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

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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