Lecture 1.1

Question 0

What is the composition of the dry mass of a lipid membrane?

Answer 0

40% lipid
60% protein
1-10% carbohydrates

Question 1

State five functions of biological membranes

Answer 1

1. Selective permeability barrier
2. Allows control of enclosed environment
3. Communication - allows information to flow between cells and their external environment
4. Recognition - cell signalling, immune surveillance, adhesion proteins
5. Signal generation in response to stimuli

N.B. Plasma membrane have all of the above functions - functions of organelle membranes may vary e.g. Mitochondrial membrane main function - ox phos to produce ATP

Question 2

How much of the mass of a lipid membrane is water?

Answer 2

Roughly 20%

Question 3

What type of molecules are the lipids that make up a membrane?

Answer 3

Amphiphatic molecules - contain both hydrophobic and hydrophilic regions. Their distribution varies on cell types.

Question 4

What is the predominant type of lipid in a membrane?

Answer 4

Phospholipid

Question 5

What is the structure of a phospholipid?

Answer 5

Polar head group e.g. choline, amines, amino acids, sugars
Fatty acid tail - C16 and C18 are most prevalent. Unsaturated fatty acids can produce a kink in the cis conformation - reduces phospholipid packing

Question 6

In what conformation can a fatty acid tail of a lipid produce a kink? What is the result of the kink?

Answer 6

Cis conformation.

Reduces phospholipid packing

Question 7

What is a sphingomyelin?

Answer 7

The only phospholipid not based on glycerol

Question 8

How do you get glycolipid from a phospholipid?

Answer 8

Replace phosphate with polar head group attached, with a sugar moiety

Question 9

What are the two types of glycolipids and what’s the difference between them?

Answer 9

Cerebrosides’ head group is a sugar monomer

Gangliosides’ head group is an oligosaccharide i.e. sugar multimers

Question 10

What is cholesterol?

Answer 10

A plasma membrane lipid that accounts for 45% of the total membrane lipids.

Note: different lipids exist in different numbers in different membranes - all down to membrane function

Question 11

What two structures can an amphipathic molecule form in water and which is the preferred one?

Answer 11

Micelles and bilayers. Bilayers are preferred for phospholipids and glycolipids.

Question 12

For which two types of lipids is the preferred structure formed in water a bilayer?

Answer 12

Phospholipids and glycolipids

Question 13

How does bilayer formation occur?

Answer 13

Spontaneously due to the attractive VDWs forces between the hydrophobic tails

Question 14

How are bilayers stabilised?

Answer 14

By non covalent forces such as hydrogen bonds and electrostatic attractive forces between hydrophilic moieties and between hydrophilic head groups and water.

Question 15

What is a characteristic of pure lipid bilayers?

Answer 15

Very poor permeability to ions and polar molecules

Question 16

How are membranes fluid structures?

Answer 16

Lipids can move in membranes in 4 ways;

1. Flip flop
2. Lateral diffusion
3. Fast axial rotation
4. Intra chain motion - kink formation in cis conformation of unsaturated fats

Question 17

How do unsaturated bonds reduce lipid fluidity?

Answer 17

Introduction of kink disrupts the hexagonal packing of phospholipids and increase membrane fluidity

Question 18

What functions do proteins serve in a membrane?

Answer 18

Transporters, enzymes, receptors, ion channels, pumps, energy transducers.