Session 1/2

Question 1

General function of Biological membranes

Answer 1

1) Continuous, highly selective permeability barrier
2) Control of the enclosed chemical environment
3) Communication
4) Recognition- signalling molecules
- Adhesion proteins
- Immune surveillance
5) Signal generation in response to stimuli (electrical, chemical)

Question 2

Membrane composition?

Answer 2

40% lipid
60% protein
1-10% carbohydrate
20% of total weight is water

Question 3

What types of lipid membranes are there?

Answer 3

Phospholipid
Glycolipids
Cholesterol

Question 4

In Phospoholipids what is it composed of?

Answer 4

• Polar head groups: Choline, serine, inositol, ethanolamine

- Fatty acid chain

Question 5

What is the similarity of glycolipids and phospholipids?

Answer 5

They both have fatty acid chains (Hydrophobic) and a hydrophilic head group

Question 6

In the lipid bilayer, how does the structure end up that way?

Answer 6

• Water makes H-bonds with head groups to stabilise membrane structure
• It needs water to drive tails together in order to stabilise head groups.
• This increases entropy

Question 7

How does a lipid micelle structure occur?

Answer 7

Hydrophobic groups are together.
Water molecules are minimised
Entropy is increased

Question 8

What does ampiphatic mean?

Answer 8

Head group is hydrophilic

Fatty acid chain is hydrophobic

Question 9

What is meant by a fluid membrane?

Answer 9

Proteins can move around within the membrane

Question 10

What does the kink in the fatty acid chain do?

Answer 10

The kink is caused by a cis double bond and it increases fluidity by decreasing the phospholipid packaging at a lower temp

Question 11

What is meant by the fluid mosaic model?

Answer 11

Structure of the plasma membrane as a mosaic of components with the phospholipid bilayer, cholesterol (sterols),proteins and carbohydrates that the membrane a fluid character.

Question 12

What types of proteins are there in the fluid mosaic model?

Answer 12

Peripheral proteins
Peripheral lipid linked proteins
Integral protein that goes through the protein with a hydrophobic helix (mainly due to hydrophobic amino acid residues) and outer hydrophilic part

Question 13

What is the paradoxical effects of cholesterol on the fluid mosaic model?

Answer 13

At lower temperatures:
Cholesterol horizontally reduces the phospholipid packaging between adjacent molecules and hence increases the fluidity
At increasing temps:
4 sterol rings reduces the phospholipid chain’s fatty acid motion, it extends membrane stability over the range of temp, reducing excess fluidity

Question 14

What are the proteins varying degrees of mobility within the membrane bilayer and what are they dependant on?

Answer 14

Conformational change- Receptor binding its ligand. Agonist causes a conformational change to signal inside of the cell
Flexion-Tails ‘wobble’ about the C-C single bonds
Rotation- Rotary motion around it’s position
Lateral diffusion- Vibration+ rotation
Flip-flop rotation- Where lipids flip between layers
Depend on Temp+ Molecular mass
Greater mass, higher melting point

Question 15

What primary physiological factors govern whether saturated membrane lipids exists in a gel or fluid state?

Answer 15

Chain length- longer fatty acid chains decreases the fluidity

Question 16

What are lipid rafts? What are the types?

Answer 16

Cholesterol complexes with glycosphingolipids.

• Makes the membrane less fluid.
• Scaffolding: proteins involved in signal transduction- makes transfer of info more efficient.
• Partitioning: changes their micro-environment, new interacting
• Raft clustering: amplify signalling by bringing signalling components together

Question 17

Hydration in maintenance of physiological membrane structure and function?

Answer 17

Bilayer head groups facing towards surrounding water and the tails inwards. Without water the cell membranes would lack structure and cells would be unable to keep important molecules inside the cell and harmful outside the cell

Question 18

Why are flip-flop movements rare?

Answer 18

Protein’s can’t flip-flop as they can’t overcome energy barrier due to extensive hydrophillic regions.

Question 19

Describe the composition of the erythrocyte skeleton and specify which are adapter proteins and which are integral membrane proteins

Answer 19

Spectrum and actin are anchored to the erythrocyte membrane via adaptor proteins called ankyrin and band 4.1 to integral membrane proteins called band 3 and glycophorin A.

Question 20

How are cytosilic and membrane proteins directed to the ER membrane? What enables insertion of the membrane protein?

Answer 20

Cytosilic and membrane proteins are synthesised by ribosomes. When the SRP recognises the leading sequence at the N terminal end, it halts protein synthesis on the ribosome. The SRP recognised by the signal sequence receptor on the ER membrane, which is part of the protein trans locator complex.

Question 21

How does water permeate a lipid bilayer?

Answer 21

Water moves via diffusion and osmosis despite the hydrophilic domain as it is small and uncharged.
Aquaporin channels are not ion channels but permit the rapid water diffusion above the rate of passive diffusion through the lipid bilayer.
Aquaporin has a hydrophillic pore.
Has positively charged residues in pore in order to main H+ ion gradients.

Question 22

What types of molecules can permeate a lipid bilayer?

Answer 22

Hydrophobic molecules- Oxygen, Carbon dioxide, nitrogen

Small uncharged polar molecules- Water, urea, flyer of.

Question 23

What are the free ion distributions across the cell membrane?

Answer 23

Sodium ions- 145mM outside, 12Mm inside
Chloride ions- 123mM outside, 4.2mM inside
Calcium ions- 1.5mM outside, times 10 to the minus number 7 inside
Potassium ions- 140mM outside, 4mM inside

Question 24

What are the roles of transport processes?

Answer 24

• Maintenance of ionic composition
• Maintenance of intracellular pH
• Regulation of cell volume
• Concentration of metabolic fuels and building blocks
• The extrusion of waste products of metabolism+ toxic substances
• The generation of ion gradients necessary for the electrical excitability of nerve and muscle

Question 25

What does passive transport depend on?

Answer 25

• Permeability
• Concentration gradient: increases linearly with increasing gradient. Goes down the conc gradient
• Gibbs free energy change is negative

Question 26

What does facilitated diffusion do and what are the modes?

Answer 26

It enhances the permeability
Ping-pong- gated pore
Ligand gated ion channels- opens upon ligand binding
Voltage-gated ion channels- Electrical signal causes a conformational change to the channel

Question 27

What is active transport?

Answer 27

Allows transport of ions/molecules against its electrochemical gradient. Requires energy directly/indirectly via ATP hydrolysis.
Gibbs free energy change is positive