Membrane Bi-layer

Question 0

What is the composition of the membrane?

Answer 0

40% lipid
60% protein
1-10% carbohydrate

Question 1

Give some of the functions of the biological membrane

Answer 1

• Continuous, highly selective permeability barrier
• Allows control of the enclosed chemical environment
• Communication
• Recognition of signalling molecules, immune surveillance and adhesion proteins
• Signal generation in response to stimuli (electrical or chemical)

Question 2

What does it mean if a molecule is amphipathic?

Answer 2

It has hydrophilic and hydrophobic regions

Question 3

How are phospholipids named?

Answer 3

Phosphatidyl-X

Where ‘x’ is whatever the head group is

Question 4

What type of bond introduces a kink in the fatty acid chain of the phospholipid?

Answer 4

A cis bond

Question 5

How is a glycolipids formed?

Answer 5

When a phosphocholine moiety is replaced with a sugar on a phospholipid

Question 6

What is the difference between cerebrosides and gangliosides?

Answer 6

They are both types of glycolipid
Cerebroside - head group is a sugar monomer
Ganglioside - head group is an oligosaccharide

Question 7

How is the lipid bilayer formed?

Answer 7

It is spontaneous in water

Driven by Van der Walls attractive forced between hydrophobic tails

Question 8

What forces is the bilayer stabilised by?

Answer 8

Electrostatic and hydrogen bonding between hydrophilic moieties
Interactions between hydrophilic groups and water

Question 9

What percentage does cholesterol make up of the total membrane lipid?

Answer 9

45%

Question 10

What is the structure of cholesterol?

Answer 10

A polar head group
A rigid, planar steroid ring structure
A non-polar, hydrocarbon tail

Question 11

What effects does cholesterol have on the bilayer?

Answer 11

It reduces phospholipid packing, increasing fluidity

It reduces phospholipid chain motion, reducing fluidity

Question 12

What could form a head group in a phospholipid?

Answer 12

Choline, amine, amino acid, sugars

Question 13

What movements can phospholipids do?

Answer 13

Lateral diffusion
Flexion
Rotation
Flip flop

Question 14

What effect do cis double bonds have on the bilayer?

Answer 14

Reduce packing

Question 15

What functional evidence is there for proteins in the bilayer?

Answer 15

They carry out facilitated diffusion, produce ion gradients and there is specificity to cell responses

Question 16

What biochemical evidence is there for proteins in the membrane?

Answer 16

Freeze fracture

Membrane fractionation and gel electrophoresis

Question 17

What movements can proteins carry out?

Answer 17

Conformational change
Rotation
Lateral diffusion

Question 18

What movement can proteins NOT carry out?

Answer 18

Flip flop

Question 19

How is membrane protein mobility restricted by other proteins?

Answer 19

Aggregates with other proteins

Tethered to basement membrane or internal structures eg cytoskeleton

Question 20

What are the lipid mediated effects in protein mobility restriction?

Answer 20

Most proteins tend to go to cholesterol-poor regions in the membrane

Question 21

How do peripheral membrane proteins associate with the bilayer?

Answer 21

Electrostatic and hydrogen bond interactions

Question 22

How can peripheral proteins be removed from the bilayer?

Answer 22

Change to pH

Question 23

How do integral membrane proteins interact with the bilayer?

Answer 23

Interact with hydrophobic regions

Question 24

How are integral proteins removed?

Answer 24

Need detergents or organic solvents that compete for the non-polar interactions in the bilayer

Question 25

Give an example of when asymmetrical protein orientation is important

Answer 25

Receptor for a hydrophilic extracellular molecule such as insulin needs to have its recognition site facing towards the extracellular space for it to function.

Question 26

Give the structure of spectrin

Answer 26

α and β subunits wind together to form an anti-parallel heterodimer.
Two heterodimers form a head-to-head association to form a heterotetramer of α2β2.

Question 27

Give properties of spectrin

Answer 27

Long and floppy

Question 28

Describe how actin and spectrin are in the cytoskeleton

Answer 28

Rods of spectrin are cross-linked by short, actin profilaments.
Band 4.1 and adducin molecules form interactions towards the ends of the spectrin rods.

Question 29

How is the actin-spectrin network attached to the membrane?

Answer 29

Through adapter proteins:
Ankyrin (band 4.9)
Band 4.1

Question 30

Why is the erythrocyte cytoskeleton important?

Answer 30

Allows erythrocytes to pass through very narrow capillaries without lysis.

Question 31

How does dominant hereditary spherocytosis lead to haemolytic anaemia?

Answer 31

Spectrin levels are depleted by 40-50%.
Cells are more round and become less resistance to lysis.
Cleared by the spleen so shortened life
Bone marrow is unable to compensate enough for shortened life span -> haemolytic anaemia.

Question 32

What type of backbone are phospholipids based on?

Answer 32

Glycerol

Question 33

Which carbon atoms of glycerol are fatty acid side chains attached to?

Answer 33

C1 and C2

Question 34

Where are phospholipids synthesised?

Answer 34

In the ER

Question 35

What are two common carbon chain lengthens in the fatty acid side chains of phospholipids?

Answer 35

C16 and C18

Question 36

Which bonds can peripheral membrane proteins form with the membrane?

Answer 36

Electrostatic interactions
Hydrogen bonds
Disulphide bonds

Question 37

Name some peripheral membrane proteins of the erythrocyte cytoskeleton

Answer 37

Spectrin
Actin
Adducin
Band 4.1 
Ankyrin

Question 38

Name two integral membrane proteins of the erythrocyte skeleton

Answer 38

Band 3 - anion exchanger

Glycophorin A

Question 39

What length are membrane spanning domains of membrane proteins normally?

Answer 39

Between 18 and 22

Question 40

Which secondary structure are membrane spanning domains normally?

Answer 40

Alpha-helix