AS-LEVEL AQA BIOLOGY NOTES
TOPIC 3: digestion and absorption
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
Lipid absorption
Amino acid absorption
- Cross epithelium via co-transport protein carrier
- A.k.a. Secondary active transport
- 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
- Emulsifying effect of bile
- Micelles dissociate at intestinal cell phospholipid bilayer
- Monoglycerides, glycerol and fatty acids dissolve in phospholipid bilayer of intestinal cells
- 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
- 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
- Not present in newborns