Lecture 2: Membrane structure and function 2

Question 1

Learning Objectives

Answer 1

• Appreciate some of the key properties of membranes
• Understand the movement of lipid molecules within the membrane
• Gain insight into the organisation of membranes
• Appreciate the importance of asymmetry of the lipid content of membranes

Question 2

What is the lipid bilayer permeable to?

Answer 2

Non-polar molecules (energetically favourable for non-polar molecules to leave aqueous environment and enter lipid membrane)

Question 3

How think are membranes?

Answer 3

5-8 nm (50-80 A)

Question 4

What is the fluid-mosaic model?

Answer 4

This model postulates that individual lipid molecules are able to diffuse freely and rapidly within lipid bilayers.

Question 5

What is a liposome?

Answer 5

An artificial circular lipid membrane

Question 6

What arrangement do phospholipids form when many are artificially placed in water?

Answer 6

Multi-layered onion-like bilayers.

Question 7

What it the solution to the unhelpful formation of multi-layered bilayer structures when phospholipids are placed in water?

Answer 7

Sonication, which uses a high frequency sound wave to separate the multi-layers into individual liposomes.

Question 8

What is a black membrane?

Answer 8

A black membrane is a membrane which has artificially formed between a two beakers of water with a small hole in their shared wall. It appears black and is about 7 nm (70 A) thick. It is used in experiments to see the effect when the solutions on either side of the membrane are changed.

Question 9

How can lipids be labelled?

Answer 9

With GFP (green fluorescent protein - isolated from jellyfish) or gold particles.

Question 10

What is photobleaching and how it is used to investigate membranes?

Answer 10

Photobleaching investigates membrane fluidity. Green Fluorescent Protein is used to label the entire liposome and then a small patch is bleached white by a laser (the laser destroys GFP). The white patch then becomes green again as GFP-tagged phospholipids diffuse within the bilayer.

Question 11

How can semi-permeability be investigated using liposomes?

Answer 11

Prepare liposomes in a solution containing the substance of interest, then separate the liposomes and transfer into a solution lacking the substance. Then measure the rate of movement of substance into the external solution from inside the liposome. It is possible to see how easily substances can move through the membrane.

Question 12

Why is flip-flop of a phospholipid from one leaflet to the other?

Answer 12

Flip-flop of a phospholipid requires the polar, hydrophilic head passing through the highly non-polar, hydrophobic lipid centre of the membrane. This is highly energetically unfavourable and so very unlikely to happen.

Flipases are energy-dependent enzymes in the membrane which can move phospholipids from one leaflet to the other. However, spontaneous flip-flop is very rare.

Question 13

Describe the majority of the interactions between the lipid/protein molecules in membranes.

Answer 13

Non-covalent, fairly weak but numerous so overall effect is strong

Question 14

Why are some membranes more likely to undergo a phase transition than others?

Answer 14

Phase transition is the change of state of the membrane from fluid to solid. This destroys the functionality of the membrane and so must be prevented in organisms.

Membranes with double bonds and short tails are resistant to phase transition and have to be at a lower temperature before they will ‘freeze’. This is because the phospholipids cannot pack as tightly together.

Bacteria, which cannot control their own temperature, can change the composition of their membranes to make them more resistant to phase transition, which is important as their membranes need to be kept fluid to be functional.

Question 15

Which lipids are lipid rafts composed of?

Answer 15

Phosphatidylcholine, sphingomyelin and cholesterol

Question 16

How can we look at lipid raft distribution over a membrane?

Answer 16

Using atomic force microscopy. It works in a similar way to a record playing (needles goes into contours).

Question 17

Describe the properties of lipid rafts.

Answer 17

Lipid rafts are thicker than the rest of the membrane and are better accommodation for membrane proteins (proteins prefer to be on lipid raft than normal membrane).
Individual lipid rafts are temporary and dynamic.
GPI anchors.

Question 18

Comment on the symmetry of plasma membranes?

Answer 18

Plasma membranes are asymmetrical and this is very important to some functions. For example, blood can only clot when PS (Phosphatidylserine) moves from the inner leaflet to the outer leaflet of a platelet membrane. This allows the platelet to clot selectively when required and requires a flipase to flip the phospholipid (expensive process).

Another example: PS exposure can indicate a cell is designated for programmed cell death (apoptosis).

Another example: Protein kinase C is activated in response to extracellular signals. It binds to the intracellular side of the membrane where PS is most abundant (negatively charged PS head group is required for interaction).

Question 19

Explain how lipids can help to relay a signal into the cell.

Answer 19

The receptor molecule is an integral protein and the signal molecule binds to it, causing a conformational change (activating the receptor). The receptor can then bind to and activate Phosphoatidylinositide 3-kinase (PI 3-kinase), which binds to lipids in the inner membrane and phosphorylates them. The extra phosphate group creates a binding site for an intracellular signalling protein; starts signalling chain, ways of getting extracellular changes to intra-molecular signalling.

Question 20

Why is rapid movement of lipid molecules within a leaflet possible?

Answer 20

There are no covalent bonds between the lipid molecules.