M&R Session 1 - The Membrane Bilayer

Question 1

Name four types of lipids found in cell membranes

Answer 1

1. Phospholipids (eg phosphatidylcholine)
2. Sphingomyelin (not based on glycerol)
3. Glycolipids (cerebrosides and gangliosides)
4. Cholesterol (40% of total membrane lipid)

Question 2

Two types of structures that amphipathic molecules form in water

Answer 2

Micelles or bilayers

Question 3

Four movements of lipid molecules in a bilayer

Answer 3

1. Intra-chain motion - kink forming
2. Fast axial rotation - spinning
3. Lateral diffusion - same plane as bilayer
4. Transverse diffusion - flip-flop

Question 4

How does cholesterol regulate membrane fluidity at extreme temperatures?

Answer 4

High temperature - decreases fluidity due to hydrogen bonding between its -OH group and the phospholipids

Low temperature - increases fluidity by reducing packing of phospholipids

Question 5

Name 5 important functions of biological membranes

Answer 5

1. Continuous, highly selectively permeable barrier
2. Control of enclosed chemical environment
3. Communication - control of flow of information between cells and their environment
4. Recognition: signalling molecules, adhesion, immune surveillance
5. Signal generation (electrical, chemical) in response to stimuli
   (6. Energy conservation in mitochondria)

Question 6

What are the two types of membrane protein (hint: differ in how they stick with the bilayer)

Answer 6

Peripheral membrane proteins

Integral membrane proteins

Question 7

How can peripheral membrane proteins be removed?

Answer 7

Changes to pH or ionic strength

Question 8

How can integral membrane proteins be removed?

Answer 8

Agents (eg. detergents, organic solvents) that compete for the non-polar interactions in the bilayer

Question 9

Three ways in which membrane proteins can move.

Answer 9

1. Conformational change
2. Rotation (spinning)
3. Lateral diffusion.

NB: no “flip-flop”

Question 10

Why are membrane proteins unable to flip-flop?

Answer 10

Hydrophilic moieties that make up part of the membrane proteins mean that too much energy is required to diffuse them transversely

Question 11

Three things that put restraints on mobility of membrane proteins

Answer 11

1. Lipids - proteins tend to move away from cholesterol
2. Membrane protein associations (eg transmembrane domains)
3. Association with extramembranous proteins

Question 12

Which two molecules make up the erythrocyte cytoskeleton?

Answer 12

Actin and Spectrin

Question 13

Name two haemolytic anaemias caused by a deformed erythrocyte skeleton.

Answer 13

1. Hereditary Spherocytosis:
   - Autosomal Dominant
   - Spectrin depleted by 40-50%
   - Cells are ball shaped
   - Less resistance to lysis in spleen
2. Hereditary Ellipticytosis:
   - Autosomal Dominant
   - Spectrin 100% expressed but has deformity
   - Weakened lattice structure, cells turn rugby ball shaped.

Question 14

Describe the process by which transmembrane proteins are synthesised from mRNA and end up within a membrane.

Answer 14

1. mRNA sequence is “read” by a cytosolic ribosome and tRNA to synthesise a sequence of between 18-30 amino acids.
2. This sequence acts as a SIGNAL SEQUENCE, and is recognised by an SRP which then binds.
3. Translation is halted and the ribosome complex is moved to the RER.
4. At the membrane translation restarts, pushing the N-terminal through the docking protein pore in the RER.
5. When the transmembrane portion gets translated, its highly hydrophobic quality results in it being thermodynamically stable “within” the membrane, so it remain there. This sequence is the STOP TRANSFER SIGNAL.
6. As translation continues, the ribosome is pushed off the membrane.
7. Signal peptidase cleaves the signal sequence no translation stops.
8. The result is a protein with its N-terminal facing into the RER, which then once added to the cell membrane will have its N terminus facing out.