Molecular Movement across Membranes

Question 1

What are the different sorts of transport across the membrane?

Answer 1

Simple diffusion, Facilitated Diffusion, Active Transport.

Question 2

What is the equation for ▲S ?

Answer 2

▲S = [S] outside - [S] inside

Question 3

What is Vinward?

Answer 3

V(inward) = P▲S
V(inward) = rate of diffusion into cell (moles/sec/cm2 of membrane)

Question 4

What is P?

Answer 4

P is the permeability coefficient (experimentally determined)

Question 5

What is ▲S equivalent to?

Answer 5

▲S = conc. gradient across the membrane
▲S = [S] outside - [S] inside

Question 6

What is partition coefficient?

Answer 6

It gives a measure of how well a substance dissolves in lipid or aqueous phase.

P. coefficient : K = [X]oil/[X]water

Question 7

what are the factors affecting the rate of transport through the lipid phase of the membrane?

Answer 7

Thickness, size, shape, polarity

Question 8

What enables the selective permeability of water through the lipid membrane?

Answer 8

Specialised channel proteins known as aquaporins - 13 different variations. They form tetramers in the membrane, where each monomer acts as a water channel. They’re too narrow for hydrated ions.

Question 9

What is the structure of aquaporins?

Answer 9

6 α-helices, connected by 3 extracellular and 2 intracellular loops.

Question 10

What is aquaporin 2?

Answer 10

AQP-2 is a channel protein in the kidney, regulated by ADH, which stimulates movement of AQP-2 to the luminal side of the renal cell membrane in late distal tubules, collecting tubules and collecting duct, increasing water absorption.

Question 11

How can protein channels be conditional?

Answer 11

They can be voltage gated (potential difference inside/outside cell causes confirmation change), or they can be Ligand gated (binding of a chemical ligand (e.g. acetyl choline) causes conformational change.

Question 12

What limits the rate of facilitated diffusion?

Answer 12

The Vmax of the carrier protein. This is the limit of the carrier protein to operate, differing in the time it takes to open and close.

Question 13

What is a symporter?

Answer 13

A symporter, or a co-transporter molecule, transports substances across the membrane, by coupling them up with another molecule and using the electrochemical gradient of a “driver” ion - like hitching a ride.

Question 14

What are antiporters?

Answer 14

Antiporters transport substances in the opposite direction of the driver ion.