lecture 27 - absorption

Question 1

what is absorption

Answer 1

the net passage of substances from the GI tract across the lining of the small intestine into the interstitial fluid and then into the blood and lymph

Question 1

what are the sites of absroption

Answer 1

• mouth esophagus and stomach = mineral absorption
• small intestine = main site of absorption
• large intestine = water and sodium

Question 2

what affect does motility have on absorption

Answer 2

• correct rate of propulsion to allow digestion and absorption
• exposure of products of digestion to absorptive surfaces = segmentation

Question 3

how is absorption maximised across available surface area

Answer 3

• reduce nutrients to smallest possible unit = chemical digestion
• specific transport proteins = absorb what is required, allows active transport

Question 4

how does removal of substance from interstitial fluid happen and why is it important

Answer 4

arrangement in villi of : blood vessels + lacteal

• prevents build up in interstitial fluid

Question 5

what is the mechanism of water absorption

Answer 5

osmosis

• passive movement of water from lumen into blood
• osmotic gradient set up by absorption of salts and nutrients
• mostly by paracellular pathway

Question 6

how does Na+ absorption occur

Answer 6

passive movement via paracellular pathway or active transport via the cells (transcellular so requires transporters)

Question 7

what is the mechanisms of Na+ absorption

Answer 7

• Na+ transport alone
• Na+ transport coupled to monosaccharides
• Na+ transport coupled to amino acids

Question 8

explain how Na+ alone transport works

Answer 8

Na+ moves down its concentration gradient (high to low)

to get out of cell (needs to go from low to high) so therefore needs active transport = Na+/K+ATPase provides driving force

Question 9

explain the passive absorption of carbohydrates

Answer 9

monosaccharides diffuse down concentration gradient via the paracellular pathway

Question 10

explain the active absorption of carbohydrates

Answer 10

cotransport via cellular pathway across the membrane

• Na+/K+ATPase = driving force
• apical membrane = Na+ glucose cotransporter
• basolateral membrane = glucose carrier

Question 11

explain the passive absorption of proteins

Answer 11

amino acids diffuse down concentration gradient via the paracellular pathway

Question 12

explain the active absorption of proteins

Answer 12

cotransport via cellular pathway across the membrane

• Na+/K+ATPase = driving force
• apical membrane = Na+ amino acid cotransporter
• basolateral membrane = amino acid carrier

Question 13

explain the absorption of di and tri peptides

Answer 13

active absorption via cellular pathway = co transport with H+

• Na+/K+ATPase = driving force
• apical membrane = H+ / peptide cotransporter
• cytoplasmic peptidases = digest small peptides into amino acids
• basolateral membrane = amino acid carrier

Question 14

absorption of fats

Answer 14

• lumen and apical membrane = free fatty acids and monglycerides move by simple diffusion from micelle into the cell
• transported to endoplasmic reticulum
• resynthesised to triglycerides to maintain gradient
• secreted from golgi as chylomicrons
• exocytosed as chylomicrons and enter lymph via lacteals of villi

Question 15

how are fat soluble vitamins absorbed

Answer 15

with fats

Question 16

how are water soluble vitamins absorbed

Answer 16

very similar to protein and carbohydrate absorption

Question 17

how is B12 absorbed

Answer 17

in the ileum, binds to intrinsic factor