Membrane transport

Question 1

What transport mechanisms exist to transport things across a membrane

Answer 1

Simple diffusion
Facilitated diffusion
Active transport (primary and secondary)

Question 2

What is simple diffusion?

Answer 2

The movement of substances from an area of high concentration to an area of lower concentration.
Rate of diffusion dependant on the partition coefficient.

Question 3

What is the partition coefficient?

Answer 3

Equilibrium constant for partitioning of molecules between oil (octanol) and water.

Question 4

What is facilitated diffusion?

Answer 4

Simple diffusion mediated by proteins.

Question 5

How does the K+ selectivity filter work?

Answer 5

A K+ ion will have a hydration shell of a certain size.
When passing through the membrane the ion will pass through a narrow passage with for =O groups facing it, the size of this passage is the same as the size of the hydration shell on the K+ ion and so it is specific to it.

Question 6

What are the types of gated ion channels?

Answer 6

Voltage gated
Ligand gated (Extracellular and intracellular)
Mechanically gated - like in soundwaves affecting the ear there is a levering mechanism.

Question 7

What do the terms uniport, symport, antiport mean?

Answer 7

One port, more than one port or at least on port in either direction.

Question 8

What are the kinetic differences between simple diffusion and facilitated diffusion?

Answer 8

On a graph of rate/[s]
Simple diffusion is directly proportional, whereas facilitated diffusion can be described by Michaelis Menton kinetics, where 1/2Vmax = Km

Question 9

How does Km represent affinity

Answer 9

The lower the Km the higher the affinity as 1/2Vmax reached at a lower Km

Question 10

Describe SLC2A1

Answer 10

GLUT 1 = abundant in erythrocytes, low in skeletal muscle. Mediates basal transport of glucose into a wide range of cells. Km = 1.8 mM.

Question 11

Describe SLC2A2

Answer 11

GLUT 2 = found in hepatocytes and pancreatic B cells. Hepatocytes make glycogen and B cells make insulin.
Km = 20 mM; this acts as a sensor for hepatocytes and pancreatic B cells as will really only start working at a high rate when glucose levels are very high. So glycogen only produced and insulin only released when glucose is high.

Question 12

Describe SLC2A3

Answer 12

GLUT 3 = mainly in neurones, low Km as has neurones only metabolise glucose.

Question 13

Describe SLC2A4

Answer 13

GLUT 4 = found in muscle and adipocytes. Activity regulated by insulin, stimulates it moving to the membrane.
Km similar to upper range of glucose concentrations.

Question 14

Describe SLC2A5

Answer 14

Fructose transporter lol

Question 15

How is GLUT4 activity regulated

Answer 15

Insulin release following high blood glucose causes to intracellular pool of GLUT4 via phosphorylation cascade.
GLUT4 translocated to membrane
When glucose levels drop it is recycled into cell.

Question 16

What is the difference between primary and secondary active transport?

Answer 16

Primary active transport uses ATP and secondary uses an electrochemical gradient to drive energetically unfavourable processes.

Question 17

Give examples of primary active transport

Answer 17

Na+/K+-ATPase in plasma membrane
H+-ATPase in lysosomal membrane
Ca2+-ATPase in sarcoplasmic reticulum of muscle

Question 18

How does a Na+/K+ pump work?

Answer 18

Na+ binds to high affinity binding site on inside of membrane, this causes autophosphorylation of the protein (uses up ATP); this then causes a conformational change in the protein, releasing the Na+ on the exterior of the membrane, then K+ binds to the now exposed high affinity binding site on the protein, this causes autodephosphorylation of the protein causing a conformational change back to its original shape, bringin K+ into the cell.
3 Na+ removed per 2K+ added -> generates a negative charge on the inside of the membrane (electrogenic)

Question 19

Electrogenic

Answer 19

Generates charge

Question 20

Why is the Na+/K+ pump targeted in the treatment of congestive heart failure?

Answer 20

Oubain is a drug that blocks Na+/K+-ATPase by preventing K+ binding.
This increases the intracellular [Na+], which reduced=s the activity the Ca2+ antiporter (Ca2+ leaves the cell due to electrochemical gradient caused by Na+ being pumped out)
This increases intracellular [Ca2+] which stimulates contraction of the cardiac muscle.

Question 21

Give an example of secondary active transport

Answer 21

Na+/glucose transporter (GLUT1)

Important in epithelial cells of gut, Na+ pumped in and glucose brought in with it. Also in PCT of kidney.

Question 22

Describe cholera and how to treat it

Answer 22

Vibrio cholerae produces a toxin which causes release of Cl- by overacting GSalpha leading to activation of adenylyl cyclase and an increase in cAMP levels.

Replacement therapy involves giving a high dose of glucose, when absorbed this will bring with it Na+ ions and create an electrochemical gradient to cause diffusion of Cl- back into the gut. Then water will also be drawn back in by osmosis.