ZIMSEC O Level · Biology · Form 3
The alimentary canal
Ingestion → digestion → absorption → assimilation → egestion.
- Mouth: teeth chew (mechanical digestion); salivary amylase breaks starch to maltose.
- Oesophagus: food moved by peristalsis.
- Stomach: hydrochloric acid kills bacteria and gives the low pH for pepsin, which digests protein.
- Small intestine (duodenum and ileum): pancreatic juice and intestinal enzymes complete digestion; bile from the liver (stored in the gall bladder) emulsifies fats.
- Ileum: absorbs digested food through villi into the blood and lacteals.
- Colon: absorbs water. Rectum and anus: egestion of faeces.
Enzymes
- Amylase: starch → maltose. Protease: proteins → amino acids. Lipase: fats → fatty acids + glycerol.
Adaptations of villi
Large surface area (villi and microvilli), one-cell-thick walls, a rich blood supply, and lacteals for fat absorption.
Teeth and diet
Incisors cut, canines tear, premolars and molars grind. Tooth decay is caused by bacteria making acid from sugars. Deficiency diseases: kwashiorkor (protein), rickets (vitamin D/calcium), scurvy (vitamin C), anaemia (iron).
Exam tip: Bile is not an enzyme – it emulsifies fats into small droplets, increasing the surface area for lipase.
Check yourself
- Name the enzyme that digests fats and its products.
- State the role of hydrochloric acid in the stomach.
- Give two adaptations of villi for absorption.
Show answers
- Lipase; fatty acids and glycerol.
- Kills bacteria and provides the acid pH for pepsin.
- Any two: large surface area, thin walls, rich blood supply, lacteals.
Questions about this topic (0)
No questions about this topic yet.
Sign in or create a free account to join in.