6. Digestion & absorption Flashcards
name 2 features of the small intestine that promote absorption
- large surface area created by PLICAE CIRCULARES (permanent folds), VILLI and MICROVILLI
- slow mov. of contents
how does the small intestine protect itself from bacteria
- mucous production (mucous cells of crypts)
2. antimicrobial secretion (Paneth cells of crypts)
how does the intestinal mucosa regenerate
Mucosa is constantly shed (3-6 days) and replaced by stem cells which migrate from base of crypts of Lieberkuhn
where does carbohydrate breakdown and absorption occur
In brush border of jejenum by transmembrane hydrolases
name the hydrolases required to breakdown common dietary carbs
- Lactose: cleaved by LACTASE into glucose + galactose
- Sucrose: cleaved by SUCRASE into glucose + fructose
- Starch - formed of:
i. amylose (glucose chain with a-1,4 bonds): cleaved by AMYLASE into glucose or maltose (glucose x2, then cleaved by MALTASE)
ii. amylopectin (glucose chain with a-1,4 and a-1,6 bonds) broken down into a-dextrins: a-1,6 bonds cleaved by ISOMALTASE
how are monosaccharides transported from gut lumen into blood
Transported into enterocyte via:
- SGLT1 transporter - glucose/galactose co-transport with Na+ (requires basolateral Na/K ATPase)
- GLUT5 transporter - fructose (passive facilitated diffusion)
Transported into blood via:
- GLUT2 channel (diffuse down gradient)
why do oral rehydratiion fluids contain Na+ and glucose?
Mix of Na+ and glucose stimulate max. water absorption:
- Na+ generates osmotic gradient followed by water
- glucose stimulated Na+ uptake (via SGLT1 co-transporter)
how are proteins broken down in GI tract to allow absorption by enterocytes
- In stomach (40% catabolism):
CHIEF CELLS release pepsinogen… converted to PEPSIN by HCl… cleaves proteins to oligopeptides/amino acids… move to jejenum for absorption - ENTEROPEPTIDASE (released by duodenal cells) converts trypsinogen (released from pancreas) to TRYPSIN… activates other pancreatic proteases:
- ENDOPEPTIDASES (trypsin, chymotrypsin, elastase): break bonds in middle of polypeptide
- EXOPEPTIDASES (carboxypeptidase A and B): break bonds at end of polypeptide
how are amino acids and di-/tri-peptides transported into enterocytes
amino acids: Na+/aa co-transporters
di-/tri-peptides: PepT1 (H+ co-transporter
then converted to aa by cytosolic peptidases)
where and how is iron absorbed - why is this affected by PPIs
Absorbed in duodenum:
- as haem by haem transporter
- as Fe2+ by DMT1 transporter (H+ co-transporter so Fe absorption inhibited by PPIs)
what happens to iron in the enterocyte
Converted to Fe3+…
- if low iron stores: Fe3+ binds transferrin and is transported to stores
- if high iron stores: Fe3+ contained in ferritin complexes and lost when enterocyte is replaced
how is calcium absorbed when intake is high or low
Normal/high Ca2+ intake: passive paracellular absorption down conc. gradient
Low Ca2+ intake:
i. facilitated diffusion (via Ca channel) into enterocyte…
ii. basolateral Ca ATPase (PMCA) removes Ca2+ into blood - requires VitD and stimulated by PTH
where and how is vitamin B12 absorbed
absorbed in terminal ileum bound to intrinsic factor secreted by gastric parietal cells
why is much water present in duodenum
digestion of chyme creates isoactive molecules so much water secretion into duodenal lumen to render hypertonic chyme isotonic
how is water reabsorbed further down GI tract
Na+ gradient needs to be created to reabsorb water: basolateral Na/K ATPase actively transports Na+ out of enterocyte to that Na+ and water can move across apical membrane into enterocyte:
- small intestine: Na+ is co-transported
- large intestine: Na+ channels induced by aldosterone
