Revision Notes

7.4 Chemical Digestion

Enzyme Action in the Alimentary Canal

  • The purpose of digestion is to break down large, insoluble molecules (carbohydrates, proteins and lipids) into small, soluble molecules that can be absorbed into the bloodstream
  • Food is partially digested mechanically (by chewing, churning and emulsification) in order to break large pieces of food into smaller pieces of food which increases the surface area for enzymes to work on
  • Digestion mainly takes place chemically, where bonds holding the large molecules together are broken to make smaller and smaller molecules
  • Chemical digestion is controlled by enzymes which are produced in different areas of the digestive system
  • There are three main types of digestive enzymes – carbohydrases, proteases and lipases

Carbohydrases: Basics

  • Amylases are produced in the mouth and the pancreas (secreted into the duodenum)
  • Amylases digest starch into smaller sugars


The digestion of starch, IGCSE & GCSE Biology revision notesThe digestion of starch

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  • Amylase is secreted into the alimentary canal in the mouth and the duodenum (from the pancreas) and digests starch to maltose (a disaccharide)
  • Maltose is digested by the enzyme maltase into glucose on the membranes of the epithelium lining the small intestine

Proteases: Basics

  • Proteases are a group of enzymes that break down proteins into amino acids in the stomach and small intestine (with the enzymes in the small intestine having been produced in the pancreas)

The digestion of proteins, IGCSE & GCSE Biology revision notesThe digestion of proteins

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  • Protein digestion takes place in the stomach and duodenum with two main enzymes produced:
    • Pepsin is produced in the stomach
    • Trypsin is produced in the pancreas and secreted into the duodenum


  • Lipase enzymes are produced in the pancreas and secreted into the duodenum
  • They digest lipids into fatty acids and glycerol


The digestion of lipids, IGCSE & GCSE Biology revision notesThe digestion of lipids

The Role of Hydrochloric Acid

  • The stomach produces several fluids which together are known as gastric juice
  • One of the fluids produced is hydrochloric acid
  • This kills bacteria in food and gives an acid pH for enzymes to work in the stomach
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How is a Low pH Helpful in the Stomach

  • The low pH kills bacteria in food that we have ingested as it denatures the enzymes in their cells, meaning they cannot carry out any cell reactions to maintain life
  • Pepsin, produced in the stomach, is an example of an enzyme which has a very low optimum pH – around pH 2
  • The hydrochloric acid produced in the stomach ensures that conditions in the stomach remain within the optimum range for pepsin to work at its fastest rate
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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 notesBile production and secretion


Bile has two main roles:

  • It is alkaline to neutralise the hydrochloric acid which comes 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. This is known as emulsification. The larger surface area allows lipase to chemically break down the lipid into glycerol and fatty acids faster

Exam Tip

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

This is an example of mechanical digestion, not chemical digestion – breaking something into smaller pieces does not break bonds or change the chemical structure of the molecules which make it up, which is the definition of chemical digestion.

Author: Jenna

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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