M&R 1 - Membrane Bilayer

Question 0

What is the composition of the membrane?

Answer 0

40% lipid
60% protein
1-10% carbohydrate

Question 1

List the 5 general function of the membrane bilayer

Answer 1

Highly selective permeability barrier
Allows control of enclosed chemical environment
Communication - allows info travel between cells and their environment
Recognition - to signal molecules, adhesion proteins
Signal generation in response to stimuli

Question 2

What type of molecule is a phospholipid?

Answer 2

Amphipathic

Question 3

Describe some features of fatty acid chains?

Answer 3

C14 —> C24

Hydrophobic hydrocarbon chains

Question 4

How does a cis double bond affect membrane stability?

Answer 4

A cis double bond introduces a kink in the fatty acid chain.
This reduces the phospholipid packaging, increasing membrane fluidity

Question 5

How is a glycolipid formed?

Answer 5

The phosphocholine moiety from sphingomyelin is replaced with a sugar.

Question 6

What are the two types of glycolipid?

Answer 6

Cerebroside - single sugar as head group

Ganglioside - complex oligosaccharide as head group

Question 7

How are bilayers formed?

Answer 7

Spontaneous in water driven by VdW forces between hydrophobic tails.
Cooperative structure is stabilised by electrostatic and H bonds between hydrophilic moieties.

Question 8

What is sphingomyelin?

Answer 8

It is the only phospholipid not based on glycerol

Question 9

What are the 3 modes of motility of proteins?

Answer 9

Change in conformation
Rotational
Lateral diffusion

Question 10

What are the restraints on protein motility

Answer 10

Lipid mediated effects - separate out into fluid phase
Membrane protein associations
Associations with extra membranous proteins (eg cytoskeleton)

Question 11

Describe peripheral proteins

Answer 11

These are proteins that are attached to the surface of membranes by electrostatic and hydrogen bonds. These can be removed by changes in pH or ionic strength.

Question 12

Describe integral proteins

Answer 12

These interact with the hydrophobic region of the lipid bilayer.
Can’t be removed by changes in ionic strength or pH but require agents (detergents) these compete for the non polar interactions in the bilayer.

Question 13

Why is orientation of membrane proteins important?

Answer 13

Receptors for hydrophilic molecules must face extra cellular space.

Question 14

How is the erythrocyte membrane prepared for analysis of protein?

Answer 14

It undergoes osmotic haemolysis to release cytoplasmic content. Then undergo gel electrophoresis to separate.

Question 15

What does it mean when most if the proteins are released with high ionic strength medium?

Answer 15

This means the proteins are peripheral in the cytoplasmic face.

Question 16

What does it mean for the proteins that are dissociated from the RBCs membrane by detergents?

Answer 16

This means these are integral proteins.

Question 17

How do glycoproteins lock orientation of lipid bilayer?

Answer 17

These contain a covalently binded carbohydrate. This is highly hydrophilic which prevents flip flop rotation.
The carbohydrates can also be useful for cellular recognition to allow cells to form an immune response.

Question 18

What is the erythrocyte cytoskeleton made of?

Answer 18

A network of spectrin and actin molecules.

Question 19

What is spectrin?

Answer 19

A long floppy rod like molecule. a and B subunits wrap round each other forming an antiparallel heterodimer. Then two of these combine head to head to form a heterotetramer.

Question 20

How does the spectrin attach to the lipid bilayer?

Answer 20

The rods of spectrin are cross linked into networks by short actin profilaments. Adducin and band 4.1 molecule attaches to the end of the spectrin networks. The spectrin actin network is attached to the lipid bilayer via Ankyrin. This way the integral proteins attach to the cytoskeleton - preventing lateral movement of membrane proteins.

Question 21

Describe the pathophysiology of hereditary spherocytosis

Answer 21

Spectrin levels are depleted by 40-50%
The cells become round, meaning less resistance to lysis
More haemolysis - cleared by spleen
Haemolytic anaemia.

Question 22

Describe the pathophysiology of hereditary elliptocytosis

Answer 22

Defect in spectrin molecule meaning unable to form heterotetramers resulting in fragile elliptiod cells.

Question 23

If the positive residues are located on the c terminal

Which side of the membrane is the n terminal targeted for?

Answer 23

The lumen side of the membrane

Question 24

Explain what the stop transfer signal is

Answer 24

The region of highly hydrophobic primary peptide sequence followed by a charged amino acid. This allows the protein to span the entire hydrophobic section of bilayer.