Intestinal absorption

Question 1

Epithelia types in sections

Answer 1

Small intestine = leaky, paracellular occurs

Large = tight epithelium, transcellular so more highly regulated

Question 2

Water reabsorption mechanism

Answer 2

Osmosis secondary to solute reabsorption
Na+ movement underlying
Through tight junctions and aquaporins

Question 3

Types of transporter

Answer 3

Passive diffusion = ENaC
Na/H exchanger = NHE
Na+ coupled exchanger
Na/K ATPase

Question 4

Aldosterone action

Answer 4

Increases Na+ absorption

Question 5

Absorption in small intestine (start)

Answer 5

Coupled transport of amino acids and glucose with Na+

NHE on apical creates gradient for Na+ to move into cell, also creates gradient for Cl- to be reabsorbed paracellularly

Question 6

Absorption in later small intestine

Answer 6

HCO3- into lumen and Cl- into cell exchanger
Cl- into cell
Cl- leaves on basolateral with K+ (KCCl)
NHE operates in parallel with an anion exchanger
CO2 hydration occurring in cell with carbonic hydrate

Question 7

Colon and rectum

Answer 7

Cl- accumulated using the gradient of HCO3- from CA reaction
Apical Na+ influx through ENaC
Cl- efflux on basolateral
NHE on basolateral removes H+
- Na+ less important, paracellular less important

Question 8

Potassium absorption/ secretion

Answer 8

Can be absorption or secretion
Passive absorption paracellular in small intestine,
Active absorption with H/K ATPase in colon, then K-Cl symporter on basolateral takes K+ into interstitium
Can have active secretion in colon, through action of Na-K ATPase and diffuse across apical
can have passive secretion through paracellular

Question 9

Calcium absorption

Answer 9

Occurs in upper duodenum, against a transepithelial gradient
Calcium enters through specific channels (ECaC), binds to protein in cytosol (calbindin) and exits on basolateral via a ATPase

Question 10

Iron absorption

Answer 10

Apical membrane divalent cation transporter, with copper and zinc as well
Becomes Fe3+ in cell
Can be stored or in the blood bound to ferritin

Question 11

Types of uptake of nutrients

Answer 11

• Hydrolysis in lumen and then uptake (proteins)
• Hydrolysis on the apical membrane (disaccharides)
• Absorption into and hydrolysis within cell (di/tri peptides)

Question 12

Carbohydrate breakdown

Answer 12

Salivary and pancreatic amylases

Brush border enzymes hydrolyse these products to give glucose, fructose, galactose

Question 13

Carbohydrate absorption

Answer 13

D-Glucose and D-galactose absorbed on apical SGLT1
Monosaccharides exit basolateral on glut2 (hexoses)
Occurs by midi-jejunum

Question 14

Protein breakdown

Answer 14

Pepsinogen -> pepsin in the stomach
Further digestion at brush border can occur
Pepsin, trypsin, chymotripsin
Enterokinase activates intestinal peptidases

Question 15

Protein absorption

Answer 15

Amino acids are absorbed by apical carriers, Na+ dependent
Di/tri peptides can be directly absorbed by H+ dependent apical carrier proteins, then broken down inside cell
By the end of the jejunum
Exit cell on basolateral by Na+ independent carrier

Question 16

Lipid breakdown

Answer 16

Muscular movements of the stomach emulsify fats, aided by lingual lipases
Most digestion in small intestine
Pancreatic lipase digest triglycerides -> monoglycerides and free FA
Products incorporated into bile salt micelles

Question 17

Lipid absorption

Answer 17

Micelles diffuse to unstirred acidic layer adjacent to apical membrane, components dissociate at cell surface
Components are reassembled in the SER, form microscopic chylomicron particles
Chylomicrons exported to Golgi and pass into lymph
Lymph -> left subclavian vein via thoracic duct

Question 18

Water soluble vitamin absorption

Answer 18

B1, B2, B12

Passive diffusion, specific Na+ coupled carrier proteins

Question 19

Lipid soluble vitamin absorption

Answer 19

Steroids, vitamin A,D,E,K

Absorbed in bile micelles, mostly exit the epithelial cell unmodified in chylomicrons

Question 20

where does protein breakdown begin

Answer 20

acidic stomach

Question 21

enzymes in protein breakdown

Answer 21

pepsin = from stomach
pancreatic peptidases (trypsin/chymotrypsin/elastase/carboxypeptidases) responsible for the bulk of digestion of proteins to oligopeptides and aa

Question 22

where does further digestion of oligopeptides to amino acids occur

Answer 22

brush border peptidases

Question 23

protein absorption

Answer 23

amino acids absorbed by apical carriers, mostly Na+ dependent, different categories of transporters
di and tri peptides directly absorbed by H+ dependent. apical career protein

Question 24

role of intracellular peptidases

Answer 24

converts the absorbed di and tri peptides into amino acids in cells

Question 25

absorption of amino acids finished by

Answer 25

end of jejunum

Question 26

break down of carbohydrates

Answer 26

- salivary amylase initiates - pancreatic amylase renders most starch in the form of disaccharides, trisaccharide and alpha limit dextrin - brush border disaccharides release glucose, fructose and galactose

Question 27

active absorption of carbohydrates

Answer 27

active absorption of D-glucose and D-galactose across apical membrane by Na+ dependent (SGLT-1)

Question 28

fructose absorption

Answer 28

facilitated diffusion GLUT 5

Question 29

exit of hexoses across basolateral

Answer 29

via GLUT2

Question 30

oral rehydration solutions

Answer 30

given for diarrhoea to provide electrolytes isosmotic salt/glucose solution (1:1) contain sodium and potassium

Question 31

what happens to unabsorbed carbohydrates

Answer 31

enter the colon as fuel for gut microflora, basically is fermented

Question 32

bicarbonate secretion in duodenum

Answer 32

neutralise stomach acid - create bicarbonate from hydration of CO2 using CA - exchanger with Cl- on apical puts HCO3- into lumen

Question 33

regulation of calcium absorption

Answer 33

ECaC and calbindin synthesis regulated by the vitamin D derived hormone calcitriol

Question 34

cell polarity in GI

Answer 34

``` apical = lumen basolateral = interstitium/ blood ```

Question 35

different methods of macromolecules absorption

Answer 35

hydrolysis in lumen followed by absorption (proteins -> aa) hydrolysis on apical membrane (disaccharides) absorption into and hydrolysis within cell (di/tri peptides) - following luminal hydrolysis, substances may be taken top and then resynthesises in the cell, eg. triglycerides

Question 36

disaccharides split into

Answer 36

lactose -> glucose and galactose (via lactase) maltose -> glucose (maltase) sucrose -> maltose (sucrase) hydrolysed before absorption in small intestine

Question 37

importance of brush border disaccharidases

Answer 37

hydrolyse products of main amylase enzymes and sucrose and lactose to give glucose, fructose, galactose

Question 38

importance of brush border disaccharidases

Answer 38

hydrolyse products of main amylase enzymes and sucrose and lactose to give glucose, fructose, galactose