lecture 29 - the gut 3: small intestine Flashcards
micro anatomy of small intestine
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
functions of the small intestine
- digestion
- nutrient absorption
- electrolyte absorption - NaCl
- water absorption - 7L/day
- secretion
key points in carbohydrate digestion and absorption
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
carbohydrate digestion
amylase cannot produce monosaccharides but produces disaccharides
• maltose
• sucrose
• lactose
they are then broken down by other enzymes to monosaccharides
what is maltose broken down by?
maltase
into 2 glucose
what is sucrose broken down by?
sucrase
into 1 glucose + 1 fructose
what is lactose broken down by?
lactase
into 1 glucose + 1 galactose
absorption of monosaccharides
there is a different in sodium content outside and inside making a gradient
energy involved
co-transporter system
how does the co-transporter system work?
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
what does SGLT1 stand for?
sodium dependent glucose co-transporter
digestion of proteins
luminal digestion by gastric and pancreatic proteases
membrane digestion by brush-border peptidases
cytosolic digestion within epithelium of small peptides
gastric pepsin
protein digestion - 10-20%
endopeptidase
pancreatic peptidases
protein digestion
endopeptidases and exopeptidases
short peptides and free amino acids
membrane and cytosolic peptidases
endo, exo or dipeptidases
many different types
free amino acids
absorption of peptides and amino acids
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
what is the H+ gradient maintained by?
by NHE3 - a sodium/H+ exchanger
types of AA transporters
neutral
cationic
anionic
overlapping specificity
AA absorption on the apical membrane
mainly coupled to ion uptake (Na+)
secondary active transport
accumulation within epithelial cells
AA absorption on the basolateral membrane
mainly facilitated diffusion
key points in fat (lipid) digestion and absorption
- 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
dietary lipids
triglycerides
phospholipids
short chain fatty acids
triglycerides
triesters of glycerol and long chain fatty acids
1 glycerol + 3 fatty acids
phospholipids
substitute 1 fatty acid chain with a phospholipase-ester of organic base
lipid digestion
1) detergent emulsification
2) digestion by lipase produces micelles
3) diffusion into epithelial cells
4) resynthesis of triglycerides
5) release as chylomicrons
lipid emulsification - stage 1
begins with chewing, grinding and mixing of food
fat droplets
increases oil-water interface area
stabilised by bile salts and phospholipids
lipid digestion - stage 2
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
micelles
polymolecular aggregates
monoglyceride, fatty acids and bile salts
cholesterol and fat-soluble vitamins (A, D, E, K)
close approximation to enterocyte microvilli for absorption
lipid absorption
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
