intestine. Teeth cut and grind; saliva starts breaking down starch; the muscular oesophagus squeezes food down (peristalsis — it works even upside down); the stomach churns it in acid; the intestines finish the job.
Enzymes: molecular scissors
Food molecules are too big to pass into the blood. ENZYMES cut them down: carbohydrases cut carbohydrates into sugars, proteases cut proteins into amino acids, lipases cut fats into fatty acids and glycerol. Each enzyme cuts only its own target — a lock-and-key fit.
Absorption needs surface area
The small intestine's wall is folded into millions of finger-like VILLI, each wrapped in blood vessels. This gives a huge surface area — spread flat it would approach the size of a tennis court — so digested food passes quickly into the blood. The large intestine then reclaims the water.
Why the same enzyme cannot work everywhere
Enzymes are fussy about conditions as well as targets. The protease in your stomach works best at about pH 2, which is why the stomach makes hydrochloric acid. Move that same enzyme into the small intestine, where bile and pancreatic juice hold the pH near 8, and it stops working — its shape changes, the lock no longer fits the key, and above roughly 40 °C it is destroyed outright. So the small intestine uses a DIFFERENT protease, one suited to alkaline conditions. Bile matters here too, and it is not an enzyme: it neutralises the acid arriving from the stomach, and it EMULSIFIES fat into tiny droplets. That does not digest the fat; it multiplies the surface area lipase can work on, which is why fat digestion collapses when the bile duct is blocked.