Membrane Transport

Question 1

Why are biological membranes referred to as being selectively-permeable?

Answer 1

They allow the free passage of gases, small, uncharged or hydrophobic/lipid soluble molecules down a concentration gradient.

Large or charged molecules require membrane proteins in the form of channels, pumps and transporters to help them cross the membrane.

Question 2

Can the following molecules freely cross the lipid-bilayer: O2,N2, Benzene, short chain fatty acids?

Answer 2

Yes - they are hydrophobic molecules

Question 3

Can the following molecules freely cross the lipid-bilayer: H2O, CO2, Urea, Glycerol?

Answer 3

Yes - they are small uncharged polar molecules

Question 4

Can the following molecules freely cross the lipid-bilayer: Glucose, Sucrose?

Answer 4

No - they are large uncharged polar molecules so require pumps or transporters

Question 5

Can the following molecules freely cross the lipid-bilayer: H+, Na+, HCO3-, K+, Ca2+, Cl-?

Answer 5

No - they are ions so require ion channels, pumps or transporters

Question 6

Can the following molecules freely cross the lipid-bilayer: Amino acids, ATP?

Answer 6

No - they are charged polar molecules

Question 7

What are intracellular and extracellular concentrations of Na+, K+, Cl-, Ca2+ ions?

Answer 7

Na+ [10]i mM [140]o mM
K+ [140]i mM [4]o mM
Cl- [4]i mM [140]o mM
Ca2+ [0.001]i mM [2.5]o mM

Question 8

Other than ions, what else inside a cell contributes to the negative membrane potential?

Answer 8

Proteins - net negative charge

Question 9

What is the difference between primary active transport and secondary active transport?

Answer 9

Primary AT - Energy comes from ATP e.g Na+/K+ ATPase

Secondary AT - Energy from stored potential energy in one of the ion’s electrochemical gradient e.g Na+/Glucose transporter

Question 10

Define Kow

What does a high Kow value mean?

Answer 10

The equilibrium constant for partitioning of a molecule between octane oil and water.

The higher the Kow, the more lipid soluble it is and thus the faster it passes through the lipid bilayer.

Question 11

How does the structure of an ion channel determine which ions can pass through?

Answer 11

In solution, the ions are surrounded by water molecules with their oxygen atoms interacting with the ion - hydration shell.

In the pore of an ion channel, there are amino acid residues that mimic the hydration shell with their side chains. For K+, the arrangement in the pore is the same as in the aqueous environment (4 interactions) so there is no energy difference and K+ pass through the channel.

For Na+, a smaller ion, the hydration shell is closer to the ion than the amino acid side chains in the pore. It is not energetically favourable for Na+ to lose two of the interactions so Na+ does not pass through the ion channel.

Question 12

What are different glucose transporters and where are they located?

Answer 12

GLUT1

• Ubiquitous; abundant in RBCs but low in skeletal muscle
• High affinity

GLUT2

• Liver and ß-cells in Pancreas
• Low affinity

GLUT3

• Highly expressed by neurons
• High affinity

GLUT4

• Muscle and Adipocytes
• Moderate affinity
• Regulated by insulin and recruited to plasma membrane

GLUT5
- Transporter of fructose

Question 13

Describe how the Na+/K+ ATPase pump functions.

Answer 13

1. Na+ binds to intracellular site
2. Triggering of autophosphorylation of the pump
3. Conformational change releases Na+ into the extracellular space, and at the same time, exposing K+ binding site
4. K+ binds to extracellular site.
5. Pump returns to original confirmation and K+ is discharged into the intracellular space.

Question 14

How is the Na+/K+ATPase used a drug target in congestive heart failure?

Answer 14

Heart muscle contains Na+/Ca2+ antiporter, which functions to remove Ca2+ ions from the cytosol following muscle contraction.

Oubain binds to the pump’s extracellular K+ binding sites, preventing K+ ions from binding and thus inhibiting the pump and Na+ extrusion out of the cell.

As a result, intracellular {Na+] increases, which reduces the activity of the Na+/Ca2+ antiporter, causing slower Ca2+ efflux and therefore prolonged high intracellular [Ca2+] to maintain cardiac muscle contraction.

Question 15

Describe how Glucose is transported from the lumen of the gut into the blood stream.

Answer 15

Glucose and Na+ are co-transported into the intestinal epithelial cell, utilising the Na+ concentration gradient.

Glucose then leaves the IEC via GLUT2 at the basal membrane.

Na+ concentration gradient is maintained by the Na+/K+ ATPase pump on the basal membrane.

Question 16

Describe how Cholera toxin causes huge electrolyte and fluid losses in the intestine.

Answer 16

1. Cholera toxin binds to the GM1 ganglioside receptor on the apical membrane of intestinal cells.
2. The toxin is internalised by endocytosis and transported through the golgi to the ER.
3. In the ER, the subunits of the toxin split, with the A1 subunit escaping into the cytosol.
4. A1 binds to and over activates the heterotrimeric GTPase Gs-alpha, which leads to the activation of Adenylyl Cyclase and an increase in cAMP levels.
5. Increased cAMP causes continuous activation of the CFTR Cl- channelled thus an increase in Cl- secretion from the cell.
6. Na+ ions follow the Cl- out of the cell down its concentration gradient.
7. H20 follows down the osmotic gradient caused by the efflux of these ions.

Question 17

What can be done to reverse the effects of the cholera toxin?

Answer 17

Give oral rehydration therapy using a solution high in glucose to drive Na+ ions back into the intestinal cells via the Na+/Glucose co-transporter, causing Cl- ions and H2O to re-enter the cells too.