MR 1

Question 1

Which is the only phospholipid not based on glycerol?

Answer 1

Sphingomyelin

Question 2

Phospholipids have a double bond - what does this introduce into the chain?

Answer 2

A kink. This increases fluidity by decreasing packing.

Question 3

What two structures do lipids form in water?

Answer 3

Micelles or bilayers.

Question 4

Name the way lipids are can move around in the bilayer.

Answer 4

Fast axial rotation
Flip flop
Lateral diffusion within the plane of the bilayer.
Intra chain motion

Question 5

What methods give us evidence for proteins?

Answer 5

SDS Page
Gel electrophoresis
Freeze fracture

Question 6

Describe the mobility of proteins

Answer 6

Conformational change - eg open/close
Rotation
Lateral movement.

Question 7

Explain why proteins cannot flip flop

Answer 7

They have too much of a hydrophilic structure that would need the pass through the lipid layer - requires too much energy. Also, a change in proteins in the bilayer would be too disruptive.

Question 8

Distinguish between peripheral and integral proteins.

Answer 8

Peripheral

• not embedded
• can be removed via changes in the ionic and pH strength
• bound to the membrane via electrostatic forces

Integral

• can’t be removed via pH/ionic changes.
• needs a detergent that can compete for the non polar interactions
• they interact with the hydrophobic regions of the bilayer extensively

Question 9

Describe in general terms the mechanism of membrane insertion of integral proteins

Answer 9

• protein synthesis occurs on ribosome. A hydrophobic N-terminal signal sequence is produced, which is recognised by Signal Recognition Particle (SRPs). SRP binds, along with GTP.
• binding of GTP-SRP stops protein synthesis and directs the ribosome to the ER. As it binds, the SRP dissociates.
• protein synthesis resumes into the lumen of the ER, via pores which are known as protein translocating complex.
• At a certain point, a highly hydrophobic sequence is encountered by the ribosome
• this sequence is 18-20 AA long which spans the membrane. It effectively locks the protein into the membrane.
• the remainder of the protein is synthesised in the cytosol.
• this way, the part before the hydrophobic sequence is inside the membrane. The other side of the sequence is outside
• hence it spans the membrane.

Question 10

Discuss membrane asymmetrically

Answer 10

Asymmetrical orientation is important for function.

Eg the receptors for hydrophilic extracellular molecules must have their recognition site directed to the surface.

Question 11

Describe the influence of FAs on membrane fluidity.

Answer 11

Increase membrane fluidity due to the kink that is introduced into the chain, which reduces packing.

Decreases packing by reduced chain motion of phospholipids.

Question 12

Describe the influence of cholesterol on membrane fluidity.

Answer 12

Increases fluidity by reducing packing.
Decreases fluidity by decreasing phospholipid chain motion
Stabilises membrane by hydrogen bonding to fatty acids. (This abolishes the endothermic phase transition).

Question 13

Why is the membrane bilayer often referred to as a ‘fluid mosaic model’

Answer 13

Fluid - as the phospholipids are mobile hence fluid m

Mosaic - the bilayer constitutes many different components like a mosaic

Question 14

Name some constituents of the membrane bilayer

Answer 14

Phospholipids
Proteins
Cholesterol

Question 15

What does the membrane cytoskeleton consist of?
(Hint: think of RBCs)
Make references of these to integral proteins.

Answer 15

Composed of spectrin and actin - these are peripheral proteins.
Ionic wash of RBCs sees these form a cytoskeleton which is facing the cytosol.
Spectrin attached to Ankyrin and then to Band 3 (integral protein)
Actin attached to Glycophorin and then to Band 4.1 (integral protein)

Question 16

Integral proteins are attached to cytoskeleton via Ankyrin and Glycophorin. What does this attachment mean for their movement ?

Answer 16

Reduces their lateral movement within the bilayer.

Question 17

Describe what Hereditary spherocytosis is, and how it leads to haemolytic anaemia.

Answer 17

H.S is the reduction of spectrin (which is connects to Band 3 via Ankyrin, forming part of the cytoskeleton) by around 40-50%.

• this reduces the lifespan of the RBC
• RBC tend to ‘round up’ and are lysed by the sheering forces in capillaries.
• increase RBC lysis by spleen - bone marrow cannot compensate
• there is a deficit of RBC
• hence, haemolytic anaemia.

Question 18

What is the main difference between TAGs and phospholipids?

Answer 18

Phospholipids only have 2 FAs whereas TAGs have 3. In the 3rd space, there is a phosphate group instead.