Ad Banner Placeholder

Cambridge IGCSE Biology · 0610

Chapter 7: Human nutrition (Part 2)

Physical digestion

Definition
The breakdown of food into smaller pieces without chemical change to the food molecules.
Importance
It increases the surface area of food for the action of enzymes in chemical digestion.
Locations
  • Mouth: Chewing and grinding by teeth.
  • Stomach: Churning by muscular walls to mix and grind food.

Role of teeth

Teeth are embedded in bone and the gums.

Types of teeth
  • Incisors and canines: Used for biting and tearing.
  • Premolars and molars: Used for chewing and grinding.
Structure
  • Enamel: Very hard outer layer.
  • Dentine: Layer under enamel, less hard and decays more easily.
  • Pulp: Contains blood vessels and nerves.
  • Cement: Covers the root and stabilises the tooth.
Cross-section of a human tooth showing enamel, dentine, pulp with nerves and blood vessels, cement, and the position of the gum and jaw bone
Diagram 1: Structure of a human tooth. Cross-section showing enamel, dentine, pulp with nerves and blood vessels, cement, and the position of the gum and jaw bone.

Chemical digestion

Definition
The breakdown of large insoluble molecules into small soluble molecules so they can be absorbed.
Specific enzymes
  • Amylase: Breaks down starch into simpler reducing sugars.
  • Proteases: Break down protein into amino acids.
  • Lipase: Breaks down fats and oils into fatty acids and glycerol.

Digestive secretions and enzymes

Starch digestion
Amylase breaks starch into maltose; then maltase (membrane-bound in the small intestine) breaks maltose into glucose.
Protein digestion
  • Pepsin: Functions in the acidic conditions of the stomach.
  • Trypsin: Functions in the alkaline conditions of the small intestine.
Hydrochloric acid
Found in gastric juice; kills harmful microorganisms and provides the optimum acidic pH for protease (pepsin) action.
Bile
Produced in the liver and stored in the gall bladder. It has two roles:
  1. Neutralisation: It is alkaline and neutralises stomach acid to provide a suitable pH for enzymes in the duodenum.
  2. Emulsification: It breaks large fat globules into smaller droplets to increase the surface area for lipase.
Where enzymes are secreted and where they act
Enzyme Secreted by Acts in
Amylase Salivary glands Mouth (starts starch digestion)
Amylase Pancreas (into duodenum) Small intestine (duodenum)
Protease (pepsin) Stomach (gastric glands) Stomach
Protease (trypsin) Pancreas (into duodenum) Small intestine (duodenum)
Lipase Pancreas (into duodenum) Small intestine (duodenum)

Pancreatic enzymes reach the duodenum through the pancreatic duct. Maltase is not secreted into the gut lumen — it is a membrane-bound enzyme on the surface of cells lining the small intestine.

Exam Traps

  • Do not say bile breaks fats into fatty acids — lipase does that after emulsification.
  • Avoid placing trypsin in the stomach or pepsin in the alkaline duodenum.
  • Do not state that all digestive enzymes come from the pancreas; salivary amylase and gastric pepsin are secreted elsewhere.

Absorption

Location
Most water and all nutrients are absorbed in the small intestine. Some remaining water and salts are absorbed in the colon.
Adaptations of the small intestine
  • Villi and microvilli: Hair-like projections that greatly increase the internal surface area for absorption.
  • Capillaries: Located in each villus to carry away absorbed nutrients and maintain a concentration gradient.
  • Lacteals: Specific vessels in the villi that absorb fats.
Structure of a villus showing epithelium with microvilli, blood capillaries, and a central lacteal
Diagram 2: Structure of a villus. The outer layer of epithelium with microvilli, the internal network of blood capillaries, and the central lacteal.

Exam Traps

  • Do not say fats enter blood capillaries directly — they enter the lacteal first.

0/15

Ad Banner Placeholder