Absorption Flashcards
Carbohydrates absorption
(broken into monosaccharides for digestion)
1. Glucose and galactose - co-transporter with Na across apical membrane and GLU2 across basolateral
2. Fructose - GLUT5 across apical and GLUT 2 across basoateral
Proteins absorption
(digested into AA/peptides in digestion)
1. Peptides - H dependant transporter across apical and hydrolysed to AA inside
2. AA - sodium co-transporter across apical and fac diff across basal
Lipids absoption
(broken into free fatty acids, glycerol, monoglycerides, and cholesterol in dig)
1. Glycerol - passive diffusion
2. short fatty acids - diffusion
3. Long fatty acids/monoglycerides - diffuse into cell and re-synthesised into triglycerides inside then exocytosed as chylomicron at the basal membrane
4. Cholesterol - endocytosed into clarthrin pits
Ca2+ absorption
- Passive transport through whole SI
- Active transport when in chyme
Iron absorption
(Fe3+ reduced to Fe2+ in digestion)
1. Fe2+ is H+ co-transported (or transported through haem) into cell and inside, combines with apoferratin to be stored as ferratin in oxidised Fe3+ form.
2. Fe2+ is protein transported into blood and oxidised by ferroxidase into Fe3+ at basolateral membrane where it is picked up by transferrin for transport
B12 absorption
When bound to intrinsic factor it is endocytosed into cell
Fat soluble vitamins (A, D, E, K) absorption
Diffuse into cell where they are incorporated into chylomicron and exocytosed into lymphatics
Water soluble vitamins (C, H, B) absorption
Na transporter into and out of cell
Sodium absoprtion
- Na/H exchange - duodenum and jejunum
- Na/glucose co-transporter - jejunum
- Na/AA co-transporter - jejunum
- parallel Na/H and Cl/HCO3 exchange - ileum and proximal colon
- Epithelial Na Channels - distal colon
- Na/K ATPase for basolateral membrane
Chlorine absorption
- Electrochemical gradient absortion due to Na/AA and Na/glucose transporter whole length of small intestine
- Electrochemical gradient absorption due to ENaC in the large intestine
- parallel Na/H and Cl/HCO3 exchange - ileum and proximal colon
- Cl/HCO3 exchange - ileum, proximal and distal colon
Chlorine secretion
Apical membrane - CTFR channels
Basolateral membrane - Na co-transport
Parietal cells - Cl and HCO3 undergo antiport and then H+ is released to bind to HCl in a canaliculus to be released
Chylomicron transport and digestion (in blood)
(chylomicron are the exocytosed form of tryglycerides)
1. they are transported into lymphatic system
2. If they are metabolised, it is by lipoprotein lipase
3. Albumin carries the products of the metabolism (free fatty acids and glycerol)