7.4 Chemical Digestion

2026 Syllabus Objectives

  1. Describe chemical digestion as the breakdown of large insoluble molecules into small soluble molecules
  2. State the role of chemical digestion in producing small soluble molecules that can be absorbed
  3. Describe the functions of enzymes as follows:
    • (a) amylase breaks down starch to simple reducing sugars
    • (b) proteases break down protein to amino acids
    • (c) lipase breaks down fats and oils to fatty acids and glycerol
  4. State where, in the digestive system, amylase, protease and lipase are secreted and where they act
  5. Describe the functions of hydrochloric acid in gastric juice, limited to killing harmful microorganisms in food and providing an acidic pH for optimum enzyme activity
  6. Describe the digestion of starch in the digestive system:
    • (a) amylase breaks down starch to maltose
    • (b) maltase breaks down maltose to glucose on the membranes of the epithelium lining the small intestine
  7. Describe the digestion of protein by proteases in the digestive system:
    • (a) pepsin breaks down protein in the acidic conditions of the stomach
    • (b) trypsin breaks down protein in the alkaline conditions of the small intestine
  8. Explain that bile is an alkaline mixture that neutralises the acidic mixture of food and gastric juices entering the duodenum from the stomach, to provide a suitable pH for enzyme action

What is Chemical Digestion? 🔑

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Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules using enzymes. This process is essential because only small, soluble molecules can be absorbed through the wall of the small intestine into the bloodstream.

Key Point: Physical digestion (e.g., by teeth and stomach churning) increases the surface area of food, allowing enzymes to make contact with molecules more easily and speed up chemical digestion.

The Role of Chemical Digestion

Chemical digestion produces small soluble molecules that can be absorbed through the epithelium of the small intestine. Large molecules like starch, proteins, and fats cannot pass through cell membranes, so they must first be broken down into their smaller building blocks:

  • Starch → glucose
  • Proteins → amino acids
  • Fats/oils → fatty acids and glycerol

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