GI: The Intestines Flashcards
What is the function of the intestines?
To absorb water/electrolytes and nutrients (can be cellular and paracellular)
What are plicae circulares?
Permanent folds of the mucosa to increase surface area for absorption
What is the role of the intestinal crypts?
The base of the crypts contain intestinal stem cells which migrate and differentiate to new epithelia. (shed every 3-6 days)
The crypts also contain goblet cells and endocrine cells.
Why type of carbohydrate can the intestine absorb?
Only monosaccharides can be absorbed eg fructose, galactose, glucose
Give an overview of the digestion of starch
20% amylose and 80% amylopectin
Amylose - has alpha-1,4 bonds
Amylopectin - has alpha-1,4 bonds and alpha-1,6 branching bonds
Amylase breaks the 1,4 bonds to form maltose (2glucose) from amylose and alpa dextrins from amylopectin
Isomaltase breaks the 1,6 bonds
How are the monosaccharides absorbed? (ion channels)
Glucose/galactose are contransported with Na+ into the enterocyte via SGLT1
Fructose enters the enterocyte via GLUT5
(Na+k+ ATPase maintains the gradients)
Glucose/ galactose and fructose symport into the blood via GLUT 2
Why are oral rehydration sachets containing glucose and salt so effective at rehydration?
Glucose uptake stimulates uptake of the Na+ from the salt which generates an osmotic gradient so water follows - this stimulates maximum water uptake
Why type of protein can the intestine absorb?
Only amino acids, dipeptides and tripeptides
What enzymes are important for protein digestion?
Pepsin - in the stomach pepsinogen is relaased from chief cells which is converted to pepsin via HCl
Trypsinogen - pancreas releases trypsinogen which is converted to trypsin by enterpeptidase. Trypsin then activated other proteases eg chymotrypsinogen
What is the role of exopeptidases and endopeptidases? Give examples of each
Exopeptidases break bonds at the ends of polypeptides to produce dipeptides or AAs.
(carboxypeptidase)
Endopeptidases break bonds in the middle of the polypeptide to produce shorter chains.
(trypsin, chymotrypsin, elastase)
How are the protein products absorbed? (ion channels)
AA are transported into the cell via Na+-AA cotransporters
Di/tripeptides are transported into the cell via a H+ co transporter (Peptide transporter 1) the inside the cell they are converted to AA
Where is the main site of water absorption?
The small intestines
How are electrolytes and water absorbed? (ion channels)
Na+ moved out of basolateral membrane by Na-K-ATPase
In small intestine - Na+ is cotransported
In large intestine - there are Na+ channels which are induced by aldosterone
How is calcium absorbed?
When calcium intake is low: there is facilitated transcellular absorption - Ca2+ ATPase removes Ca2+ from basolatersal membrane creating a gradient
(this requires vitamin D)
When calcium intake is normal or high:
Passive paracellular absorption
How is iron absorbed?
Iron is absorbed mostly in the state of haem or Fe2+ and cotransported with H+
When iron levels are low: iron binds to transferrin and transported to stores eg Hb, or in bone marrow, liver, spleen
When iron levels are high: iron is kept in the enterocyte and lost when the cells is replaced (3-6 days)
