The Membrane Bilayer

Question 1

What are 5 functions of the membrane bilayer?

Answer 1

1. Continuous highly selective permeable barrier
2. Control of enclosed chemical environment
3. Communication - control the flow of information between cells and their environment
4. Recognition: signalling molecules, adhesion proteins and immune surveillance
5. Signal generation in response to stimuli - electrical/chemical

Question 2

What is the composition of the membrane in terms of dry weight

Answer 2

40% lipid
60% protein
1-10% carbohydrate

Question 3

What is the purpose of the Cis double bond in membrane lipids

Answer 3

Introduces a kink that reduces phospholipid packing

Question 4

What is a cerebrosides and gangliosides

Answer 4

Cerebrosides: glycolipid with head group sugar monomer
Ganglioside: glycolipid with head group oligosaccharides

Question 5

What are the 4 phospholipid motions

Answer 5

1. Intra-chain motion
2. Fast axial rotation
3. Fast lateral diffusion
4. Flip-flop (rare) - requires energy

Question 6

What are the actions of cholesterol in the plasma membrane

Answer 6

Increases fluidity by reducing phospholipid packing

Decreases fluidity by reducing phospholipid chain motion (H-bonding between head ends)

Question 7

What is the motility of proteins in membranes

Answer 7

Conformational change
Rotational and lateral
Mobility restrained due to association with other proteins

Question 8

What is the biochemical evidence for proteins in the membrane

Answer 8

Membrane fractionation and gel electrophoresis

Freeze fracture

Question 9

What is the functional evidence for proteins in the membrane

Answer 9

Facilitated diffusion
Ion gradients
Specificity of cell responses

Question 10

How are peripheral membrane proteins bound to the surface

Answer 10

Electrostatic and hydrogen bonds

Question 11

How can peripheral proteins be removed from the membrane and what is this evidence for

Answer 11

Changes in pH or ionic

Shows that the proteins are not embedded

Question 12

How are integral proteins associated with the membrane

Answer 12

Interact with hydrophobic domains of the lipid bilayer

Question 13

How can integral proteins be removed from the membrane

Answer 13

They are not removed by changes in pH or ionic strength

Removed by agents that compete for non-polar interactions e.g. detergents

Question 14

What is the erythrocyte skeleton a network of

Answer 14

Spectrin and actin molecules

Question 15

What is the structure of spectrin

Answer 15

Rod-like with alpha and beta subunits wound together to form an anti-parallel heterotetrama alpha2beta2

Question 16

How are the spectrin molecules held onto the membrane?

Answer 16

By ankyrin, band 4.1, band 3 and glycophorin A

Question 17

What is hereditary spherocytosis and what happens

Answer 17

Disease where spectrin levels are decreased to 40-50%
Cells become more rounded and therefore are less resistant to lysis when passing through capillaries
Patients are easily fatiguable

Question 18

What is the treatment for hereditary spherocytosis

Answer 18

Blood transfusion

Question 19

What is hereditary elliptocytosis

Answer 19

Defect in spectrin which prevents heterotetramers from forming which causes ellipsoid shaped cells

Question 20

What recognises the signal sequence of a protein

Answer 20

Signal recognition particle (large protein/RNA complex)

Question 21

What happens when the signal sequence is bound to SRP

Answer 21

The ribosome locks the ribosome complex and prevent further protein synthesis in the cytoplasm

Question 22

What recognises the SRP on the ER

Answer 22

SRP receptor/docking protein

Question 23

What happens once the SRP is released from the signal sequence

Answer 23

The signal sequence interacts with the signal sequence receptor (SSR) within the protein translocater complex in the ER membrane

Question 24

How is passage of the protein through the membrane arrested

Answer 24

There is a stop transfer signal - highly hydrophobic primary sequence of 18-22 aa (transmembrane domain)

Question 25

Where are phospholipids synthesised?

Answer 25

Endoplasmic reticulum

Question 26

What are used as head groups on phospholipids

Answer 26

• Amino acids
• Choline
• Amines
• Sugars

Question 27

What are the two most common carbon atom chain lengths of phospholipids

Answer 27

16 and 18

Question 28

The presence of what decreases the ability of phospholipids to form 2D crystal structure

Answer 28

Double bonds in the fatty acid chains

Question 29

Which movement of phospholipids is the most thermodynamically unfavourable

Answer 29

Flip-flop

Question 30

What is the ratio of cholesterol to phospholipids?

Answer 30

1:1

Question 31

How does cholesterol bind to phospholipid in the membrane

Answer 31

Hydrogen bonding between the hydroxyl group of cholesterol and the double bonded oxygen of the ester in the phospholipid

Question 32

Which proteins are the peripheral membrane proteins in the erythrocyte membrane

Answer 32

• Spectrin
• Actin
• Adducin
• Band 4.1
• Ankyrin

Question 33

Which proteins are the integral membrane proteins in the erythrocyte membrane

Answer 33

• Band 3

- Glycophorin A

Question 34

How many amino acids are the membrane spanning domains of proteins

Answer 34

18-22

Question 35

Where are mitochondrial membrane proteins made

Answer 35

Cytoplasm

Question 36

What anchors the ribosomes to the endoplasmic reticulum

Answer 36

Ribophoryns

Question 37

What often follows the stop transfer sequence of membrane proteins on the C terminal side

Answer 37

1 or 2 basic residues

Question 38

What process needs to happen in order for membrane proteins to be targeted to the lysosomal membrane

Answer 38

Glycosylation