Digestion and absorption processes in the GI tract Flashcards
Where does absorption mainly take place?
the small intestine
Where is the first place that we can start digesting proteins?
in the stomach
What are the 2 types of digestive activity?
cavital (luminal) digestion
Membrane (contact) digestion
What are the longitudinal folds on the surface of the small intestine called?
folds of Kerckring
Where are villi longest and shortest?
longest in the duodenum and shortest in the terminal iileum
What are enterocytes?
eptihelial cells of the intestine
What are globlet cells?
mucus secreting cells
-they protect us
What are Paneth cells
Part of the mucosal defenses against infection
What does the enterocyte membrane do to control the flux of solutes and fluid between the lumen and blood?
- pinocytosis @ base of microvilli, major mechanism in uptake of protein
- Passive diffusion: particles move through prose in cell membrane or through intercellular spaces
- Facilitated diffusion
- active transport
What is the big deal if the terminal ileum is resected?
we can’t absorb vitamin B12 and bile salts
What kind of carb is the only one that is absorbed by enterocytes?
ONLY MONOSACCHARIDES!
-we have to get everything to that point somehow
What is starch broken down into?
amylase breaks it down to Dextrin, maltose, and maltotriose
-those all get turned into glucose
What do trehalose, lactose, and sucrose get broken down into?
trehalose— glucose
lactose= gluc and galactose
sucrose= gluc and fruct
What two sugars does sucrase cut?
maltotriose and sucrose
What are the 3 end products of carb digestion?
glucose, galactose, and fructose
What is the special transporter that Fructose gets?
GLUT 5
What transports glucose and galactose into the cell?
SGLT 1
-uses 2ndary active transport with Na
What exports all three digested sugars out of the basolateral side?
GLUT 2
What results from the lack of the brush border enzyme lactase?
osmotic diarrhea and excess gas
what is pepsin made by?
chief cells
What are the 4 endo peptidases?
pepsin, trypsin, chymotrypsin, and elastase
What are the exopeptidases?
Carbosypeptidase A and B
In the small intestine, what activates trypsin?
Enteropeptidase (brush border) and trypsin
What does trypsin do?
activates a lot of other enzymes
What do we end up breaking proteins into when we are in the small intestine?
aa’s, di and tri peptides
What kind of transport is used to bring in aa’s
2ndary active using Na,
-there are 4 separate sotransporters, one for each neutral, acidic, basic, and imino aa
What kind of transporter is used to bring in di and tri peptides
H+ cotransporter
-then, a peptidase inside the cell cuts them up and then they go out the basolateral side like all the others through facilitated diffusion
What is Cystinuria?
- when we don’t have that transporter for the di-basic amino acids
- those aa’s will be excreted in feces
What is Hartnup disease?
when we cannot absorb neutral amino acids like trp
-ppl will have trp and serotonin in their pee or poo
Which organ is the first one to fail in CF?
the pnacreas because the stuff is too thick so WE CAN’T FLUSH OUT ACTIVE ENZYMES OUT OF THE DUCT
-may lead to acute pancreatitis
What makes the assimilation of lipids rather difficult?
their insolubility
-we need to stick them in micelles
What initiates lipid digestion in the stomach?
lingual and gastric lipases
What performs the emulsifying action of lipids in the stomach
dietary proteins
-NOT BILE
What allows sufficient time for lipid to get digested properly by slowing the rate of gastric emptying?
CCK
-it’s secreted when fat hits the small intestine
Where does the most lipid digestion occur?
the small intestine
-that’s where bile salts will emusilfy the lipids
What are the 4 pancreatic enzymes that help complete the digestive work in the small intestine?
- pancreatic lipase: inactivated by bile salts, secreted as active form
- Colipase: activated by trypsin, it binds pancreatic lipase displacing bile salts
- cholesterol ester hydrolase: catalyzes production of cholesterol and hydrolzes triglycerides to produce glycerol
- PLA2: also activated by trypsin
What is the optimum pH for pancreatic lipase?
6
What is the mechanism for processing a lipid?
- solubize in bile salts
- diffusion of micellar content across apical membrane
- reesterification
- chylomicron formation (apoB)
- exocytosis of chylomicron
What happens if there’s no Apo B?
there’s no absorption of dietary lipids
- abetalipoproteinemia
- we need that for the chylomicron
What is the result of any disorder of lipid assimilation?
steatorrhea
What are the key steps in lipid formation?
- pancreatic enzyme secretion
- bile acid secretion
- emulsification
- micelle formation
- diffusion of lipids into intestinal epithelial cells
- chylomicron formation
- transfer of chylomicrons into lymph
What is Zollinger Ellison syndrom?
a gastrin-secreting tumor that makes parietal cells secrete a lot of H+ and there’s an overload of acid in the DUODENUM
What do bacteria do to bile salts?
deconjugate them… causing a failure of micelle formation
What are the 2 examples of deficits in bile salts?
Ileal resection and Small intestinal bacterial overgrowth
What is Tropical sprue?
reduction inthe # of intestinal epithelial cells which in turn reduces the microvillar surface area
-lipid absorption is impaired because the surface area for absorption is decreased… steatorrhea
What is non-troical (celiac) sprue?
autoimmune disorder
- Ab’s against a component of gluten:GLIADIN
- destruction of villi in SI
- can’t absorb B12 very well
What are the fat soluble vitamins?
ADEK
-absorbed in same way as lipids
What are the water soluble vitamins?
B1,2,3 and 12, C, biotin, folic acid, nicotinic acid, and pantothenic acid
How are the water soluble vitamins absorbed?
via a Na+ dependent cotransport mechanism inthe small bowel
What does Vit B12 complex with in order to be absorbed?
- R proteins secreted by slaivary juices
- IF secreted by gastric parietal cells
- and Transcobalamin II
What happens if we take the stomach out or bypass it?
lose parietal cells or we can’t absorb Vit B12 anymore
What is pernicious anemia?
Where B12 doesn’t get absorbed because IF isn’t there
- could be from chronic inflammation of stomach mucosa leading to no parietal cells
- or could be an autoimmune condition
Describe the mechanism of the absorption of Vitamin D
- 7-dehydrocholesterol gets turned into cholecalciferol by the sun
- liver changes that to 25 hydroxy-D3 and it goes to the kidney
- that gets turned into 2 different forms of D3 by 1-alpha-hydroxylase
- increases gut Ca absorption, bone calcification, and bone resorption
