Enzymes & Digestion (AQA GCSE Biology: Combined Science)

• 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 enzymes – carbohydrases, proteases and lipases

Carbohydrases

• 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

Diagram showing the digestion of starch

Proteases

• 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

Diagram showing the digestion of proteins

Lipases

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

Diagram showing the digestion of lipids

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

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)

• 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