L30 - GI absorption : nutrients Flashcards
name the inactive precursor of pepsin
pepsinogen
where is pepsinogen released from
chief cells of stomach
how is pepsinogen converted to pepsin
pepsinogen + HCL from stomach acid forms pepsin
this is also +ve feedback
describe characteristics of chyme
- acidic
- contains slightly digested fats proteins and sugars
- contains intrinsic factor (enables b12 absorption in SI)
after leaving stomach, what is needed to allow further digestion
pancreatic enzymes
neutral pH
bile salts for fat digestion
what enzyme involved in protein digestion is found on the brush border of enterocytes (duodenum) and what is its function
Enterokinase
- converts the inactive trypsinogen into active trypsin
- trypsin then activates inactive pancreatic enzymes
what type of enzymes are involved in protein digestion, give examples
endopeptidases
trypsin
elastase
chymotrypsin
two examples of exopeptidases and their function
- carboxypeptidase - removes AA’s from carboxyl ends of the recently cleaved short peptides
- aminopeptidase - removes AA’s from amino end of the recently cleaved short peptides
explain the process of digestion of proteins in the duodenum
- enterokinase found on the brush border of enterocytes in the duodenum convert active trypsinogen to the inactive trypsin
- trypsin can then activate other pancreatic proteolytic enzymes (that follow)
- pancreatic trypsin, elastase, chymotrypsin cleave peptide bonds producing short peptides
- carboxypeptidase removes AA’s from the carboxyl end of the newly cleaved peptide
- aminopeptidase removes AA’s from the amino end of the newly cleaved peptide
leaves AA’s / di / tri peptides
examples of endopeptidases
trypsin
elastase
chymotrypsin
how are AA’s absorbed from duodenum
via Na+ linked cotransporters then diffuses through cell into capillary
how are di/tri peptides absorbed into blood from duodenum
- absorbed into cell via H+ linked transporters
- hydorlised further into AA’s
- diffuse into capillary
a small amount can diffuse as di/tri
where does carbohydrate digestion start?
mouth (amylase in saliva)
explain action of pancreatic amylase
cleaves straight chain polysaccharides (cant cleave branches) into short oligosaccharides
what oligosaccharides are produced after pancreatic amylase cleavage
maltose (disaccharide
maltriose (trisaccharide)
how are other disaccharides digested (eg lactose/sucrose)
brush border enzymes e.g si membrane bound disaccharidases cleave into monosaccharides
describe digestion of carbohydrates in the SI
- pancreatic amylase cleaves straight chain polysaccharides (only) into di and tri saccharides (maltose and maltriose)
- brush border disaccharidases further digest these and other disaccharides (eg lactose and sucrose) into monosaccharides
what transporter is involved in Glucose and Galactose absorption from SI lumen
SGLT Na linked co transporter (secondary active transport)
what pump maintains the Na gradient needed for the SGLT transporter?
Na/k ATPase pump
what transporter is involved in the absorption of fructose from the SI lumen
GLUT 5 (facilitated diffusion)
what transporters allow absorption of fructose, glucose and galactose from enterocyte into capillary?
GLUT 2 for all 3 (facilitated diffusion)
explain absorption of sugars in SI
SGLT Na cotransporter (lumen) for glucose&galactose GLUT 5 (lumen) for facilitated D of fructose GLUT 2 (basolateral) for facilitated D of all 3 into capillary
what enzymes are involved in fat digestion
mainly pancreatic lipase
gastriclipase
what is needed for fat digestion
colipase
bile salts
correct pH (>7)
b-lipoprotein (involved in packaging)
explain process of fat digestion in SI
- fat emulsified in lumen by bile salts
- lipase can now convert TG to 1x MG and 2x FAs
- these can form micelles in an equilibrium
- micelles diffuse to brush border
what are micelles made of?
FAs and MGs
explain absorption of fats from lumen of SI into blood
- 2xFA and 1xMG diffuse across lipid bilayer
- they are resynthesized into TG in SER
- TG is incorporated into chylomicrons (with B-lipoprotein, cholesterol etc)
- chylomicrons transported into lacteals (lymph ssystem)
what happens to the MG and FA’s once inside the enterocyte
they are resynthesized to TG in the SER
what is the role of B-lipoprotein in fat absorption
aids packaging of TG’s into chylomicrons
where are chylomicrons transported to?
lacteals (lymph system)
what % of bile salts are reabsorbed and where does reabsorption occur
95%
terminal ileum
explain enterohepatic recirculation
- 95% of bile salts reabsorbed in terminal ileum
- transported to liver via portal vein
- re-extracted and exported into bile juice
explain de novo synthesis
liver resynthesises bile to compensate for loss (from faeces)
what else is absorbed in GI tract
- minerals (Ca/Fe etc)
- vitamins (lipid soluble ones absorbed with fats, specific mechanisms for others)
- drugs (absorbed best when non ionised
explain water absorption in the colon
- driven by Na+
- Na/K ATPase pumps Na from cell into interstitial fluid setting up Na gradient
- Na into cell from lumen via channel and Na/H antiport
- water travels into interstitial fluid following osmotic gradient
what are the 2 routes water takes to enter interstitial fluid
- paracellular
2. through enterocyte via aquaporins
what are 2 causes of malabsorption
- failure to digest macromolecules
2. failure to absorb
what are causes of failure digesting chyme
- insufficient bile salts
- failure to deliver pancreatic enzymes
- poor co-ordination of secretions and gastric emptying
what can cause a failure to absorb lipids
lack of B-lipoprotein
what can cause a failure to absorb water
- increased solutes in lumen
2. infection causing increased solute and water secretion
explain osmotic diarrhoea as a result of malabsorption
- incomplete absorption means increased [solutes] in lumen
- this causes water retention in lumen
—–> increased vol of luminal contents stimulates
peristalsis
—–> undigested fats can have laxitive effect
—–> partially digested food may be fermented by
gut flora causing gas distension (trapped wind)
explain secretory diarrhoea
in response to infection
toxins eg E.coli increase the secretions of intestinal crypt cells
what stimulates water and Na reabsorption
aldosterone
