Digestion & absorption

• Why is digestion necessary?

  Large biological molecules to big to cross cell membranes (can't be absorbed from gut into blood).
• What is amylase?


  Digestive enzyme that catalyses breakdown of starch via hydrolysis. Produces disaccharide (maltose). Produced by salivary glands & released into mouth & also produced by pancreas & released into small intestine.
• What are membrane-bound disaccharidases?

  Enzymes attached to cell membranes of epithelial cells lining ileum. Help break disaccharides into monosaccharides.
• Which disaccharidase breaks down maltose?

  Maltase
• Which disaccharidase breaks down sucrose?

  Sucrase
• Which disaccharidase breaks down lactose?

  Lactase
• What is lipase?


  Enzyme that catalyses break down of lipids into 2 fatty acids & 1 monoglyceride. Hydrolysis of ester bonds in lipids. Lipases mainly made in pancreas & secreted into small intestine.
• What are bile salts?

  Produced by liver- emulsify lipids (cause them to form small droplets). Several small lipid droplets have bigger surface area than a single large droplet ( for same volume of lipid). Increases surface area available for lipases to work on.
• What happens after the lipid has been broken down by lipase?

  Monoglycerides & fatty acids stick with bile salts to form tiny structures (micelles)- help products of lipid digestion to be absorbed.
• What are peptidases?


  Enzymes catalysing conversion of proteins to amino acids by hydrolysing peptide bonds.
• What are endopeptidases?


  Hydrolyse peptide bond within protein
• What are exopeptidases?


  Hydrolyse peptide bonds at ends of protein molecules- remove single amino acids from proteins
• What are dipeptidases?


  Exopeptidases that work specifically on dipeptides. Often located in cell-surface membrane of epithelial cells in small intestine.
• How are monosaccharides absorbed across ileum epithelium?

  Glucose absorbed via active transport with sodium ions via a co-transporter protein. Galactose absorbed in same way. Fructose absorbed via facilitated diffusion through different transporter protein.
• How are monoglycerides & fatty acids absorbed?

  Micelles help move them towards epithelium. As micelles constantly break up & reform they can 'release' them, allowing them to be absorbed- whole micelles are not taken up across the epithelium. Monoglycerides & fatty acids are lipid soluble, so diffuse directly across epithelial cell membrane.
• How are amino acids absorbed?

  Sodium ions actively transported out epithelial cells into ileum. Diffuse back into cells through sodium-dependent transporter proteins in epithelial cell membranes, carrying amino acids with them.