Digestion and Absorption

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  • Digestion and Absorption
    • Food broken down
      • 1)Large biological molecules too big to cross cell membranes- can't be absorbed from gut into blood
      • 2) During digestion, large molecules broken down into smaller ones e.g glucose, amino acids- can be easily absorbed from gut to blood and transported around body as they can cross cell membrane
      • 3) Hydrolysis reactions used to break down large molecules
      • 4) During hydrolysis, carbohydrates broken down into disaccharides then monosaccharides. Fats broken down into fatty acids and  monoglycerides. Proteins broken down into amino acids
    • Digestive enzymes:   Carbohydrates
      • 1) Amylase catalyses conversion of starch into maltose- hydrolysis of glycosidic bonds
      • 2) Amylase produced by salivary glands (secrete into mouth) and pancreas (secrete into small intestine)
      • 3) Membrane bound disaccharides- enzymes that attach to cell membranes of epithelial cells lining ileum. Help to break down dissacharides into monosaccharides.
      • 4)    Monosaccharides transported across cell membrane of ileum by specific transporter proteins
    • Digestive enzymes: Lipids
      • 1) Lipase catalyse breakdown of lipids into monoglycerides/fatty acids. Hydrolysis of ester bonds
      • 2) Made in pancreas. Work in small intestine
      • 3) Bile salts produced in liver, emulsify fats- cause lipids to form small droplets
      • 4) Several small lipid droplets have larger SA than single large droplet. Greatly increases SA for lipase to work on
      • 5) Monoglycerides/fatty acids stick with bile salts to form micelles
    • Digestive enzymes: Proteins
      • 1) Endopeptidases hydrolyse peptide bonds within a protein
      • 2) Trypsin/chymotrypsin: synthesised in pancreas, secreted into small intestine
      • 3) Pepsin: released into stomach by cells in stomach lining. Only works in acidic conditions- HCl in stomach
      • 4) Exopeptidases hydrolyse peptide bonds at end of proteins- remove single amino acids
      • 5) Dipeptidases work specifically on dipeptides- hydrolyse peptide bond between two amino acids
      • 6) Dipeptidases located at cell surface membrane of epithelial cells/small intestine
    • Products of digestion
      • Monosaccharides: Glucose absorbed by AT w/sodium ions via co transporter proteins. Fructose absorbed via FD by different transporter protein
      • Monoglycerides/fatty acids: Micelles move them towards epithelium. Release them to be absorbed, micelles not taken up. MG/FA are lipid soluble, can diffuse directly across epithelial cell membrane
      • Amino acids: Na ions actively transported out of epithelial cells into ileum. Diffuse back into cells through sodium dependent transporter proteins in epithelial cell membranes, carrying amino acids
      • Absorbed across ileum epithelium into bloodstream


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