Edexcel IGCSE Biology: Double Science 复习笔记 2.5.9 Humans: Digestive Enzymes

• 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 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
• 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 are enzymes that break down carbohydrates to simple sugars such as glucose
  • Amylase is a carbohydrase which 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 are a group of enzymes that break down proteins into amino acids
  • Pepsin is an enzyme made in the stomach which breaks down proteins into smaller polypeptide chains 
  • Proteases made in the pancreas and small intestine break the peptides into amino acids

   

Proteins are broken down using pepsin and other proteases

• 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

   

Diagram showing the digestion of lipids

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