GI 08 Flashcards
Absorption
mvt of nutrients, water, and electrolytes from intestinal lumen to blood (paracellularly or cellularly)
How are carbohydrates ingested (saccharide types)
Poly, mono, and disaccharides
What kinds of saccharides are absorbed through intestinal lumen
Monosaccharides (esp. glucose, fructose, galactose)
What is the major dietary carbohydrate, and what does this mean?
Starch - mix of straight and branched chain polymers of glucose
Major 4 disaccharides in food
Trehalose, sucrose, maltose, and lactose
Maltose
2 glucose
sucrose
glucose and fructose
lactose
glucose and galactose
Monosaccharies in our diet are mainly (3)
glucose, fructose, galactose
Cellulose
Glucose linkage that we cannot break (b1,4 linkage) - excreted or used for bacteria in gut
where does digestion of carbs begin
mouth - salivary amylase (breaks a1,4-linkage)
What happens to salivary amylase in stomach
shit gets fucked (no worky)
pancreatic amylase
most significant amylase in digestion, digests a1,4-bonds and gives mixture of disaccharides, trisaccharides, and oligosaccharides - alpha-limit dextins, maltose, and maltotriose must break them down further, and then down more by a-dextrinase (isomaltase), maltase, and sucrase to get glucose which can be absorbed by intestinal epithelium
Do disaccharies require amylase igestion
no
Where are the enzymes for carbohydrate digestion
brush border epthelium
How do we bring monosaccharides into the cell
SGLT1- Glucose and galactose, share a ride with Na
GLUT5 (for fructose)
requires atp
how do we get monosaccharides across into the blood
GLUT2 (glucose, galactose, fructose) - this is facilitated diffusion
Lactose intolerance
Lack/deficiency in lactase in brush border
result of lactose intolerance
osmotic diarrhea because sugar holds onto water and you poop it out
Congenital lactose intolerance
Lack of jejunal lactase - rare and serious, replace lactose with sucrose or fructose diet
Glucose-galactose malabsorption
Mutation of SGLT1 leads to inability to absorb these - can result in severe diarrhea and thus dehydration and other consequences - fructose only diet is recommended
Essential amino acids
Cannot be synthesized by body; must be obtained in diet
Where does protein digestion start
Stomach (pepsin)
Where is protein digestion completed, and how?
Small intestine with pancreatic and brush-border proteases (endopeptidases and exopeptidases)
endopeptidase
hydrolyzes the interior peptide bonds of proteins (pepsin, trypsin, chymotrypsin, elastase)
Exopeptiases
hydrolyze one amino acid at a time from C terminal ends of proteins and peptides (carboxypeptidases A and B)
Is pepsin active in the small intestine
no (high pH will inactivate it and small intestine should have pancreatic HCO3-)
Is pepsin essential for protein digestion in a healthy individual
no
Where are proteases that act in small intestine
Brush border and secreted from pancreas
5 major pancreatic proteases that are secreted as zymogens
Trypsinogen chymotrypsinogen proelastase procarboxypeptidase A Procarboxypeptidase B
What is the first step in protein digestion in the intestine
activate trypsinogen to trypsin by enterokinase (located on brush border)
What enzyme catalyzes the conversion of inactive intestinal precursors to active enzymes
trypsin - it also autocatalyzes any remaining trypsinogen to active trypsin
What are the absorbable kinds of protein digestion products
amino acids, dipeptides, tripeptides
Oligopeptides are not absorbable - they must be hydrolyzed by brush border proteases to yield smaller absorbable molecules
Products of the 5 pancreatic proteases
amino acids, dipeptides, tripeptides, and oligopeptides
How do we get amino acids, dipeptides, and tripeptides from GI lumen to blood
There are AA transporters that will transpot a specific kind of amino acid (ex, neutral, basic whatever) from lumen - these will sometimes take up drugs (clinically relevant). Inside the cell, peptides will be hydrolyzed into amino acids by cytosolic peptidases, nd amino acids exit cell via facilitated diffusion on blood side
Cystinurea
Transporter for dibasic amino acids (cystine, lysine, arginine, ornithine) is absent in small intestine and kidney, so no or low absorption of these happens in SI / kidney. This results in failure to absorb them and they are excreted in feces
Major categories of lipid in diet (3)
Triglycerides, phospholipids, cholesterol
There are also many vitamins that ae along for the ride
fat soluble vitamins
A, D, E, K
What problem do we face when digesting lipids
lipids and water hate each other so we need to make them kiss to give lipolytic enzymes access
Emulsification
the mixing action of stomach and small intestine churns dietary lipids into a suspension of fine droplets, this greatly increases surface area for digestion enzymes (also, bile salts are considered emulsifiers)
In stomach, what emulsifies lipids
dietary proteins
in small intestine, what emulsifies lipids
bile acids
Where does lipid digestion start
There is a lingual lipase, but she says stomach
Gastric lipase is released by chief cells nd hydrolyzes about 10% of dietry triglycerides
Major contribution of stomach in lipid digestion
Slow emptying into the small intestine, which allows adequate time for pancreatic enzyme action
What hormone is released when dietary lipids enter the small intestine and what is the result
CCK - this will slow down gastric emptying to give time for these fats to be broken down
Where does most lipid digestion occur
small intestine
What are the 3 lipolytic enzymes that work at neutral pH that are released from the pancreas
pancreatic lipase
phospholipase A2
cholesterol ester hydrolase
What is colipase?
Cofactor that keeps pancreatic lipase active even when it interacts with bile acids - binds to both and anchors lipase to fat droplet even when bile acids are present
What activates phospholipase A2
trypsin
What is the one FA product that is not going to be part of micelles
Glycerol (this is water soluble)
Structure of micelle
Products of lipid digestion (monoglycerides, FAs, cholesterol, lysolecithin) are solublized in intestinal lumen in mixed micelles (notice no glycerol, this is water soluble). The core of a micelle has the products of this digestion - exterior is lined with amphipathic bile salts. The hydrophilic portion of bile salts dissolves in aqueous solution of intestinal lumen and shields the hydrohobic region - solubilizes the lipids in the micelle core
Micelles get into brush border epithelium – how?
Diffusion - but its more like the membranes mingle and the lipids are yeeted inside because the micelle itself does not get in. Bile salts are left in intestinal lumen to be reabsorbed in ileum, and inside the cell lipid digestion products are re-esterified with FFAs on smooth endoplasmic reticulum to form TGs, cholesterol ester, and phospholipids, then repackaged as a chylomicron with apoproteins (like ApoB) and sent out into the lymph system
can chylomicrons get into blood capillaries
nope must go through lymph system
Where are chylomicrons going to join the blood
when lymph is yeeted into the thoracic duct that empties lymph to vena cava
Where is most sodium reabsorbed
Jejunum - water will follow
Transporters of sodium + other electrolytes and water in jejunum
Sodium + sugar / AA transporter and Na/H exchanger bring Na in, NKA helps Na leave
Also, bicarbonate is absorbed here from CA activity
Net absorption: NaHCO3
Transporters of sodium + other electrolytes and water in ileum
Na + sugar / AA trnsporters, Na/H exchanger will bring sodium in from GI, and NKA will let it leave into blood.
Cl is absorbed here instead of HCO3
Net absorption: NaCl
Crypts (secrete / absorb) fluid and electrolytes
Secrete
Lining of villi cells (secrete / absorb) electrolytes and fluid
absorb
Apical membrane of crypt cells has what kind of anion channel
chloride
basolateral membrane of crypt cells have what 2 transporters
NKA, Na/K/Cl-
Net change in water in crypts is (loss from body, absorption into body)
loss / secretion
Cholera impact on water
adenylyl cyclase will be highlly activated in crypt cells and lead to fluid secretion by crypt that overwhelms absorptive capacity of villus
Are chloride channels in apical membranes of crypt cells usually open?
No they are usually closed, but will open because of neurotransmitters like Ach after sensing chyme in small intestine
Osmotic diarrhea
Diarrhea caused by presence of nonabsorbable solutes in intestinal lumen (ex. someone shitting their brains out after having ice cream if they’re lactose intolerant)
Secretory diarrhea
like cholera, this is caused by excessive secretion of fluid by crypt cells (major cause is overgrowpth of enterohepatic bacteria like cholera or E coli)
Calcium absorption
Actively absorbed in small intestine but requires active form of vitamin D, which needs the vitamin D dependent calcium binding protein calbindin D-28K
How does calcium get into cell on luminal side
diffusion down electrochemical gradient
how is calcium absorbed into blood
Out a calcium ATP-ase
How do we absorb vitamin B12
B12 is released by food from pepsin in stomach. In stomach it binds to R proteins which will stabilize it.
In stomach, parietal cells create intrinsic factor.
In duodenum, pancreatic proteases degrade R proteins and B12 is transfered to intrinsic factors.
These B12-intrinsic factors are absorbed by a specific transport mechanism in ileum - without B12 absorption you get pernicious anemia
