The Membrane Bilayer

Question 1

Give 5 functions of biological membranes

Answer 1

Highly selective permeability barrier , control of the enclosed chemical environment, communication, recognition- signalling molecules, signal generation in response to stimuli

Question 2

What makes up the majority of the membrane?

Answer 2

Proteins

Question 3

Membrane lipids are amphipathic, what is mean by this term?

Answer 3

They have hydrophilic and hydrophobic ends

Question 4

What is a phospholipid made up of?

Answer 4

A hydrophilic phosphate head and a hydrophobic tail

Question 5

What are glycolipids?

Answer 5

Sugar containing lipids

Question 6

What are cerebrosides and gangliosides?

Answer 6

They are both glycolipids. Cerebrosides head group is a sugar monomer whereas gangliosides head group is an oligosaccharide

Question 7

What is it called when the lipid bilayer forms a ring?

Answer 7

A liposome

Question 8

What are the 4 ways a phospholipid can move?

Answer 8

Laterally (diffuse across same side of phospholipid bilayer), rotation around their own axis, flexion (tails wiggle) and flip flop (very rare phospholipids switch sides of the bilayer)

Question 9

What is the influence of the cis double bond in bilayer structure?

Answer 9

It makes a kink meaning the phospholipids cant pack as tightly together.

Question 10

Describe the structure of cholesterol

Answer 10

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

Question 11

How does cholesterol stabilise the membrane?

Answer 11

It reduces the effect of the external environment. At high temps it stabilises the membrane by raising its melting point and at low temps it intercalates between phospholipids and prevents them clustering together and stiffening

Question 12

What is the evidence for proteins in membranes?

Answer 12

Facilitated diffusion
Ion gradients
Specificity of cell responses

Question 13

What is an SDS page and how does it work?

Answer 13

It is a type of gel electrophoresis that separates proteins based on their Mr. The proteins are given a negative charge and the base of the gel is positively charged so the proteins move through the gel. The mesh only allows the smallest proteins to travel the furthest so the proteins separate out in the distance travelled along the gel by Mr

Question 14

How does freeze fracture work?

Answer 14

Freeze sample to immobilise cell constitutes.
Push sharp knife against the frozen section and it will fracture between bilayers as the hydrophobic attraction between the two tails is the weakest part. The structure can then be viewed using an electron microscope

Question 15

Biochemically how can we prove the presence of proteins in a membrane?

Answer 15

Cell fractionation and gel electrophoresis

Freeze fracture

Question 16

What are the 3 types of motion provided to proteins in the membrane?

Answer 16

Conformational change
Rotation
Lateral
NOT flip flop as proteins eg receptors must be on the surface

Question 17

What is the purpose of the cytoskeleton?

Answer 17

Maintains cell shape and protects integrity of membrane

Question 18

What causes haemolytic anaemia?

Answer 18

When the cytoskeleton doesn’t form properly.

Question 19

What is spherocytosis?

Answer 19

When spectrin is depleted , RBCs round up and are more likely to undergo lysis. Only treatment is regular blood transfusions

Question 20

What is hereditary Elliptocytosis?

Answer 20

When the spectrin molecule is defective and RBCs look like rugby balls.

Question 21

Describe the sequence of events that leads to a protein being secreted into the ER lumen

Answer 21

1. SRP (signal recognition protein) binds to signal sequence on the ribosome in order to stop protein synthesis.
2. Docking protein recognises SRP and delivers the ribosome to the signal sequence receptor.
3. Protein synthesis continues into the ER
4. The signal sequence is cut off by the enzyme signal peptidase
5. Protein excreted into ER
6. Ribosome moves back into the cytoplasm

Question 22

How are intrinsic proteins secreted?

Answer 22

The protein will have a hydrophobic region which prefers to stay in the membrane so stops there. The protein synthesis continues forming a loop. Signal peptidases cut the protein so the non hydrophobic region is excreted into the ER and the hydrophobic region stays in the membrane and goes on to form intrinsic proteins