M&R - Lipid Bilayer and Lipids/Proteins

Question 1

Do all regions of plasma membrane have the same function?

Answer 1

No
Dynamic environment, constantly changing
Functions include - secretion, transport, interaction with adjacent cells, interaction with basement membrane

Question 2

What is the composition of a membrane?

Answer 2

40% lipid
60% protein
1-10% carbohydrate

20% of total weight = water

Question 3

Membranes are described as amphipathic molecules, what does this mean?

Answer 3

Contain both hydrophobic and hydrophillic moiety

Question 4

What are the three predominant lipids found in a cell membrane?

Answer 4

Phospholipid (Small head groups - choline, amine, )
Sphingomyelin - doesn’t have glycerol backbone
Glycolipid - two types = cerebrosides and ganglioside

Question 5

What is the length of a fatty acid chain?

Which is more prevalent?

Answer 5

C14-C24

C16-C18

Question 6

Name some of the roles of membrane proteins

Answer 6

Functional = Enzymes
Transporters
Pumps
Ion channels 
Receptors
Energy transducers
Facilitated diffusion

Biochemical = membrane fractionation, freeze fracture

Question 7

What are the four ways lipids can mobilise in a cell membrane?

Answer 7

Flexion
Rotation
Lateral diffusion
Flip Flop - flip head group through hydrophobic domain to the other side

Question 8

What effect does the presence of a double bond in a fatty acid side chain have?

Answer 8

Introduces a kink into the chain that reduces the ability of the phospholipid to pack in crystalline array and so contributes to increased membrane fluidity

Question 9

What is the role of cholesterol in the lipid bilayer?

What effect does cholesterol have on the fluidity of the membrane bilayer?

Answer 9

Stabilises the structure

Cholesterol reduces phospholipid packing and maintains membrane in fluid phase = increasing fluidity

Rigid cholesterol ring structure held closely to fatty acid chains reduces intrachain vibrations = reducing fluidity

Question 10

What ways can a protein mobilise in a cell membrane?

Answer 10

Conformational change
Rotational
Lateral

NO FLIP FLOP - energetically unfavourable

Question 11

How are membrane proteins restricted in movement?

Answer 11

Lipid mediated effects - proteins tend to seperate into fluid phase or cholesterol poor region

Membrane protein associations

Association with extra membranous proteins e.g. Cytoskeleton

Question 12

What are the two types of membrane proteins and describe?

Answer 12

Peripheral proteins - bound to surface.
Electrostatic or hydrogen bond interactions.
Removed by changes in pH or ionic strength

Integral proteins
Interact with hydrophobic domains of bilayer
Removed by detergents/organic solvents which dissolves bilayer to release protein

Question 13

What is the length of amino acids that tend to span bilayer?

Answer 13

18-22 amino acids

Question 14

What arrangement are the transmembraneous domains?

Answer 14

Alpha helical

Question 15

What is the name of the plots used to determine if a membrane is membrane spanning?

Answer 15

Hydropathy plots

High hydrophobic score = membrane spanning

Question 16

What are the functions of a biological membrane? (5)

Answer 16

Highly selective permeability barrier
Control of enclosed chemical environment
Communication
Recognition - signalling molecule, adhesion proteins, immune surveillance
Signal generation in response to stimuli - electrical/chemical

Question 17

How many hydrophobic domains are there in bacteriorhodopsin?

Why is this molecule important?

Answer 17

7 domains

Bacteria that traps light. Can be grown in a dish to make lots of protein. Has the same structure as a lot of the receptors.

Question 18

What terminal of the protein faces in and which faces out?

Why is orientation important?

Answer 18

C terminal out
N terminal in cytosol

Important for function

Question 19

What are the integral proteins of the erythrocyte skeleton?

Answer 19

Glycophorin A

Band 3

Question 20

What are the peripheral membranes of the erythrocyte skeleton?

Answer 20

Actin
Ankyrin
Adducin
Band 4.1

Question 21

What are the two haemolytic anaemias?

Answer 21

Hereditary spherocytosis - Spectrin depleted by 40-50%, erythrocytes round up, less resistant to lysis, cleared by spleen = anaemia

Heridtary elliptocytosis - defect in spectrin molecule, unable to form stable heterotetramers, fragile elliptoid cells

Question 22

Describe the secreted protein biosynthesis

Answer 22

Signal sequence recognised by SRP
Binding of SRP prevents further protein synthesis
SRP recognised by receptor - directs SRP ribosome complex to protein translocator complex
SRP released
Protein synthesis continue
Translocates through until stop sequence reached
Protein released
Ribosome detaches from ER
Signal sequence cleaved by signal peptidase

Question 23

What anchors proteins in the bilayer?

Answer 23

Stop sequence in the middle of the protein

Ribosome continues to make protein, cannot force through bilayer so rest of protein made in cytoplasm

Question 24

What direction would the protein enter bilayer if there are more positive residues at N terminus?

Answer 24

C terminal section will pass into lumen

Question 25

What happens in the presence of N terminal signal sequence in absence of signal peptidase cleavage?

Answer 25

C terminal to ER lumen - C terminus flops in

Question 26

What happens when there are more positive residues at the C terminal end of the start transfer sequence?

Answer 26

N terminal translocated and in ER lumen

Protein orientation reversed
No signal peptidase