Lecture 3 Biological Membranes

Question 1

what are the functions of biological membranes

Answer 1

1. a continuous highly selective permeability barrier
2. control enclosed chemical enviroment
3. recognition - signalling molecules and receptors
4. signal generation in response to stimuli
5. communication

Question 2

what is the composition of a membrane?

Answer 2

60 % proteins, 40% lipid membrane, 1-10% carbohydrates - water forms 20% of membrane weight
Different regions of the membrane will have different functions - ie neural synapses, tight junctions on side/attach to basement membrane

Question 3

Membrane lipids are amphiphathic molecules, what does this mean? what is the main membrane lipid and its srtucutre? Draw if you can .

Answer 3

have hydrophobic and hydrophillic regions.

a phospholipid - a glycerol backbone with fatty acids attached (hydrophobic) and a phosphate head group(hydrophillic).

Question 4

what are possible variations in the phospholipid?

Answer 4

Head Groups can have amines, choline, amino acids or sugars attached.
Head groups are generally small.
the fatty acid chain length can vary c14-c24
c16 and c 18 most common
cis double bond gives a kink in the acid

Question 5

what are the types of glycolipid? and what is glycolipid structure

Answer 5

cerebroside - which as a sugar monomer attached
gangloside - will have sugar multimer attached (many sugars)
Glycolipds are based on sphingomyelin, they have a fatty acid tail, a carbon chain and then the sugars.

Question 6

What are the 4 rotations possible in the phospholipid bi layer, and how difficult are they.

Answer 6

Flexion, Rotation - low energy
Lateral Diffusion - medium energy
Flip Flop - High energy and hence are rare in actuality

Question 7

How do cis double bonds influence bi layer structure?

Answer 7

they reduce phospholipid packing, producing a more dynamic, fluid, less crystalline environment.

Question 8

Explain the role of cholesterol in the bi layer, and its effect on temperature.

Answer 8

humans insert cholesterol into the lipid bi layer.
it is amphipathic aliphatic (ring based) molecule. it is very rigid due to its ring structure, with a polar head group. Cholesterol lowers the phase transition energy (crystalline to fluid) as its concentration is increased in the membrane.

it bonds via H bonds to the phosopholipds -
this restricts phospholipid chain motion, reducing fluidity
it also reduces phospholipid packing which increases fluidity
So at high temps it stabilises by reducing fluidity and at low temps it reduces packing, keeps movement and increases fluidity. Keeps the systems fluidity stable