Enzymes in Digestion (AQA A Level Biology): Revision Note

Enzymes in digestion

• Digestive enzymes are extracellular enzymes, meaning that they function outside the body cells

• There are three main types of digestive enzymes:

  • carbohydrases

  • lipases

  • proteases

Carbohydrate digestion

• Carbohydrase enzymes are a group of enzymes involved in carbohydrate digestion; examples include:

  • amylase

  • maltase

  • lactase

• The process of digesting starch into simple carbohydrates is as follows:

  1. Amylase hydrolyses starch into the disaccharide maltose

     • Amylase is made in the salivary glands, the pancreas and the small intestine

  2. Maltose is then hydrolysed into the monosaccharide glucose by maltase

• Maltase is a membrane-bound disaccharidase, meaning that it:

  • is attached to the cell-surface membranes of the epithelial cells lining the small intestine

  • breaks down disaccharides into monosaccharides

Lipid digestion

• Lipid digestion involves the action of:

  • lipase enzymes

  • bile salts

• The process of lipid digestion is as follows:

  1. emulsification

     • Partially digested food arrives in the small intestine and mixes with bile

     • Bile salts bind to large lipid droplets and breaks them into smaller droplets; this is emulsification

     • The resulting small lipid droplets have a large surface area on which lipase enzymes can act

  2. Lipase enzymes in the lumen of the small intestine break down lipids to glycerol, monoglycerides and fatty acids

Protein digestion

• Protein digestion involves the action of different types of protease enzymes:

  • endopeptidases

  • exopeptidases, including dipeptidases

• Protein digestion involves the following:

  • Endopeptidase enzymes in the stomach and small intestine hydrolyse peptide bonds within polypeptides, creating shorter polypeptide chains

  • Exopeptidases hydrolyse peptide bonds at the ends of polypeptide chains, producing single amino acids

    • Dipeptidases are a type of exopeptidase that break down dipeptides into individual amino acids

• Membrane-bound dipeptidases are attached to the cell surface membrane of epithelial cells in the small intestine