Carbohydrate and Protein Digestion

Question 1

What form of carbohydrates are able to be digested?

Answer 1

Monosaccharide hexose sugars (6C) - glucose, galactose and fructose

Question 2

What are disaccharides?

Answer 2

Two monosaccharides linked together by a glycosidic bond which are broken down by enzymes to be absorbed

Question 3

Give 3 examples of disaccharides

Answer 3

Lactose
Sucrose
Maltose

Question 4

What enzyme breaks down lactose and what does it become?

Answer 4

Lactase: lactose -> glucose + galactose

Question 5

What enzyme breaks down sucrose and what does it become?

Answer 5

Sucrase: sucrose -> glucose and fructose

Question 6

What enzyme breaks down maltose and what does it become?

Answer 6

Maltase: maltose -> glucose + glucose

Question 7

Give 3 examples of a polysaccharide

Answer 7

Starch
Cellulose
Glycogen

Question 8

What is starch?

Answer 8

Plant storage form of glucose, with glucose monomers being linked by alpha-1-4 glycosidic bonds

Question 9

Can starch be digested by humans?

Answer 9

Yes, bonds hydrolysed by amylase in saliva and pancreas

Question 10

What is cellulose?

Answer 10

Constituent of plant cell walls, with glucose monomers are linked by beta-1-4 glycosidic bonds

Question 11

Can cellulose by digested by human?

Answer 11

No, as we don’t produce enzyme cellulase

Question 12

What is glycogen?

Answer 12

Animal storage form of glucose, with glucose monomers linked by alpha-1-4 glycosidic bonds

Question 13

What are the two membranes across the epithelium of the small intestine?

Answer 13

Apical (brush border) and bass-lateral membrane

Question 14

What occurs to nutrients after passing through the wall of the small intestine?

Answer 14

Most move into capillaries, and as all gut tube venous blood passes through the hepatic portal system, absorbed material travels through liver for filtration

Fats pass into the lacteal and through the lymphatic systems which drains into the venous system

Question 15

Describe process of monosaccharides (and galactose) absorption across intestinal epithelial cells

Answer 15

1. Apical Na-glucose symporter transports the two molecules into the cell, but only if both are present in lumen
2. Na is pumped straight out of the cell via Na-K – ATPase transporter
3. Once the concentration of glucose in the cell exceeds the blood, glucose diffuses into the blood via the GLUT-2 transporter
4. The uptake of nutrients into the bloodstream increases the osmostic gradient, so water moves thought the highly water permeable tight junctions and into the bloodstream

Question 16

Where does most absorption occur and what shape are the epithelial cells?

Answer 16

Small intestine - cuboidal epithelial cells with villi

Question 17

How is fructose absorbed?

Answer 17

It is not Na-dependent, and moves across the apical membrane by facilitated diffusion via the GLUT5 transporter, and into the blood by GLUT2 transporter on the basolateral membrane

No water drawn in by osmosis as it is immediately broken down in the blood and therefore no gradient formed

Question 18

How are proteins broken down?

Answer 18

Enzymes hydrolyse the peptide bonds between the amino acids

Enzymes: proteases and peptidases

Question 19

What are the two groups of enzymes for protein digestion?

Answer 19

Endopeptidases and exopeptidases

Question 20

What are endopeptidases?

Answer 20

Protease enzymes: breaks down peptide bond in the interior of the AA chain and break a long peptide chain into smaller fragments

Question 21

What are exopeptidases?

Answer 21

Peptidase enzymes: acts on terminal amino acids on either end and removes one AA at a time

Aminopeptidases: works on amino-terminal end
Carboxypeptidases: works on carboxylate-terminal end

Question 22

How are amino acids absorbed?

Answer 22

Most free AA absorbed via Na-dependent co-transport, but some transporters are H dependent

Question 23

How are AA absorbed via H dependent transporters?

Answer 23

Some epithelium exists in an acid micromlimate where H ions to maintain a low pH

1. Dipeptides and tripeptides are transported via PepT1 which uses H-dependent co-transport.
2. The H- ion taken up is then used in the NHE3 transporter also on the apical membrane, to pump Na into the cell
3. Na is moved out of the cell via the Na-K ATPase
4. AA moved out of cell into blood