Enzymes & Digestion (AQA GCSE Combined Science: Trilogy: Biology): Revision Note

Chemical digestion

• Digestive enzymes 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

• There are three main types of digestive enzymes:

  • carbohydrases

  • proteases

  • lipases

Carbohydrases

• Carbohydrases break down carbohydrates to simple sugars

  • E.g. amylase is a carbohydrase that breaks down starch

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• Amylase is made in the:

  • salivary glands

  • pancreas

  • small intestine

Proteases

• Proteases are a group of enzymes that break down proteins into amino acids

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• Proteases are produced in the:

  • stomach

  • pancreas

  • small intestine

Lipases

• Lipases break down lipids (fats) to glycerol and fatty acids

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• Lipase enzymes are produced in the:

  • pancreas

  • small intestine

The role of bile

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

• Bile does not contain digestive enzymes, but it aids in the digestion of lipids as follows:

  • It is alkaline to neutralise hydrochloric acid from the stomach

    • The enzymes in the small intestine have a more alkaline optimum pH than those in the stomach

  • It breaks down large drops of fat into smaller ones, increasing surface area, a process known as emulsification

• The alkaline conditions and larger surface area allows lipase to chemically break down fat into glycerol and fatty acids at a higher rate

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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