Digestion and Absorption Flashcards
What is digestion?
The process by which ingested food is broken down chemically into absorbable molecules.
What is absorption?
Is the movement of nutrients, water and electrolytes from the lumen of the intestine into blood.
What are the 2 pathways of absorption?
Cellular path - the substances enter the intestinal epithelial cells via the apical (luminal
membrane) and are extruded via basolateral membrane to enter blood.
Paracellular path - substances move across the tight junctions, through the lateral interspaces into blood.
What must carbohydrates be digested to in order to be absorbed?
Monosaccharides
What are the monosaccharides that can be absorbed?
Fructose, Galactose and Glucose
What is amylose?
It is the straight chain polymer
What is amylopectin?
It is the branched chain polymer
What does trehalose break down to?
2 Glucose
What does sucrose break down to?
1 Glucose 1 Fructose
What does lactose break down to?
1 Glucose 1 Galactose
Why can’t cellulose be broken down?
It has beta-1,4 linkages which cannot be broken down in humans due to the lack of the enzyme
What is the function of the enzyme alpha-amylase?
Cleaves the α-1,4 linkages in carbohydrates
What is the result of digestion with pancreatic amylase?
It yields a mix of disaccharides, trisaccharides
and oligosaccharides
What further digests the saccharides after the amylase does?
These disaccharides are further digested to monosaccharides by intestinal brush border enzymes:
- α-dextrinase (isomaltase)
- maltase
- sucrase
What is unique about α-limit dextrins?
Alpha-dextrins have the α-1,6 linkage that
can only be broken with alpha-dextrinase
What is the function of SGLT1?
SGLT1 transports glucose and galactose against their concentration gradients by coupling their transport to that of Na+ (active process).
What is the function of GLUT5?
Transports fructose across the apical membrane and it does not require an Na+ ion
What is the function of GLUT2?
It transports glucose, galactose and fructose across the basolateral membrane
What is the function of sucrase?
Digests sucrose
What is the function of lactase?
Digests lactose
What is lactose intolerance and what are symptoms of it?
Lack or deficiency of lactase in the brush border- lactose is not digested to glucose and galactose. If lactose is ingested (in milk or milk products), lactose remains undigested and unabsorbed in the intestinal lumen – and retains water causing osmotic diarrhea.
What is congenital lactose intolerance?
It is a lack of jejunal lactase and is much rarer than normal lactose intolerance. It can be dangerous so replacement with a sucrose/fructose diet may be necessary
What is glucose-galactose malabsorption?
It is due to a mutation in SGLT1 and a fructose diet is recommended
What enzyme starts protein digestion in the stomach?
Pepsin
What are endopeptidases?
They hydrolyze the interior peptide bonds of proteins
Example – pepsin, trypsin, chymotrypsin, elastase.
What are exopeptidases?
They hydrolyze one amino acid at a time from the C-terminal ends of proteins and peptides
Example - carboxypeptidases A&B.
What activates trypsinogen?
Enterokinase. It will then become trypsin and activate the other proteases.
Is pepsin in the stomach essential for protein digestion?
Pepsin is not essential for normal protein digestion, since in people whose stomach has been removed or who d not secrete gastric H+, have normal protein digestion and absorption.
Where does the major protein digestion occur?
In the small intestine with the pancreatic enzymes
Besides enterokinase, what else can activate trypsinogen?
Trypsin. It can auto catalyze itself.
What proteins can be absorbed?
Only tri, di and single amino acids can be absorbed
How are proteins absorbed?
- Some are symported with Na+
- Others symport with H+
What happens to the absorbed peptides in the cell before transfer through the basolateral membrane?
They will be cleaved into amino acids by peptidases
Describe the effects of diseases related to single AA absorption.
These are usually mild as many amino acid transporters have redundant function
What is cystinuria?
The transporter for dibasic amino acids (cystine, lysine, arginine, ornithine) is absent in small intestine and kidney so there is low or no absorption of these in intestine or kidney. The intestinal defect results in failure to absorb amino acids – excreted in feces.
How are lipids digested more effectively as a fatty meal floats on the surface of gastric contents?
The mixing action of the stomach churns the dietary lipids into a suspension of fine droplets via emulsification
What is the lipid digestion enzyme in the stomach?
Gastric Lipase
What is the main emulsification method in the small intestine?
Bile Acids
What are the 3 important lipase enzymes in pancreatic juice?
o Pancreatic lipase
o Phospholipase A2
o Cholesterol ester hydrolase
What is the function of pancreatic lipase?
Hydrolyzes triglycerides to monoglycerides and fatty acids.
What is the function of phospholipase A2?
Hydrolyzes phospholipids such as those present in cell membranes to lysolecithin and fatty acids.
What is the function of cholesterol ester hydrolase?
Hydrolyzes cholesterol ester to free cholesterol and fatty acids - also hydrolyzes ester linkages of triglycerides, producing glycerol.
What happens to pancreatic lipase in the presence of bile acids and how is this overcome?
Bile acids inactivate pancreatic lipase- which is overcome by an important cofactor colipase. Colipase binds to both bile acids and lipase – anchors lipase to the fat droplet even in the presence of bile acids.
How are the products of lipid digestion solubilized?
They are solubilized in the intestinal lumen in mixed micelles. Core of a micelle contains the products of lipid digestion – the exterior is lined with amphipathic (has both hydrophilic and hydrophobic face) bile salts.
How are lipids absorbed?
Lipids are released from micelles at the apical membrane – they diffuse down their concentration gradient into
the cells. The micelles do not enter the cells. The bile salts are left in the intestinal lumen – reabsorbed in the ileum.
What happens to the lipids inside the cells?
They are re-esterified with free fatty acids on the smooth endoplasmic reticulum – to form triglycerides, cholesterol ester and phospholipids.
The re-esterified lipids are packaged with apoproteins to form chylomicrons.
How are the chylomicrons absorbed into the blood?
Due to the large size they cannot enter blood vessels and enters the lymphatic capillaries. Chylomicrons are transported by lymphatic circulation to the thoracic duct – that empties into bloodstream.
What is pancreatic insufficiency?
Results in defects of pancreatic enzyme secretion. Abnormality of lipid digestion/absorption – results in steatorrhea.
What happens with a deficiency of bile salts?
No micelle formation – interferes with lipid digestion/absorption.
What may be a cause of bile salt deficiency?
In ileal resection (removal of ileum) enterohepatic circulation of bile is interrupted – bile is lost in feces.
What is abetalipoproteinemia?
Failure to synthesize Apo B so chylomicrons are either not formed or are unable to be transported into lymph.
What is the importance of fluidity in the GI tract?
Fluidity provides lubrication
Where do the fluids in the GI tract come from?
Part of it is ingested, the rest comes from GI secretions
How is the fluid level maintained?
Most of the fluid is reabsorbed, with most of it being done by the small intestine. Lack of reabsorption will lead to diarrhea.
Where in the GI tract does paracellular movement of fluid occur?
Most of it occurs in the small intestine as tight junctions of small intestine are leaky and permit significant paracellular movement. The tight junctions of the colon are tight and do-not permit paracellular movement.
What is the major site of Na absorption?
Jejunum
How does Na enter the apical membrane of the jejunum?
Na-Monosaccharide Cotransporters
Na-Amino Acid Cotransporters
Na/H+ Exchangers
How is Na extruded across the basolateral membrane of the jejunum?
Na/K ATPAse
Where does the H+ for the H/Na exchanger in the jejunum come from?
Carbonic acid. The HCO3- will be reabsorbed into the blood
What transport mechanism does the apical membrane of the ileum has that makes it different from the jejunum?
The ileum has all of the same transporters as the jejunum. It has an additional HCO3-/Cl- exchanger on the apical membrane though.
What does the basolateral membrane of the ileum has that makes it deferent from the jejunum?
Cl- transporter
What is there a net absorption of in the jejunum?
HCO3-
What is there a net absorption of the ileum?
Cl-
What activates the normally closed Cl- channels of the apical membrane in the intestine?
ACh
VIP
How do ACh and VIP activate the Cl- channels?
These bind to the basolateral receptors and activate adenylyl cyclase – generates cAMP in the crypt cells. cAMP opens the Cl- channels and causes Cl- secretion.
How does the disease cholera work?
Adenylyl cyclase is highly activated - fluid secretion by the crypt overwhelms the absorptive capacity of villus cells causing life-threatening diarrhea.
What is osmotic diarrhea?
It is caused by the presence of non-absorbable solutes in the intestinal lumen.
EX: Lactose intolerance
What is secretory diarrhea?
It is caused by excessive secretion of fluid by crypt
cells.
EX: Cholera
What is required for Ca absorption?
Vitamin D
How is vitamin D3 converted to its active form?
D3 ->
25-hydroxycholecalciferol ->
1,25-dihydroxycholecaiferol (PTH is required for
the conversion here)
What is necessary for the absorption of vitamin B12?
Intrinsic Factor
What is the problem in gasrectomy?
Parietal cells are lost - IF is not released – no vitamin B12 absorption
What is the problem in pernicious anemia?
Vitamin B12 deficiency can lead to this condition
Can oral vitamin B12 aid with gastrectomy?
No. There is no IF to absorb B12. Needs to be treated with B12 injections.
