Physiology of Digestion and Absorption 2

Question 1

What glucose polymers are present in starch?

Answer 1

Amylose and amylopectin.

Question 2

What are the links between glucose subunits in amylose and amylopectin?

Answer 2

Amylose - alpha-1,4 linkage.

Amylopectin - alpha-1,4 linkage and alpha-1,6 linkage (branches).

Question 3

Is glycogen or amylopectin more branched?

Answer 3

Glycogen.

Question 4

What are the linkages in glycogen molecules?

Answer 4

Alpha-1,4 and alpha-1,6 (branches).

Question 5

What are oligosaccharides?

Answer 5

Sugar dimers.

Question 6

What makes up sucrose and what are the bonds between the monomers?

Answer 6

Glucose and fructose. Alpha-1,2 linkages.

Question 7

What makes up lactose and what are the bonds between the monomers?

Answer 7

Glucose and galactose. Beta-1,4 linkages.

Question 8

What breaks down starch into oligosaccharides and where does this occur?

Answer 8

Alpha-amylase (salivary and pancreatic). In the lumen (intra-luminal hydrolysis).

Question 9

What breaks down oligosaccharides into monosaccharides and where does this occur?

Answer 9

Oligosaccharidases (lactase, maltase, sucrase-isomaltase).

Question 10

Is alpha-amylase and exoenzyme or an endoenzyme?

Answer 10

Endoenzyme (not sure about this).

Question 11

What bonds can amylase break and not break?

Answer 11

Can: internal alpha-1,4 linkages. Can’t: terminal alpha-1,4 linkages, alpha1-6 linkages or alpha-1,4 linkages adjacent to branch points.

Question 12

What is produced from amylase digestion of starch?

Answer 12

Maltotriose, maltose and alpha-limit dextrins.

Question 13

What is the substrate of lactase?

Answer 13

Lactose (the only one).

Question 14

What are the functions of the other oligosaccharidases?

Answer 14

They cleave the terminal alpha-1,4 linkages.

Question 15

What are the specific functions of maltase, sucrase and isomaltase?

Answer 15

Maltase - can degrade alpha-1,4 linkages in straight chain monomers up to 9 in length.
Sucrase - hydrolyses sucrose.
Isomaltase - only one that can split branching alpha-1,6 linkages of alpha-limit dextrins.

Question 16

What is the only oligosaccharidase where the hydrolysis by the enzyme is the rate limiting step in assimmilation?

Answer 16

Lactase.

Question 17

What gene affects lactase persistence (LP)?

Answer 17

MCM6 gene (regulates expression of the lactase gene).

Question 18

Describe primary, secondary and congenital lactase deficiency.

Answer 18

Primary - lack of lactase persistence allele (commonest).
Secondary - damage to/infection of the proximal small intestine.
Congenital - rare autosomal recessive disease, no ability to digest lactose from birth.

Question 19

What is the name for lactase insufficiency?

Answer 19

Hypolactasia.

Question 20

In hypolactasia, what do the colonic microflora produce when lactose is delivered to the colon?

Answer 20

Short chain fatty acids (absorbed), hydrogen (can be detected in breath following lactose challenge), carbon dioxide, methane.

Question 21

What symptoms do the substances produced by the colonic microflora cause?

Answer 21

Bloating, abdominal pain, flatulence.

Question 22

What does undigested lactose cause?

Answer 22

Acidification of the colon, an increased osmotic load (loose stools and diarrhoea).

Question 23

How are glucose and galactose absorbed from the lumen?

Answer 23

Secondary active transport mediated by SGLT1 (sodium symporter).

Question 24

How is fructose absorbed from the lumen?

Answer 24

Facilitated diffusion mediated by GLUT5.

Question 25

How do monosaccharides get transported across the basolateral membrane of the enterocyte?

Answer 25

All by facilitated diffusion mediated by GLUT2.

Question 26

What are the 2 conditions for a substrate to go through the SGLT1 transporter?

Answer 26

Must be a hexose in the D-confomation and one that can form a pyranose ring.

Question 27

What maintains a low sodium concentration in the enterocytes?

Answer 27

Na+/K+ATPase.

Question 28

What binds first to the SGLT1 transporter?

Answer 28

Sodium.

Question 29

What 3 types of enzymes can break down peptides?

Answer 29

Luminal enzymes, brush border enzymes and intracellular enzymes.

Question 30

How much of daily energy intake does protein assimilation account for in developed countries?

Answer 30

10-15%.

Question 31

What is the difference between an endopeptidase and an exopeptidase?

Answer 31

Endopeptidase - attacks bonds in middle of chain.

Exopeptidase - attacks bonds at end of chain.

Question 32

What pancreatic enzymes are endopeptidases and exopeptidases?

Answer 32

Endopeptidases - trypsin, chymotrypsin, elastase.

Exopeptidases - procarboxypeptidase A+B.

Question 33

What are aminopeptidases and carboxypeptidases?

Answer 33

Aminopeptidases - cleave off amino acid at N-terminal.

Carboxypeptidases - cleave off amino acid at C-terminal.

Question 34

Why are there numerous brush border peptidases?

Answer 34

Each enzyme attacks a limited number of peptide bonds and the oligopeptides to be digested are extremely varied in their structure.

Question 35

What types of peptides do brush border peptidases have a higher affinity for?

Answer 35

Larger oligopeptides (3-8 amino acids).

Question 36

At the brush border how are amino acids absorbed?

Answer 36

5 Na+-dependent co-transporters (secondary active transport), 2 Na+ independent transporters (facilitated diffusion of cationic amino acids).

Question 37

What diseases does dysfunction of the Na+ dependent and Na+independent transporters cause?

Answer 37

Na+ dependent: Hartnup disease.

Na+ independent: cystinuria.

Question 38

How are amino acids transported over the basolateral membrane?

Answer 38

3 mechanisms mediate efflux and are sodium independent, 2 mediate influx and are sodium dependent.

Question 39

What is the name of mechanism of transport of oligopeptides into the enterocyte?

Answer 39

H+-dependent mechanism called PepT1.