Digestion and Absorbtion Flashcards
what are the two seemingly paradoxical tasks of intestinal epithelial barrier tissue?
what are the cell types here?
what are 4 features of ^ of their cytoskeleton proteins?
- *intestinal epithelial barrier (IEB)** has two paradoxical tasks:
- enable absorption of nutrients (permeable)
- control passage of pathogens (impermeable
due to enterocytes (highly specialised cells of intestinal lining) having specialised cytokskeleton proteins:
a) brush border - increase SA for absorb
b) tight junctions - to be impermeable to toxins
c) polarity complexes (lets the cell know whats inside / outside)
d) lots of ECM to hold them in place
surface area of skin v mucusoa?
skin - 25m2
mucosae - 32m2 - 95% of absorption of nutrients occurs in the small intestine
why does the small intestine absorb the most?
- *Anatomical features** make the small intestine so specialised for absorption. The human small intestines have the surface area of a studio apartment **(between 30 and 40 square meters).
- intestine has folds:villi& the cells inside the villi containmicrovilli**
how are cells made in villus?
at the crypt:
- stem cells make new cells - differentiate into enterocytes and other intestinal cells like goblet cells
- get pushed up to the top of villus and die: microbiome feed on dead cells
- whole process is very dynamic
what specifically is absorbed in the ileum? (5)
and the colon (3)?
ileum
- water
- Na+
- B12
- intrinsic factors
- bile acids
colon
- water
- **electrolytes
- **bile acids (less)
what is the difference in the structure of colon and small intestine/
small intestine: villus & crypts
colon: no villus & crypts
explain what the 3 different phases of digestion?
-
luminal phase: - ingested food broken down by:
- acid in stomach
- alkali in small intest.
- substrate specific enzymes from gastric, small bowel mucosa & pancreas
- *2. mucosal phase:**
- pre-digested nutrients are taken up by brush border membrane of enterocytes and enter
- *3. post absorptive:** nutrients absorbed via
- lymphatics
- portal circulation
why is water needed in the digestive system? (4)
- Hydrolysis reactions of digestion
- Facilitation of absorption (brings products of digestion into close proximity to microvilli)
- Facilitation of propulsion of gut contents
- Combination with mucin granules to make mucus
explain luminal, mucosal and post absorptive phases of carb digestion
- *luminal phase:**
- carboyhydates: enzymes secreted by saliva and pancreas = disaccaharides + limit dextrins
mucosal phase:
- brush border enzymes: sucrase, lactase, maltase, limit desxtrinase, glucoamylase break down molecules into glucose and galactose
- glucose and galactose enter epithelial cells via Na linked secondary transport acorss apical membrane using SGLT1
- fructose enters by fac. d.
post absorptive phase
- The sugars exit the cells across the basolateral membrane by facilitated diffusion to the portal vein - GLUT 1/2
( The Na/K ATPase pump drives the entrance of the sugars into the blood capillary. It will be drained to the portal vein, straight into the liveR)
explain luminal, mucosal and post absorptive phases of llipid digestion
luminal phase
- mouth: lingual lipases
-stomach:gastric lipases
- pancrease: pancreatic lipase bile salts & bile salts
= triglycerides -> free fatty acids & monoglycerides. then form micelles (contain fat soluble vitamins and cholesterol)
- *mucosal phase:**
- simple diffusion (bc membrane of enterocytes are also lipids - so can just diffuse through)
- within enterocytes: molecules are reassembled by GA = chylomicrons
- *post absorptive phase:**
- chylomicrons secreted across basolateral membrane, but are too big to enter blood: enter lympahtic fluid
what is role of bile salts
what is the enterohepatic circulation?
(why bad if doesnt work)
bile salts: cause emulsification: breaking down of fat globules into smaller droplets
Enterohepatic circulation: recycling of bile salts
- liver: makes bile salts
- stored in gall bladder
- goes into duodenum: help digest fats
- reabsorbed in ileum
- bulk of bile salts are absorbed in liver
how do commensal bacteria regulate digestion?
what happens if we have bacterial overgrowth?
dynamic equilibrium between diet-gut microbiome-bile acid pool size:
normally - we have conjugated bile acids, created by liver. Conjugated bile acids (primary bile acids): more efficient in emulsifying fats because at intestinal pH they become more ionized than the unconjugated bile acids.
Commensal bacteria: participate in the synthesis of bile acids. Microbial enzymes de-conjugate bile acids & make them less effecient: (secondary bile acids).
so we have a pool of primary and secondary bile acids: if have bacterial overgrowth in gut: form too much secondary bile acids = struggle to digest fats
explain luminal, mucosal and post absorptive phases of protein digestion
luminal phase
- stomach: pepsin released by pepsinogen (zymogen / proenzyme). pepsinogen is activated by HCl, which is releaed from parietal cells in the gastric pits.
- small intestine: further digestion from pancreatic enzymes
mucosal phase:
- brush border enzyme: enterokinase converts trypsinogen -> trypsin
then:
trypsin activates:
a) chymotrysinogen -> chymotrypsin
b) procarboxypetidae -> carboxypeptidase
- a.a. enter the epithelial cells via Na-linked secondary active transport across the apical membrane (same system for sugar)
- *post-absorptive phase;**
- a.a. transporte across basolateral membrane by fac. d
** why do vitamins need to be absorbed from the food?
what are two type of vitamins?
how are each absorbed?^
into which system are they absorbed? **
vitamins cant be manufactured by body
1. fat soluble vitamins: A, D, E & K
- absorbed with lipids: readily dissolve in **lipid droplets, micelles and chylomicrons
- absorbed into lymph fluid**
2. water soluble vitamins: B & C
- follow flux of water (B&C)
- absorbed into portal vein
B12 can be only observed where? & what must it first be complexed with?
what is B12 aka?
describe the absorptive pathway of B12 :)
- B12: absorbed only in terminal ileum, after being complexed with stomach-derived intrinsic factor
- B12: aka cobalamin
absorptive pathway:
- bound to dietary protein
- first dissociated by HCl and pepsin, in stomach
- reattaches itself via haptocorrin (from saliva thats now in stomach)
- dissociated from haptocorrin and binds with stomach-derived intrinsic factor
- absorbed only in terminal ileum in enterocytes (although 60-80% still goes into faeces)
- *-** reassociates with transcobalamin and then goes to portal circulation
*important*
most common cause of malabsorption in developed world is = ?
most common cause of malabsorption in developing world is = ?
developed world = coeliac disease
developing world = post-infectious malabsoprtion and tropical spure
what causes coeliac disease? - what isoform do they have? which 3 antibodies this this create?
what does it cause in the cells ? (3)
- *coeliac disease:**
- *- autoimmune disease**
- (95% of CD) have isoform of DQ2 or DQ9 of human leukocyte antigen (HLA) HLA-DQ protein
- due to HLA-DQ, get high levels of antibodies: antigliadin, tissue transgluataminase, antiendomyisial
causes
- complete villi atrophy
- marked crypt hyperplasia (bc trying to replace the villi, which are being replaced)
- major inflammation
why do unabsorbed dissacharides cause diarrhoea? (2)
add to the osmotic load of the colon
AND
sugars get fermented - causes gaseous distension (e.g. H2) :(
