Membranes and transport

Question 1

Are membranes self-repairing?

Answer 1

Yes, they allow breakage and fusion of different membranes.

Question 2

Describe the selective permeability trait of cell membranes?

Answer 2

Selectively permeable to retain compounds and ions.

This is to exclude harmful chemicals, maintain concentrations of reagents and allow electrochemical gradient formation.

Question 3

What is the other lipid aggregate that can form aside from normal?

Answer 3

Micelles

Micelles are spherical and a buried tail and exposed polar head,e.g fatty acid/detergent.

Vesicles

Formed when bilayers folds in on itself, spherical, no exposed hydrophobic region.

Question 4

What happened in the 70’s that affected fluid mosaic understanding?

Answer 4

Experiment demonstrates that the lipid is able to freely diffuse in the membrane.

Question 5

What are the 4 types of membrane proteins?

Answer 5

Polytopic
Monotopic
Amphitropic
Peripheral

Question 6

Describe the function of the polytopic membrane protein?

Answer 6

Integral membrane protein that interacts with both sides of the membrane several times (bitopic only once).

Question 7

Describe the function of the monotopic membrane protein?

Answer 7

Interact with only one leaflet

Question 8

Describe the function of the amphitropic membrane protein?

Answer 8

reversibly associate with both sides of the membrane so can be found in the cytoplasm as well

Question 9

Describe the function of the peripheral membrane protein?

Answer 9

Interact with integral membrane proteins or with the membrane directly through electrostatic and hydrogen bonds.

Question 10

Membranes are formed by what two things?

Answer 10

2 leaflets that form spontaneously in aqueous solution.

Question 11

Describe the asymmetrical distribution of lipids.

Answer 11

They have variable membrane fluidity/stability.

Question 12

How is the asymmetry of membranes maintained?

Answer 12

Energised membrane proteins.

Question 13

Membrane ______ with proteins.

Answer 13

purify

Question 14

Where does the flexibity of cell membranes come from?

Answer 14

• each lipid having a large degree of freedom of movement.
• Each lipid has free diffusion in the membrane (more or less)
• The non-covalent interactions of each lipid

Question 15

What are the two states of temperature dependent phase transitioning membranes?

Answer 15

Ordered state(gel phase)
- Headgroups are uniformly distributed
- Lipid tails are neatly packed
- Membrane nearly motionless

Disordered state(fluid phase)
- No regular organisation
- Everything’s moving.

Question 16

What are the associated issues for membranes when they are too rigid or too loose?

Answer 16

(Too rigid)
Membrane embedded enzymes can’t move
Transport across membrane stops

(Too loose)
Membrane loses integrity and becomes leaky
No ion gradient = no energy production

Question 17

How do bacteria adjust for fluctuations in temperature?

Answer 17

Bacteria modify their membrane composition in response to temperature.

Question 18

Give an example of a polytopic integral membrane protein?

Answer 18

Bacteriorhodopsin

Question 19

On average how many helices does an integral membrane protein have?

Answer 19

Helices of around 20-25 residues to span the bilayer.

Question 20

How can you predict the topology of the IMP?

Answer 20

Hydropathy profile analysis.

Question 21

What is the common trait for membrane-buried amino acids?

Answer 21

They are all hydrophobic.

Question 22

What are the common traits of integral membrane proteins?

Answer 22

• Charged residues face the solvent.
• Positive charges are found on the cytoplasmic side(positive inside rule).
• Aromatic belt, Tyr and Trp anchor the proteins.