Digestion and Absorption of Carbohydrates and Proteins

Question 1

Which chemical form must carbohydrates be in to be absorbed by the small intestine?

Answer 1

Monomers.

Question 2

Are cellulose and hemicellulose soluble or insoluble fibres?

Answer 2

Insoluble.

Question 3

Are pectins soluble or insoluble fibres?

Answer 3

Soluble.

Question 4

What is starch composed of?

What is its function in the body?

Answer 4

Amylose and amylopectin.

Functions as a storage form for carbohydrates in plants.

Question 5

How does amylose differ from amylopectin?

Answer 5

Amylose:

• Straight chain.
• Only α-1,4 linkages.

Amylopectin:

• Branched chains.
• Both α-1,4 and α-1,6 linkages.

Question 6

How does glycogen differ from amylopectin?

What is its function?

Answer 6

• More highly branched than amylopectin due to more α-1,6 linkages.
• Functions as a storage form for carbohydrates in animals.

Question 7

Which monomers compose sucrose?

Answer 7

Alpha glucose and beta fructose.

Question 8

Which monomers compose lactose?

Answer 8

Beta glucose and beta galactose.

Question 9

Which monomers compose maltose?

Answer 9

Alpha glucose x2.

Question 10

What are the two steps involved in the conversion of polysaccharides to monosaccharides.?

Answer 10

1 - Intraluminal hydrolysis.

2 - Membrane digestion.

Question 11

What occurs in intraluminal hydrolysis?

Answer 11

• 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.

Question 12

What occurs in membrane digestion of carbohydrates?

Answer 12

Oligosaccharides are converted to monosaccharides by brush-border disaccharidases.

Question 13

What is CCK?

What is its function?

Where is it secreted?

Answer 13

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.

Question 14

Why does starch hydrolysis terminate at maltose, maltotriose and α-limit dextrins?

Answer 14

Because amylase is and endoenzyme that hydrolyses internal α-1,4 links only (cannot hydrolyse external links).

Question 15

What is an α-limit dextrin?

Answer 15

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.

Question 16

What are the 3 brush-border oligosaccharidases?

Answer 16

1 - Lactase.

2 - Maltase.

3 - Sucrase-isomaltase (two enzymes).

Question 17

Which enzyme can split branching α-1,6 linkages of α-limit dextrins?

Answer 17

Isomaltase.

Question 18

How does oligosaccharidase activity and distribution differ across the small and large intestine?

Answer 18

• Highest in the jejunum.
• Less in the duodenum and ileum.
• None in the large intestine.

Question 19

What are the two steps involved in the absorption of monosaccharides in the small intestine?

Answer 19

1 - Uptake across the apical membrane into the enterocyte.

2 - Exit across the basolateral membrane.

Question 20

What is SGLT1?

Answer 20

• A Na/Glucose transporter that is responsible for glucose and galactose uptake at the apical membrane.
• An example of secondary active transport.

Question 21

Why is glucose transport across the apical membrane an example of secondary active transport?

Answer 21

As sodium is actively pumped out of the enterocyte, but then passively cotransports glucose into the enterocyte back down sodium’s electrochemical gradient.

Question 22

What is GLUT5?

Answer 22

A protein that transports fructose across the apical membrane via facilitated diffusion.

Question 23

What is GLUT2?

Answer 23

A protein that transports all three monosaccharides (glucose, fructose and galactose) across the basolateral membrane via facilitated diffusion.

Question 24

Why is breath hydrogen increased in individuals with primary lactase deficiency?

Answer 24

Non-absorbed lactose is metabolised by colonic bacteria to hydrogen gas, which is absorbed into the blood and excreted by the lungs.

Question 25

Which chemical form must proteins be in to be absorbed?

Answer 25

Amino acids or oligopeptides.

Question 26

What are the 4 ways in which digestion and absorption of proteins occurs in the GIT?

Answer 26

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.

Question 27

What activates the pro-enzyme (pepsinogen) secreted by chief cells of the stomach? What is the product of activation of pepsinogen?

Answer 27

- Low pH. | - Pepsin.

Question 28

Which linkages is pepsin specific for? How much protein digestion does pepsin account for?

Answer 28

- Linkages of aromatic and large neutral amino acids. | - Pepsin accounts for 10% - 15% of protein digestion.

Question 29

What is the activating agent for trypsinogen? Is this an endoenzyme or exoenzyme in its active form?

Answer 29

Enterokinase. Trypsin is an endoenzyme.

Question 30

For which 4 proenzymes is trypsin an activating agent? Which of these proenzymes are exopeptidases in their active form?

Answer 30

1 - Chymotrypsinogen. - endo. 2 - Proelastase. - endo. 3 - Procarboxypeptidase A. - exo 4 - Procarboxypeptidase B. - exo

Question 31

How do endopeptidases and exopeptidases differ in function? What about dipeptidases?

Answer 31

- 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.

Question 32

What is the function of brush border peptidases?

Answer 32

To hydrolyse oligopeptides formed during luminal digestion to amino acids.

Question 33

How do brush-border peptidases differ from cytosolic peptidases?

Answer 33

Brush border peptidases have a gretaer affinity for relatively larger oligopeptides (3-8 AAs) compared to cytosolic peptidases.

Question 34

How does PEPT1 transport amino acids across the apical membrane?

Answer 34

It uses secondary active transport driven by a proton gradient.

Question 35

What is iminoglycinuria?

Answer 35

A mutation in the IMINO amino acid transport system, which is responsible for the cotransport of glycine and proline with Na+ into enterocytes.

Question 36

What is cystinuria?

Answer 36

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.

Question 37

What is Hartnup disorder?

Answer 37

A mutation in the B0 transporters responsible for the cotransport of non-polar amino acids with Na+ into enterocytes.

Question 38

What is dicarboxylic aminoaciduria?

Answer 38

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.

Question 39

What is lysinuric protein intolerance?

Answer 39

A mutation in the transporters at the basolateral membrane that are responsible for the transport of lysine, arginine and ornithine.

Question 40

Which transporter mediates the release of cationic amino acids at the basolateral membrane?

Answer 40

4F2hc/y+LAT1 antiporters.

Question 41

What property do all transporters of amino acids at the basolateral membrane have?

Answer 41

They are Na+ independent.

Question 42

Which transporter mediates the release of neutral amino acids at the basolateral membrane?

Answer 42

4F2hc-independent LAT transporters.

Question 43

List 3 signs of Hartnup disease.

Answer 43

1 - Skin changes of pellagra. 2 - Cerebellar ataxia (lack of fine control of voluntary movements). 3 - Psychiatric abnormalities.

Question 44

What are the two routes by which in tact proteins / polypeptides are absorbed in adults?

Answer 44

1 - Enterocyte phagocytosis. 2 - M cells (in Peyer's patches).

Question 45

Where are Peyer's patches?

Answer 45

In lymphatic tissue in the ileum.

Question 46

How do M cells facilitate protein uptake? What is the importance of protein uptake in adults? Where else is in tact protein uptake seen?

Answer 46

- 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).