Nutrient Digestion 1 - carbs and proteins

Question 1

Types of disaccharides and their constituents

Answer 1

Lactose - glucose + galactose
Sucrose - glucose + fructose
Maltose - glucose + glucose

Question 2

Effect of lactose intolerance on water secretion?

Answer 2

Absence of lactase leaves patient unable to break down lactose, so it remains in the gut - where it acts osmotically to draw water into lumen, resulting in increased water secretion

Question 3

Types of polysaccharide

Answer 3

Starch - glucose polymer in plants, linked by alpha 1,4 glycosidic bonds - hydrolysed by amylase (saliva/pancreas)
Cellulose - in plant cell wall, beta 1,4 bonds - not digestible by amylase
Glycogen - animal storage form of glucose, alpha 1,4 bonds

Question 4

Types of amylase

Answer 4

Salivary amylase - produced by salivary glands

Pancreatic amylase

Question 5

Which membranes must a substrate cross to get into the body from the gut lumen?

Answer 5

Apical membrane - on the surface of microvilli in the gut lumen, sugar usually in via Na paired transporter

Basolateral - Membrane away from the gut lumen, sugar usually out via Na/K ATPase paired glucose transporter

Question 6

Types of transport from gut lumen into body

Answer 6

paracellular - around the cells past tight junctions
transcellular - through cells in either direction, usually with gradient
Vectorial - unidirectional through cell membranes (usually via transporter)

Question 7

How does glucose get across apical membrane?

Answer 7

SGLT1 is a Na/glucose symporter, binds both to move them into cell

Question 8

How does glucose/galactose cross basolateral membrane?

Answer 8

Na in the cell from the SGLT1 symporter generates ATP via the Na/K ATPase by moving out of the cell with it’s gradient while K moves in (K back out via leaky channels - maintains gradient)
ATP powers GLUT-2 transporter - glc across membrane

Question 9

Effect of glucose import on water?

Answer 9

Water in gut lumen gets reabsorbed as well due to the net movement of Na and glucose into the body

Question 10

How does fructose move across apical membrane?

Answer 10

Via GLUT-5 transporter

Then crosses basolateral via GLUT-2 like glucose

Question 11

Describe protein structure

Answer 11

Polymers of AA’s linked by peptide bonds, large variations in chain length
Can be post-translationally modified - eg. glycoproteins and lipoproteins

Question 12

Which enzymes hydrolyze peptide bonds?

Answer 12

Proteases and peptidases

Question 13

Types of peptidase

Answer 13

Endopeptidase - hydrolyze peptide bonds within peptide
Exopeptidase - bonds at end of peptide
Carboxypeptidase - carboxyl end of peptide
Aminopeptidase - amino end of peptide

Question 14

Describe Amino Acid transport from gut lumen into body

Answer 14

Move across apical membrane by SAAT1 Na/AA paired transporter
Na then powers the Na/K ATPase, moves Na out and generates ATP to power AA specific transporters that moves the AA’s across basolateral membrane

Question 15

What is the maximum peptide size that can be imported by a cell in the gut lumen?

Answer 15

Di and tri - peptides can be symported across apical membrane with H (proton motive force)

Question 16

Describe process of moving a di/tri - peptide across the apical membrane

Answer 16

The peptide is symported in with H, via PepT1 symporter
The H is moved back out of the cell via the Na/H exchanger NHE3
Na is exchanged for K by the ATPase, generating ATP to transport the peptide across the basolateral membrane

Question 17

Why is there an acid microclimate at the mucosal lining of the apical membrane of some gut lumen cells?

Answer 17

Because during the transport of di and tri peptides, H ions are exchanged for Na and move into the mucosal lining, causing it to acidify

Question 18

How is the Na/K gradient that powers the ATPase maintained?

Answer 18

Na gets transported in from the gut lumen, and K that gets imported moves back out of the epithelial cells via leaky K channels