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A-LEVEL NOTES

AS-LEVEL AQA BIOLOGY NOTES 
TOPIC 3: digestion and absorption

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Picture
Digestion = breaking down large, non-absorbable molecules by hydrolysis into smaller molecules that can be absorbed by the body.
Substrate
Enzyme
Products
Comments
Carbohydrates
​Amylase and membrane-bound dissaccharidases
Monosaccharides
 
Lipids
Lipase
Fatty acids
Bile acids involved in emulsifying fats making them more absorbable ​
Proteins
Endopeptidases, exopeptidases and membrane-bound dipeptidases ​
Amino acids
 
Glucose absorption
  • Cross epithelium via co-transport protein carrier
    • A.k.a. Secondary active transport
  • Glucose and Na+ move into intestinal cell
  • Glucose diffuses from intestinal cell into capillaries

Lipid absorption
  • Triglycerides broken down by lipases into monoglycerides, glycerol and fatty acids
  • Bile binds to the monoglycerides to form micelles
    • Emulsifying effect of bile
    • Contains fatty acids and glycerol
  • Micelles dissociate at intestinal cell phospholipid bilayer
    • Monoglycerides, glycerol and fatty acids dissolve in phospholipid bilayer of intestinal cells
  • Triglycerides are re-synthesised from these materials and move into lymph capillaries
  • Bile remains in (small) intestine

Amino acid absorption
  • Secondary active transport
  • Carrier protein transport amino acids and Na+ into the intestinal cell
    • Carrier proteins specific to amino acid type
  • Facilitated diffusion of amino acids into capillaries from intestinal cell
  • Trypsin and HCl responsible for breaking down proteins into amino acids
    • Not present in newborns
    • Therefore entire proteins absorbed by endocytosis into intestinal cells
    • Exocytosis from cell into capillaries
    • E.g. antibodies from breast milk - passive immunity

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