lecture 29 - the gut 3: small intestine

Question 1

micro anatomy of small intestine

Answer 1

intestinal surface area is enhanced by finger like villi and crypts

gut is lined by a single cell thick epithelium

3 sections
• duodenum
• jejunum
• ileum

Question 2

functions of the small intestine

Answer 2

• digestion
• nutrient absorption
• electrolyte absorption - NaCl
• water absorption - 7L/day
• secretion

Question 3

key points in carbohydrate digestion and absorption

Answer 3

carbohydrate in diet mainly polysaccharide
• only absorbed as monosaccharide

luminal digestion of starch-type polysaccharides
• amylases

membrane digestion by disaccharides

absorption of monosaccharides in the small intestine

Question 4

carbohydrate digestion

Answer 4

amylase cannot produce monosaccharides but produces disaccharides
• maltose
• sucrose
• lactose

they are then broken down by other enzymes to monosaccharides

Question 5

what is maltose broken down by?

Answer 5

maltase

into 2 glucose

Question 6

what is sucrose broken down by?

Answer 6

sucrase

into 1 glucose + 1 fructose

Question 7

what is lactose broken down by?

Answer 7

lactase

into 1 glucose + 1 galactose

Question 8

absorption of monosaccharides

Answer 8

there is a different in sodium content outside and inside making a gradient

energy involved

co-transporter system

Question 9

how does the co-transporter system work?

Answer 9

uses movement of Na+ to drive the intake of glucose or galactose by SGLT1 - secondary active transport
• fructose uses GLUT5 by facilitated diffusion

GLUT-2 transporter takes glucose out of the cell

Na+ exchanged for K+ using K+/Na+ ATPase

Question 10

what does SGLT1 stand for?

Answer 10

sodium dependent glucose co-transporter

Question 11

digestion of proteins

Answer 11

luminal digestion by gastric and pancreatic proteases

membrane digestion by brush-border peptidases

cytosolic digestion within epithelium of small peptides

Question 12

gastric pepsin

Answer 12

protein digestion - 10-20%

endopeptidase

Question 13

pancreatic peptidases

Answer 13

protein digestion

endopeptidases and exopeptidases

short peptides and free amino acids

Question 14

membrane and cytosolic peptidases

Answer 14

endo, exo or dipeptidases

many different types

free amino acids

Question 15

absorption of peptides and amino acids

Answer 15

uses a H+ gradient - pH gradient

in the lumen you have an acid microclimate - more H+ outside the cell so outside has a lower pH

H+ move down gradient and drive peptide movement by PepT1
• H+ coupled di/tripeptide transport

there are transporters for AA in the basolateral membrane

Question 16

what is the H+ gradient maintained by?

Answer 16

by NHE3 - a sodium/H+ exchanger

Question 17

types of AA transporters

Answer 17

neutral
cationic
anionic

overlapping specificity

Question 18

AA absorption on the apical membrane

Answer 18

mainly coupled to ion uptake (Na+)

secondary active transport

accumulation within epithelial cells

Question 19

AA absorption on the basolateral membrane

Answer 19

mainly facilitated diffusion

Question 20

key points in fat (lipid) digestion and absorption

Answer 20

• low water solubility - needs to be increased
• solubilisation - bile salts
• digestion - pancreatic lipase
• absorption - passive
• re-esterification of fatty acids - maintains gradient for absorption
• transport to blood - chylomicrons via lymph system

Question 21

dietary lipids

Answer 21

triglycerides

phospholipids

short chain fatty acids

Question 22

triglycerides

Answer 22

triesters of glycerol and long chain fatty acids

1 glycerol + 3 fatty acids

Question 23

phospholipids

Answer 23

substitute 1 fatty acid chain with a phospholipase-ester of organic base

Question 24

lipid digestion

Answer 24

1) detergent emulsification
2) digestion by lipase produces micelles
3) diffusion into epithelial cells
4) resynthesis of triglycerides
5) release as chylomicrons

Question 25

lipid emulsification - stage 1

Answer 25

begins with chewing, grinding and mixing of food

fat droplets

increases oil-water interface area

stabilised by bile salts and phospholipids

Question 26

lipid digestion - stage 2

Answer 26

gastric and pancreatic lipase
• work at oil-water interface

co-lipase
• prevents inhibition of lipase by bile acids

triglycerides to 2 fatty acids and a monoglyceride

Question 27

micelles

Answer 27

polymolecular aggregates

monoglyceride, fatty acids and bile salts

cholesterol and fat-soluble vitamins (A, D, E, K)

close approximation to enterocyte microvilli for absorption

Question 28

lipid absorption

Answer 28

stage 3 - diffusion down gradient

stage 4 - driven by re-esterification of fatty acids to triglycerides within epithelial cells

stage 5 - released across basolateral membrane as chylomicrons

enter blood via lymph system