7.3 Physical Digestion

2026 Syllabus Objectives

  1. Describe physical digestion as the breakdown of food into smaller pieces without chemical change to the food molecules
  2. State that physical digestion increases the surface area of food for the action of enzymes in chemical digestion
  3. Identify in diagrams and images the types of human teeth: incisors, canines, premolars and molars
  4. Describe the structure of human teeth, limited to: enamel, dentine, pulp, nerves, blood vessels and cement, and understand that teeth are embedded in bone and the gums
  5. Describe the functions of the types of human teeth in physical digestion of food
  6. Describe the function of the stomach in physical digestion
  7. Outline the role of bile in emulsifying fats and oils to increase the surface area for chemical digestion

Understanding Digestion 🔑

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The food we eat contains large molecules of starch (a carbohydrate), protein, and fat. These molecules are too large to pass through the walls of the intestines into the bloodstream. They must be broken down into smaller molecules through a process called digestion, which allows them to be absorbed.

Digestion occurs in two distinct stages:

Physical Digestion

Physical digestion is the breakdown of large pieces of food into smaller pieces without any chemical change to the food molecules.

Key mechanisms:

  • Primarily performed by the teeth through biting and chewing
  • Also assisted by the churning movements of the stomach
  • Does not alter the chemical components of the food

📌 Important: Physical digestion only changes the size of food pieces, not their chemical structure.

Chemical Digestion

Chemical digestion involves breaking apart large molecules into smaller molecules through chemical reactions catalysed by enzymes.

The smaller molecules produced are:

  • Soluble in water
  • Easier to absorb into cells

Summary of chemical digestion:

Large MoleculeEnzymeSmall Molecules Produced
StarchAmylaseSimple reducing sugars
ProteinProteaseAmino acids
FatLipaseFatty acids and glycerol

Note: Vitamins, water, and mineral ions are already small enough to be absorbed directly and do not require chemical digestion.

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