ICPP 2 - The Membrane Bilayer

Question 1

What are the 3 main general functions of biological membranes in cells and organelles?

Answer 1

1) To form a continuous, highly selective permeability barrier which controls the internal environment
2) Communication - with outside environment and recognition of signalling molecules
3) Signal generation - in response to chemical and electrical stimuli.

NB - different regions of plasma membrane have different functions.

Question 2

What is the general composition of membranes?

Are all membranes the same?

Answer 2

• In general - 40% lipid, 60% protein, 1-10% carbohydrate. Remember it is a hydrated structure so 20% of the weight is water.
• No, membrane composition varies on the source of membrane.

Question 3

Membranes are amphipathic, what does this mean?

Answer 3

They contain both a hydrophilic, and hydrophobic moiety.

Question 4

What are the 3 main types of lipid present in plasma membranes?

Answer 4

1) Phospholipids
2) Glycolipids
3) Cholesterol

Question 5

Describe the general structure of phospholipids

Answer 5

Phospholipids have a 3-carbon glycerol backbone, with two fatty acids and a phosphate head attached.

• Can have a range of polar head groups and fatty acid chains (usually 16-18 C’s)
• A cis double bond introduces a kink into the fatty acid chain.

Question 6

Describe the general structure of glycolipids

What are the two main kinds of glycolipids?

Answer 6

Same as phospholipids but phosphocholine moiety replaced with sugar. Therefore they are sugar containing lipids.

1) Cerebrosides - head group is sugar monomer
2) Gangliosides - head group is sugar oligosaccharide

Question 7

What is a lipid micelle?

What do phospholipids form instead of micelles?

Answer 7

• Lipid micelles are spherical structures that lipids form in aqueous solutions, with hydrophilic heads facing outwards and hydrophobic tails facing inwards.
• Phospholipids cannot form micelles so instead form a bilayer in aqueous solutions, with hydrophilic heads facing out and hydrophobic tails inward (sea of lipids).

Question 8

Lipid bilayers are dynamic, what are the 4 forms of motion possible?

Answer 8

1) Flexion - the tails of lipids vibrate.
2) Rotation - individual lipids can rotate
3) Lateral diffusion - due to vibration and rotation, lipids can change places with each other
4) Flip Flop (rare!) - enough energy gained to get hydrophilic head group through the hydrophobic part and out the other side.

Question 9

Why is a cis double bond in the bilayer structure important?

Answer 9

A cis double bond reduces phospholipid packing (saturated hydrocarbons are much straighter) - this bond allows for conformational changes needed to bring about function.

Question 10

What is the structure of cholesterol?
Why does cholesterol have paradoxical effects on the membrane?
How does cholesterol contribute to membrane stability?

Answer 10

• Amphipathic molecules, ridig planar steroid ring with a polar head and non-polar hydrocarbon tail.
• Cholesterol reduces membrane fluidity due to reduced chain motion but also increases membrane fluidity by reducing phospholipid packing.
• Cholesterol stabilises a constant membrane environment by preventing it becoming too fluid or too crystalline

Question 11

Describe the structure of lipid rafts.

Answer 11

Lipid rafts are dynamic, cholesterol rich structures, composed of:

• Sphingolipids w/saturated fatty acids chains
• Tightly intercalated cholesterol

NB: proteins can move in and out of lipid rafts.

Question 12

What is the main function of lipid rafts?

Answer 12

They allow for scaffolding of proteins to be involved in signal transduction - such as receptors or signalling pathway components.