Digestion and Absorption of Carbohydrates and Proteins Flashcards
Which chemical form must carbohydrates be in to be absorbed by the small intestine?
Monomers.
Are cellulose and hemicellulose soluble or insoluble fibres?
Insoluble.
Are pectins soluble or insoluble fibres?
Soluble.
What is starch composed of?
What is its function in the body?
Amylose and amylopectin.
Functions as a storage form for carbohydrates in plants.
How does amylose differ from amylopectin?
Amylose:
- Straight chain.
- Only α-1,4 linkages.
Amylopectin:
- Branched chains.
- Both α-1,4 and α-1,6 linkages.
How does glycogen differ from amylopectin?
What is its function?
- More highly branched than amylopectin due to more α-1,6 linkages.
- Functions as a storage form for carbohydrates in animals.
Which monomers compose sucrose?
Alpha glucose and beta fructose.
Which monomers compose lactose?
Beta glucose and beta galactose.
Which monomers compose maltose?
Alpha glucose x2.
What are the two steps involved in the conversion of polysaccharides to monosaccharides.?
1 - Intraluminal hydrolysis.
2 - Membrane digestion.
What occurs in intraluminal hydrolysis?
- Starch is converted to oligosaccharides by salivary and pancreatic α amylases.
- Salivary amylase initiates starch digestion, but is inactivated by gastric acid.
- Pancreatic amylase (induced by CCK) completes starch digestion in the lumen of the small intestine.
What occurs in membrane digestion of carbohydrates?
Oligosaccharides are converted to monosaccharides by brush-border disaccharidases.
What is CCK?
What is its function?
Where is it secreted?
Cholecystokinin, a peptide hormone that:
- Stimulates the release of bile into the intestine.
- Stimulates the secretion of enzymes by the pancreas.
- Is secreted by duodenal cells.
Why does starch hydrolysis terminate at maltose, maltotriose and α-limit dextrins?
Because amylase is and endoenzyme that hydrolyses internal α-1,4 links only (cannot hydrolyse external links).
What is an α-limit dextrin?
The remaining polymer with an α-1,6 bond, produced by hydrolysis of amylopectin with amylase, which cannot hydrolyse the alpha-1,6 bonds at branch points.
What are the 3 brush-border oligosaccharidases?
1 - Lactase.
2 - Maltase.
3 - Sucrase-isomaltase (two enzymes).
Which enzyme can split branching α-1,6 linkages of α-limit dextrins?
Isomaltase.
How does oligosaccharidase activity and distribution differ across the small and large intestine?
- Highest in the jejunum.
- Less in the duodenum and ileum.
- None in the large intestine.
What are the two steps involved in the absorption of monosaccharides in the small intestine?
1 - Uptake across the apical membrane into the enterocyte.
2 - Exit across the basolateral membrane.
What is SGLT1?
- A Na/Glucose transporter that is responsible for glucose and galactose uptake at the apical membrane.
- An example of secondary active transport.
Why is glucose transport across the apical membrane an example of secondary active transport?
As sodium is actively pumped out of the enterocyte, but then passively cotransports glucose into the enterocyte back down sodium’s electrochemical gradient.
What is GLUT5?
A protein that transports fructose across the apical membrane via facilitated diffusion.
What is GLUT2?
A protein that transports all three monosaccharides (glucose, fructose and galactose) across the basolateral membrane via facilitated diffusion.
Why is breath hydrogen increased in individuals with primary lactase deficiency?
Non-absorbed lactose is metabolised by colonic bacteria to hydrogen gas, which is absorbed into the blood and excreted by the lungs.
Which chemical form must proteins be in to be absorbed?
Amino acids or oligopeptides.
What are the 4 ways in which digestion and absorption of proteins occurs in the GIT?
1 - Luminal proteases from the stomach and pancreas: proteins -> peptides -> amino acids -> absorbed.
2 - Luminal proteases: protein -> peptides,
Enzymes at the brush border: peptides -> amino acids -> absorbed.
3 - Luminal enzymes: proteins -> oligopeptides -> uptake by enterocytes,
Cytosolic enzymes: peptides -> amino acids -> absorbed.
4 - Luminal enzymes: proteins -> oligopeptides -> absorbed.
What activates the pro-enzyme (pepsinogen) secreted by chief cells of the stomach?
What is the product of activation of pepsinogen?
- Low pH.
- Pepsin.
Which linkages is pepsin specific for?
How much protein digestion does pepsin account for?
- Linkages of aromatic and large neutral amino acids.
- Pepsin accounts for 10% - 15% of protein digestion.
What is the activating agent for trypsinogen?
Is this an endoenzyme or exoenzyme in its active form?
Enterokinase.
Trypsin is an endoenzyme.
For which 4 proenzymes is trypsin an activating agent?
Which of these proenzymes are exopeptidases in their active form?
1 - Chymotrypsinogen. - endo.
2 - Proelastase. - endo.
3 - Procarboxypeptidase A. - exo
4 - Procarboxypeptidase B. - exo
How do endopeptidases and exopeptidases differ in function?
What about dipeptidases?
- Endopeptidases hydrolyses peptide bonds between specific amino acids, resulting in oligopeptides with 2-6 amino acids.
- Exopeptidases hydrolyse peptide bonds near the ends of the peptides, resulting in single amino acids.
- Dipeptidases hydrolyse peptide bonds in dipeptides.
What is the function of brush border peptidases?
To hydrolyse oligopeptides formed during luminal digestion to amino acids.
How do brush-border peptidases differ from cytosolic peptidases?
Brush border peptidases have a gretaer affinity for relatively larger oligopeptides (3-8 AAs) compared to cytosolic peptidases.
How does PEPT1 transport amino acids across the apical membrane?
It uses secondary active transport driven by a proton gradient.
What is iminoglycinuria?
A mutation in the IMINO amino acid transport system, which is responsible for the cotransport of glycine and proline with Na+ into enterocytes.
What is cystinuria?
A mutation in the antiporters responsible for the absorption of cationic amino acids such as cysteine and efflux of neutral amino acids, leading to increased cysteine in the urine.
What is Hartnup disorder?
A mutation in the B0 transporters responsible for the cotransport of non-polar amino acids with Na+ into enterocytes.
What is dicarboxylic aminoaciduria?
A mutation in the XAG transporters responsible for the cotransport of glutamate and aspartate with 3Na+ into enterocytes, leading to increased concentrations of these amino acids in the urine.
What is lysinuric protein intolerance?
A mutation in the transporters at the basolateral membrane that are responsible for the transport of lysine, arginine and ornithine.
Which transporter mediates the release of cationic amino acids at the basolateral membrane?
4F2hc/y+LAT1 antiporters.
What property do all transporters of amino acids at the basolateral membrane have?
They are Na+ independent.
Which transporter mediates the release of neutral amino acids at the basolateral membrane?
4F2hc-independent LAT transporters.
List 3 signs of Hartnup disease.
1 - Skin changes of pellagra.
2 - Cerebellar ataxia (lack of fine control of voluntary movements).
3 - Psychiatric abnormalities.
What are the two routes by which in tact proteins / polypeptides are absorbed in adults?
1 - Enterocyte phagocytosis.
2 - M cells (in Peyer’s patches).
Where are Peyer’s patches?
In lymphatic tissue in the ileum.
How do M cells facilitate protein uptake?
What is the importance of protein uptake in adults?
Where else is in tact protein uptake seen?
- M cells take up proteins by phagocytosis and package them in clathrin-coated vesicles.
- The vesicles are secreted at their basolateral membranes into the lamina propria.
- Immunocompetent cells process the target antigens and transfer them to lymphocytes/
- An immune response is initiated.
- Important in disease processes of the mucosa.
- In tact protein uptake also seen in neonates (from colostrum / breast milk).
