Intestinal absorption of nutrients

Question 1

Describe the digestion of carbohydrates?

Answer 1

starts in mouth
- saliva rich in amylase - starch into glucose
- exocrine pancreas secretes alpha amylase

Question 2

Describe how carbs are absorbed across the villi?

Answer 2

glucose diffuses into enterocyte via Na cotransport from lumen
As Na accumulates in cell - its conc. gradient is maintained via Na/K ATPase pump - causes Na to exit basolateral membrane in exchange for K

GLUT2 transporters - basolateral carriers efflux glucose out of cell into blood
- they sense glucose conc. if its high in cell - will go into blood
- if low food intake/low glucose conc. in lumen - glucose will go in opp direction to nourish epithelial cells

Question 3

describe the digestion of proteins?

Answer 3

pepsin - digests proteins in stomach

pancreas secretes 24 proenzymes which pass into small intestine where they get activated into carboxypeptidases, trypsin, chymotrypsin
- pepsin + carboxypeptidases break down proteins into polypeptides
- trypsin and chymotrypsin break down PP into tri and dipeptides

In enterocyte - aminopeptidases + dipeptidases - break down tri and dipeptides into single amino acids

Question 4

describe the absorption of amino acids and oligopeptides?

Answer 4

tri and dipeptides enter cell via H+ cotransport
- higher conc. of H+ in lumen allows it to enter

within cell - aminopeptidase + dipeptidase break tri and dipeptides into single AAs
- AAs enter via Na cotransport
- Na conc. gradient maintained via Na/K ATPase on basolateral membrane
- AAs effluxed into bloodstream by specific transport proteins on basolateral surface

if no food intake - gradient reverse - intestinal cell gains AA, carbs, fats

Question 5

describe the digestion and absorption of fats?

Answer 5

starts in mouth - by lipases of tongue glands

10-30% broken down in stomach

70-90% broken down by pancreatic lipase + bile salts in duodenum + jejunum - upper part of small intestine
- bile salts form micelles around fat - makes it miscible in aq. solution of intestinal fluid

Question 5

describe absorption of fats?

Answer 5

glycerol + short/medium chain fatty acids - readily soluble in aqueous solution - pass through into enterocyte, diffuse through and exit basolateral membrane - small so can fit into capillary fenestrations - RAPIDLY PERMEATE

mixed lipid micelles will pass through unstirred water layer - as they give aqueous solution an effective lipid concentration - diffusion is efficient

bile salt micelles will encounter low pH, cause micelle to open - fats permeate into epithelial cell

long chain fatty acids + cholesterol - re-esterified by enterocyte, assembled with specific apoproteins (RER) into emulsion like droplets = CHYLOMICRONS
- chylomicrons too large to pass through basolateral membrane
- vesicles containing chylomicrons fuse and chylomicrons are released
- chylomicrons go into lymphatic vessels then into blood

Question 6

Describe vitamin transport + absorption

Answer 6

vitamin transported into intestinal cell from lumen via Na cotransport
- Na conc. gradient maintained via Na/K ATPase pump

Vitamin A, E and K - incorporated into newly synthesised chylomicrons or VLDLs - taken up by liver