Enzymes as Biological Catalysts (Edexcel 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
• Food is partially digested mechanically (by chewing, churning and emulsification) in order to break large pieces of food into smaller pieces of food
  • This 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 that are produced in different areas of the digestive system
• 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 are enzymes that break down carbohydrates into simple sugars such as glucose
  • Amylase is a carbohydrase that is made in the salivary glands, the pancreas and the small intestine
  • Amylase breaks down starch into maltose
  • Maltase then breaks down maltose into glucose

Starch is broken down into glucose using two enzymes: amylase and maltase

Proteases

• Proteases are a group of enzymes that break down proteins into amino acids
  • Pepsin is an enzyme made in the stomach that breaks down proteins into smaller polypeptide chains

  • Proteases made in the pancreas and small intestine break the polypeptides into amino acids

Proteins are broken down using pepsin and other proteases

Lipases

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

Lipids are broken down by lipase enzymes

Synthesis of carbohydrates, proteins and lipids

• Enzymes are not just important in breaking down larger molecules into smaller ones
• They are also required for the synthesis of larger molecules (building small molecules back up into bigger ones)
• Enzymes are required by organisms to synthesise carbohydrates, proteins and lipids
  • Carbohydrates are synthesised by joining simple sugars together
    • For example, glycogen synthase is an enzyme that joins together many chains of glucose molecules to form glycogen (an energy-storage molecule in animals)

  • Proteins are synthesised by joining amino acids together
    • Again, enzymes catalyse the reactions required to do this

  • Many enzymes are involved in the synthesis of lipids from fatty acids and glycerol