M&R 1.1 The Membrane Bilayer

Question 1

Name 5 general functions of biological membranes

Answer 1

1. Selective permeability barrier
2. Control enclosed chemical environment
3. Communication (of info between cells and environment)
4. Recognition (signalling molecules, adhesion proteins etc..)
5. Signal generation in response to stimuli (APs, 2nd messengers)

Question 2

How much of total membrane weight is water?

Answer 2

20%

membranes are hydrated structures - water needed to make hydrophilic interactions

Question 3

What is the composition of membrane DRY weight?

Answer 3

Protein ~60%
Lipid ~40%
Carbohydrate ~1-10%

Question 4

All membrane lipids are ___________

Answer 4

amphipathic (have both a a hydrophilic and a hydrophobic moiety)

Question 5

Name 3 main types of membrane lipid

Answer 5

Phospholipids
Sphingolipids
Glycolipids

Question 6

Describe the structure of a phospholipid

Answer 6

Glycerol backbone with 2 fatty acid chains (hydrophobic) and a phosphate head group (hydrophilic)

Question 7

How are phospholipids named?

Answer 7

After their head groups (e.g. if the phosphate group has choline attached, the phospholipid is called phosphatidycholine)

Question 8

Describe the fatty acid chains in a phospholipid

Answer 8

Most often C16 or C18 in length (can range C14-24)

One chain has a cis double bond which introduces a kink

Question 9

What is the advantage of FA chains generally being of similar lengths?

Answer 9

It means that the thickness of the bilayer is always about the same

Question 10

What effect does the cis double bond (on one of the fatty acids of phospholipids) have?

Answer 10

It reduces phospholipid packing, and therefore increases membrane fluidity

Question 11

Name a sphingolipid

Answer 11

Sphingomyelin

Question 12

Describe the structure of sphingomyelin

Answer 12

Sphingosine backbone
1 Fatty acid chain (hydrophobic)
Phosphocholine head group (hydrophilic)

Question 13

Why is sphingomyelin important?

Answer 13

Its composition allows a role in signalling pathways - degradation and synthesis of sphingomyelin produces important 2nd messengers
It is one of the few membrane lipids not synthesised from glycerol

Question 14

Where is sphingomyelin prominently found?

Answer 14

In myelin 
(therefore disorders of sphingolipid metabolism predominantly affect neural tissue)

Question 15

Describe the structure of a glycolipid

Answer 15

Backbone - usually sphingosine or glycerol
2 FA tails
Head group is a sugar

Question 16

What is the difference between cerebrosides and gangliosides?

Answer 16

Cerebrosides have a sugar monomer head group (usually galactose in neural tissue and glucose elsewhere)

Gangliosides have a sugar oligosaccharide head group (and are predominantly found in neural tissue)

[Both have a sphingosine backbone and FA tails]

Question 17

Is the composition of a lipid membrane the same in every cell?

Answer 17

No, the distribution of different membrane lipids varies depending on cell type

Question 18

Amphipathic molecules in water can form either _________ or _________ in water

Answer 18

Micelles

Bilayers

Question 19

Describe the forces involved in bilayer formation in water

Answer 19

Spontaneous formation driven by attractive VDW forces between hydrophobic tails
Structure stabilised by non-covalent forces (electrostatic and H-bonding) between hydrophilic moieties, and between hydrophilic moieties & water

Question 20

Describe the 4 permitted modes of mobility for lipids in a bilayer

Answer 20

1. Flexion (moving their aliphatic tails around)
2. Rotation (fast axial rotation of a single lipid molecule)
3. Lateral diffusion (lipid molecules exchanging place with each other within the same lamella)
4. Flip-flop (exchange of a lipid from one lamella of the bilayer to another. Rare because not thermodynamically favourable)

Question 21

Describe the structure of cholesterol

Answer 21

Polar head group
Rigid planar steroid ring structure (can’t twist it)
Mostly single bonds
Non-polar hydrocarbon tail

Question 22

Can the body produce all the cholesterol it needs?

Answer 22

No - the body can only make ~50% of the cholesterol it needs, and the rest must be obtained from the diet

Question 23

In general, how does cholesterol interact with phospholipids?

Answer 23

It is inserted in the bilayer, and H-bonds to every other phospholipid

Question 24

Describe the paradoxical effects of cholesterol on membrane fluidity. How is this helpful?

Answer 24

1. Its rigid sterol ring reduces phospholipid chain motion and therefore reduces fluidity
2. Its presence in the membrane reduces packing between phospholipids and therefore increases fluidity

This allows cholesterol to buffer any changes and stop the membrane becoming either too fluid or too solid