Solutes, membranes and membrane transport

Question 1

What is Fick’s law of diffusion?

Answer 1

J = D x A x (dC/dx)

J is diffusion flux
D is diffusion coefficient 
A is surface area
dC is concentration difference
dx is membrane thickness

Question 2

What factors is the diffusion coefficient dependent on?

Answer 2

Molecular weight and temperature

Question 3

What are lipid rafts, and hence caveolae?

Answer 3

Specialised membrane microdomains that are composed of significantly more cholesterol and sphingolipids than the rest of the plasma membrane, thus making them more tightly packed.
They compartmentalise cellular processes by serving as organising centres for the assembly of signalling molecules to promote protein receptor interactions and therefore signal transduction.

Caveolae are specialised lipid rafts. They cause small invaginations of the plasma membrane due to the presence of an integral protein caveolin. They are involved in signal transduction as well as endocytosis.

Question 4

What is the main difference between a channel and carrier protein?

Answer 4

Channel - undergoes a conformational change which either gates it or opens it. The molecules can then diffuse through the channel passively

Carrier - undergo cyclical conformational change which carries the ligand from one side of the membrane to another (ligand binds, conformational change occurs moving ligand attached to binding site across membrane, ligand unbinds, conformational change occurs moving binding site back across membrane, cycles cycles cycles) - can be active or passive depending on what causes conformational change