2 - The Membrane Bilayer

Question 1

What are the general functions of biological membranes?

Answer 1

1. Highly selective permeability barrier
2. Control of enclosed environment
3. Communication
4. Recognition of signalling molecules
   > adhesion proteins
   > immune surveillance
5. Signal generation in response to stimuli

Question 2

Give examples of more specific functions for specific regions of plasma membrane.

Answer 2

• Adhesion to basement membrane or adjacent cells
• Absorption
• Secretion
• Transport
• Electrical conduction

Question 3

What is the approximate composition of the membrane (dry weight)?

Answer 3

40% lipid
60% protein
1-10% carbohydrate

Question 4

Membranes are hydrated, what % is water?

Answer 4

20%

Question 5

Membrane lipids have hydrophilic and hydrophobic parts, they are said to be ……………..

Answer 5

Amphipathic

Question 6

What is the structure of a phospholipid?

Name a phospholipid.

Answer 6

Two fatty acids bound to glycerol. Glycerol also binds a phosphate group with a head group attached.

Phosphatidylcholine - head group is choline

Question 7

The head group of a phospholipid is ……. and hydro………….

Give some examples of possible head groups.

Answer 7

Polar, hydrophilic

Choline, amino acids (e.g. serine), amines (e.g. ethanolamine), sugars (e.g. inositol)

Question 8

Fatty acid chains in phospholipids range in length from …….. to ………

What lengths are most prevalent?
What type of bond would introduce a kink in the chain?

Answer 8

Varies from C14 to C24

C16 and C18 are most prevalent (gives constant thickness)

A cis double bond would introduce a kink

Question 9

A sphingolipid has no glycerol backbone.

What is a sphingolipid and what does it do?

Answer 9

Contains a head group, phosphate and fatty acid chain

Sphingolipids stabilise the outer leaflet of the plasma membrane. They can form lipid rafts to organise membrane proteins in signal transduction.

Question 10

What is a glycolipid?

What is the difference between a cerebroside and ganglioside?

Answer 10

Like a phospholipid but with a sugar instead of the head group.

Cerebroside - head group is a sugar monomer
Ganglioside - head group is oligosaccharide

Question 11

What is a lipid micelle?

Answer 11

The hydrophobic tails of phospholipids cluster to exclude water, creating a spherical micelle with the hydrophilic heads on the outside.

Question 12

What is the difference between a micelle and a liposome?

Answer 12

A micelle is a single layer of phospholipids (hydrophilic heads outside and hydrophobic tails inside)

A liposome is a double-layer of phospholipids creating a spherical bilayer for transport of substances

Question 13

What are the forms of phospholipid motion?

Answer 13

• Flexion
• Rotation
• Lateral diffusion
• Flip-flop (very rare)

Question 14

Mechanisms exist to keep phospholipid distribution ………….

Certain phospholipids prefer to be on either the ……. or ………. leaflet.

Answer 14

Constant

Outer or inner

Question 15

What effect do cis double bonds in the fatty acid chains have on the bilayer?

Answer 15

Cis double bonds introduce kinks in the fatty acid chains (unsaturated). This reduces phospholipid packing. The phospholipids can move around more easily.

Question 16

What are the main parts of the cholesterol molecule?

Answer 16

Polar head group
Rigid planar steroid ring structure
Non-polar hydrocarbon tail

Molecule is amphipathic

Question 17

Where does cholesterol sit in the lipid bilayer?

Answer 17

At the same position as the fatty acid chains of the phospholipids

OH group of cholesterol head H bonds to the carboxyl group of the fatty acid

Question 18

What effect does cholesterol have on the membrane bilayer?

Answer 18

Paradoxical effects

Rigid sterol ring reduces phospholipid motion - reduced fluidity

Cholesterol between phospholipids reduces packing - increases fluidity

Question 19

How do membrane proteins become correctly oriented (i.e. N and C termini on the correct side)?

Answer 19

The N terminal will fold into the membrane keeping terminal positive residues on the cytoplasmic side.

If a signal peptidase is present it will cleave off the signal peptide (containing the N terminus) and the new N terminal will proceed into the lumen of the ER.