Week 1 - Lipids, proteins and membrane structure

Question 1

Why is the movement of protein in lipid bilayers more restricted than that of the lipid constituents?

Answer 1

Restraints on membrane protein mobility:

1. Lipid mediated effects: Proteins tend to separate out into the fluid phase of the bilayer or into cholesterol poor regions
2. Membrane protein associations within the bilayer: May reduce rotation and lateral diffusion (the latter due to size)
3. Associations with peripheral membrane proteins: e.g. cytoskeleton, cell to cell protein interactions

Question 2

Describe the main functions of biological membranes.

Answer 2

1. Continuous highly selective permeability barrier.
2. Allows control of the enclosed chemical environment
3. Communication - control the flow of information between cells and their environment
4. Recognition - signalling molecules, adhesion proteins, immune surveillance
5. Signal generation in response to stimuli - electrical, chemical

Question 3

Describe the composition of biological membranes.

Answer 3

Varies with source, but generally membranes contain approximately 40% lipid, 60% protein, and 1-10% carbohydrate (dry weight)

The membrane bilayer is a hydrated structure and hence 20% of total membrane weight is water

Question 4

What are the four permitted modes of mobility for lipid molecules in a lipid bilayer?

Answer 4

1. Intra-chain motion (flexion): kink formation in the fatty acyl chains
2. Fast axial rotation
3. Fast lateral diffusion within the plane of the bilayer
4. Flip-flop: movement of lipid molecules from one half of the bilayer to the other half on a one for one exchange basis

Question 5

Give examples of possible polar head groups that can be found in phospholipids.

Answer 5

1. Choline
2. Amines
3. Amino acids
4. Sugar

Question 6

What are the most common lengths of fatty acid chains? (in terms of number of C atoms)

Answer 6

C16 and C18

Question 7

Differentiate between the two types of glycolipid: cerebrosides and gangliosides.

Answer 7

Cerebroside: Head group sugar monomer
Ganglioside: Head group oligosaccharides (sugar multimers)

Question 8

What are the functions of membrane proteins?

Answer 8

1. Enzymes
2. Transporters
3. Pumps
4. Ion channels
5. Receptors
6. Energy transducers

Question 9

What are the permitted modes of mobility for proteins in membrane bilayers?

Answer 9

1. Lateral motion
2. Conformational change
3. Rotation

(no flip flop!)

Question 10

What are the restraints on mobility on proteins in the lipid bilayer?

Answer 10

1. Lipid mediated effects: proteins tend to separate out into the fluid phase or cholesterol poor regions
2. Membrane protein associations
3. Association with extra-membranous proteins (peripheral proteins): e.g. cytoskeleton