The Membrane Bilayer, Main Biophsical Properties

Question 1

General functions of biological membranes

Answer 1

1. Continuous, highly selective permeability barrier
2. Control of the enclosed chemical environment
3. Communication
4. Recognition - signalling molecules
   i. adhesion proteins,
   ii. immune surveillance
5. Signal generation in response to stimuli (electrical, chemical)

Question 2

Membrane composition

Answer 2

Varies with source of membrane
Generally membranes contain approximately (dry weight)
-40% lipid
-60% protein
1-10% carbs
NB. Membranes are hydrated structures, thus, 20 % of total weight is water

Question 3

Membrane lipids

Answer 3

They are re amphipathic molecules - they contain both hydrophobic and hydrophilic properties

Phospholipid is main membrane lipid - phosphate head is hydrophilic, fatty acid tail is hydrophobic

Head groups
– range of polar head groups
– e.g. choline, amines, amino acids, sugars

Fatty acid chains:
– Length between C14 and C24
– C16 and C18 most prevalent
– Cis double bond introduces a kink

Question 4

Glycolipids

Answer 4

Similar to a phospholipid - just repack the phosphocreatine moiety (head ) with a sugar

Leads to cerebrosides and ganglioside
Cerebrosides = head group with only a single sugar - sugar monomer
Gangliosides = head group with multiple sugars - oligosaccharides

Question 5

Lipid micelle and bilayer

Answer 5

A bilayer is 2 layers of phospholipids coming together to form a membrane - each phospholipid has 2 tails

A micelle is a single layer of phospholipids - that form a 3D spherical shape as a membrane with the head groups forming the outside and the hydrophobic tails in the middle

Question 6

Phospholipid motion

Answer 6

In the phospholipid bilayer - the lipids can undergo some form of movement

They can rotate in place
They can flip flop to the top from the bottom side of the phospholipid bilayer
They can diffuse across the membrane in the same plane
They can flex their fatty acid tails in different directions

Question 7

Influence of cis double bonds in bilayer structures

Answer 7

In straight chained tails there are only saturated straight hydrocarbon chains

In bent chained tails there are also unsaturated hydrocarbons chains with cis bonds - which reduce phospholipid packing

Question 8

Cholesterol

Answer 8

Has a polar head group with a rigid planar steroid ring structure and a non polar hydrocarbon tail

Cholesterol is also present in phospholipid bilayer

They are inserted by binding via a hydrogen bond (OH—O) to the O present in the ketone group before the fatty acid chains

Cholesterol can both increase and decrease fluidity due to their variance in structure and at different temperatures

Due to the presence of the singular flexible non-polar hydrocarbon tail they reduce phospholipid packaging therefore increasing fluidity - acts to stabilise membrane and raises its melting point

However due to the rigid planar steroid ring structure it reduces the phospholipid chain motion therefore reducing fluidity - at low temperatures it intercalates between the phospholipids and prevents them from clustering together and stiffening