Digestion and absorption

• Digestion


  Process where large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes
• Digestion in mammals


  • Carbohydrates by amylases and membrane-bound disaccharidases
  • Lipids by lipase including the action of bile salts
  • Proteins by endopeptidases, exopeptidases, and membrane-bound dipeptidases
• Co-transport mechanisms


  Mechanisms for the absorption of monosaccharides and amino acids
• Micelles


  Role in the absorption of lipids
• Food molecules need to be broken down by enzymes into smaller molecules
• These molecules can then be absorbed into the bloodstream
• The large biological molecules (e.g. starch, proteins) in food are too big to cross cell membranes
• During digestion, large molecules are broken down into smaller molecules (e.g., glucose, amino acids)
• Most large biological molecules are polymers, which can be broken down into smaller molecules (monomers) using hydrolysis reactions
• Hydrolysis reactions


  Break bonds by adding water
• Digestive enzymes


  Used to break down biological molecules in food
• Amylase


  A digestive enzyme that catalyses the breakdown of starch
• Amylase works by catalysing hydrolysis reactions that break the glycosidic bonds in starch to produce maltose
• Amylase is produced by the salivary glands and the pancreas
• Membrane-bound disaccharidases


  Enzymes attached to the cell membranes of epithelial cells lining the ileum that help to break down disaccharides into monosaccharides
• Lipase


  Enzymes that catalyse the breakdown of lipids into monoglycerides and fatty acids
• Bile salts are produced by the liver and emulsify lipids
• Emulsification


  Process that causes lipids to form small droplets, increasing the surface area for lipases to work on
• Endopeptidases


  Enzymes that hydrolyse peptide bonds within a protein
• Pepsin


  An endopeptidase released into the stomach that works in acidic conditions
• Exopeptidases


  Enzymes that hydrolyse peptide bonds at the ends of protein molecules
• Dipeptidases


  Exopeptidases that work specifically on dipeptides
• Dipeptidases are often located on the cell-surface membrane of epithelial cells in the small intestine
• Monosaccharides absorption


  Absorbed across the ileum epithelium into the bloodstream
• Glucose is absorbed by active transport with sodium ions
• Fructose is absorbed via facilitated diffusion through a different transporter protein
• Micelles help to move monoglycerides and fatty acids towards the epithelium
• Monoglycerides and fatty acids can diffuse directly across the epithelial cell membrane
• Amino acids are absorbed in a similar way to glucose and galactose
• Sodium ions are actively transported out of the epithelial cells into the ileum
• Transporter proteins carry amino acids into the epithelial cells
• Micelles constantly break up, allowing monoglycerides and fatty acids to be absorbed