Enzymes as Biological Catalysts

Enzymes as Biological Catalysts

• 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

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).