AQA GCSE Biology

Revision Notes

2.2.4 Enzymes & Digestion

Chemical Digestion

  • The purpose of digestion is to break down large, insoluble molecules into smaller, soluble molecules that can be absorbed into the bloodstream
  • Large insoluble molecules, such as starch and proteins, are made from chains of smaller molecules which are held together by chemical bonds. These bonds need to be broken
  • Enzymes are biological catalysts – they speed up chemical reactions without themselves being used up or changed in the reaction
  • There are three main types of digestive enzymescarbohydrases, proteases and lipases


  • Carbohydrases break down carbohydrates to simple sugars. Amylase is a carbohydrase which breaks down starch into maltose, which is then broken down into glucose by the enzyme maltase
  • Amylase is made in the salivary glands, the pancreas and the small intestine


The digestion of starch, IGCSE & GCSE Biology revision notes

Diagram showing the digestion of starch


  • Proteases are a group of enzymes that break down proteins into amino acids in the stomach and small intestine
  • Protein digestion takes place in the stomach and small intestine, with proteases made in the stomach (pepsin), pancreas and small intestine


The digestion of proteins, IGCSE & GCSE Biology revision notes

Diagram showing the digestion of proteins


  • Lipases break down lipids (fats) to glycerol and fatty acids.
  • Lipase enzymes are produced in the pancreas and secreted into the duodenum


The digestion of lipids, IGCSE & GCSE Biology revision notes

Diagram showing the digestion of lipids

Exam Tip

The pancreas is an accessory organ in the digestive system. Food does not pass directly through it, but it has a key role in producing digestive enzymes as well as the hormones that regulate blood sugar (insulin and glucagon).

The Role of Bile

  • Cells in the liver produce bile which is then stored in the gallbladder


Bile production and secretion, IGCSE & GCSE Biology revision notes

Bile production and secretion

  • Bile has two main roles:
    • It is alkaline to neutralise hydrochloric acid from the stomach. The enzymes in the small intestine have a higher (more alkaline) optimum pH than those in the stomach
    • It breaks down large drops of fat into smaller ones, increasing surface area. This is known as emulsification.
  • The alkaline conditions and larger surface area allows lipase to chemically break down fat (lipids) into glycerol and fatty acids faster (the rate of fat breakdown by lipase is increased)

Exam Tip

Emulsification is the equivalent of tearing a large piece of paper into smaller pieces of paper.

Products of Digestion

  • The products of digestion are used to build new carbohydrates, lipids and proteins required by all cells to function properly and grow
  • Some glucose released from carbohydrate breakdown is used in respiration to release energy to fuel all the activities of the cell
  • Amino acids are used to build proteins like enzymes and antibodies
  • The products of lipid digestion can be used to build new cell membranes and hormones

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Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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