The Membrane Bilayer:

Question 1

Gives 5 functions of biological membranes:

Answer 1

• Highly selective permeability barrier.
• Control of the enclosed chemical environment.
• Communication.
• Recognition of signalling molecules.
• Signal generation in response to stimuli.

Question 2

What is the general composition of membranes by dry weight?

Answer 2

40% lipid
60% protein
1-10% carbohydrate

(If hydrated - 20% water)

Question 3

What is the name give to a molecule which is both hydrophobic and hydrophilic?

Answer 3

Amphipathic molecules

Question 4

Describe the structure of a phospholipid?

Answer 4

Glycerol backbone, 2 fatty acid tails, a phosphate group and a head group.

Question 5

Describe the structure of phosphatidylcholine:

Answer 5

Glycerol backbone, 2 fatty acid chains, phosphate group and a CHOLINE head group.

Question 6

What is characteristically present in the fatty acid chains of a phospholipid?

Answer 6

A cis double bond that introduces a kink into the molecule

Question 7

What is the difference between a phospholipid and a sphingolipid?

Answer 7

In a sphingolipid the glycerol is replaced by a sphingolipid backbone.

Question 8

What is a glycolipid?

Answer 8

Same structure as a phospholipid but the phosphate group is replaced with a sugar.

Question 9

What are the two classes of glycolipids? Describe these.

Answer 9

Cerebrosies - head group sugar monomer (single monosaccharide head group).
Gangliosides - Head group oligosaccharide (sugar multimers).

Question 10

Describe the structure of a lipid micelle?

Answer 10

Hydrophobic tails and hydrophilic head groups in a circular arrangement.

Question 11

What are the 4 modes of phospholipid motion?

Answer 11

• flexion
• rotation
• lateral diffusion
• flip flop

Question 12

How does the presence of a non saturated phospholipid regulate fluidity of a membrane?

Answer 12

Cis double bond causes a kink in the molecule and so the phospholipid molecules are unable to pack together as tightly, so it increases membrane fluidity.

Question 13

What effect does adding cholesterol into the phospholipid bilayer have?

Answer 13

• Reduces phospholipid chain motion and so reduces fluidity.
• Reduces phospholipid packing and so increases the fluidity.

Question 14

What are lipid rafts composed of?

Answer 14

• Sphingolipids with saturated fatty acid chains
• Tightly intercalated cholesterol
• can include or exclude membrane proteins.

Question 15

How do lipid rafts affect signal transduction?

Answer 15

• Have scaffold proteins that are many proteins that have been brought together in the raft.
• allows better signal transduction - can enhance signalling.

Question 16

What modes of motion are permitted for a protein within the bilayer?

Answer 16

• conformational change
• rotational
• lateral
  (NO FLIP-FLOP)

Question 17

What are the restraints on membrane protein mobility?

Answer 17

• membrane protein associations
• association with extra membranous proteins (e.g. cytoskeleton).
• lipid mediated effects

Question 18

Describe the main differences between peripheral and integral membrane proteins:

Answer 18

• Peripheral are bound to the surface whereas integral proteins lie within the membrane.
• P proteins are removed by changes in PH or ionic strength, I proteins are not.
• Integral proteins interact with the hydrophobic domains of the lipid bilayer. Peripheral proteins have electrostatic and hydrogen bond interactions.

Question 19

What is the characteristic property of the R groups of amino acid resides in transmembrane polypeptides?

Answer 19

Largely hydrophobic.

Question 20

What does membrane protein topology refer to?

Answer 20

Orientation of the protein within the bilayer - all face in same direction.

Question 21

What is the evidence for membrane proteins?

Answer 21

Gel electrophoresis and freeze fracture.

Question 22

What are the main proteins present in the cytoskeleton of a red blood cell?

Answer 22

• Band 3
• Ankyrin (attached spectrin to band 3)
• Spectrin (forms a lattice structure)
• Band 4.1

Question 23

What shape are erythrocytes in hereditary spherocytosis?

Answer 23

• spherical not biconcave disc shaped.

- therefore less resistant to lysis.

Question 24

What shape are cells in hereditary eliiptocytosis?

Answer 24

• Fragile elliptoid shaped cells form as they are unable to form heterotetramers due to a defect in spectrin molecules.

Question 25

What part of a protein ‘locks’ it into the membrane during the formation of a transmembrane protein?

Answer 25

• Hydrophobic stop transfer sequence.

Question 26

Give an example of a sphingolipid:

Answer 26

Sphingomyelin - no glycerol backbone (brought together by a fatty acid chain and a sphingosine molecule).

Question 27

What is a glycolipid?

Answer 27

• classed as sphingolipids.
• sphingosine backbone
• no phosphate group - instead connected to a mono or oligosaccharide sugar group.

Question 28

How does cholesterol affect how the lipid bilayer reacts to temperature changes?

Answer 28

• Adding cholesterol not only increases fluidity but it also extends the temperature range over which it remains stable.
• it reduces excess fluidity at higher temperatures and increases membrane stability at lower temperatures.

Question 29

What are the effects of lipid rafts?

Answer 29

• reduce mobility compared to the rest of the membrane.

- host specific proteins

Question 30

What charge do the hydrophilic heads and the hydrophobic tails have in a phospholipid bilayer?

Answer 30

• hydrophilic heads = positive (due to phosphate group)

- hydrophobic tails = negative