Membrane Structure and Function II

Question 1

What is the function of the plasma membrane, golgi, nuclear membranes, lysosomes and peroxisomes?

Answer 1

1. Plasma membrane - Barrier, transport, signal transduction
2. Golgi - Post-translational modification, processing for secretion
3. Nuclear membranes - Attachment of chromatin
4. Lysosomes - Hydrolytic enzymes
5. Peroxisomes - Fatty acid oxidation

Question 2

What is the function of the mitochondria inner + outer membrane?

Answer 2

Inner membrane - Energy transduction

Outer membrane - Barrier

Question 3

What is the function of the RER and SER

Answer 3

RER - Translation protein processing

SER - Synthesis of complex lipids

Question 4

Why do concentrations of proteins and lipids in membranes vary?

Answer 4

Different functions of membranes (Erythrocyte and hepatocyte -> More protein for more interactions in the blood)

Inner mitochondrial membrane - More protein needed (75%) needs to synthesise ATP

Question 5

Which molecules cannot pass through a membrane

Answer 5

1. Large uncharged or polar molecules e.g. glucose
2. Ions

Question 6

What two ways of passive diffusion can occur into a membrane

Answer 6

Nonmediated simple diffusion through the membrane and nonmediated diffusion through a channel protein

Question 7

What is the name for carrier proteins in carrier mediated transport

Answer 7

Uniporters

Question 8

What are the two systems of co-transport and state the difference

Answer 8

Symporter and antiporter

In the symporter, A and B molecules are transported in the same direction. It requires both molecules A and B to function

In the antiporter, A will move in one direction while B will move in the opposite direction

Question 9

Carrier mediated facilitative diffusion vs simple diffusion

Answer 9

Simple diffusion - Linear graph, constant rate of transport (straight line)

Carrier mediated facilitative diffusion - As concentration of solute increases, response gets saturated (plateaus) as carriers get filled up.

Both do not require energy.

Carrier mediated is specific. It is also faster than simple diffusion

Question 10

Which two glucose transporters are expressed on all mammalian tissues? What are their Kts and why is this important?

Answer 10

GLUT1 and GLUT3
Kt -> 1 millimolar
This corresponds to the circulating conc of glucose (resting) 4mM, so all tissues would have a supply of glucose at all concentrations under physiological conditions.

Question 11

What is Kt

Answer 11

Measure of affinity of the transporter for the molecule that it transports

Question 12

When glucose conc rises, which transporters are activated and what is their significance?

Answer 12

GLUT2 - Liver and pancreatic beta cells

Produce insulin, which stimulates the lowering of glucose conc. The liver takes up glucose to store as glycogen or as fatty acids.

Question 13

When glucose conc rises, which transporters are activated and what is their significance?

Answer 13

GLUT2 - Liver and pancreatic beta cells

Produce insulin, which stimulates the lowering of glucose conc. The liver takes up glucose to store as glycogen or as fatty acids.

Kt - 15-20 mM

Question 14

How do carrier mediated transporters function? (Using glucose as an example)

Answer 14

1. Glucose binds to transporter, then undergoes conformational changes.
2. Glucose passes across membrane, into cell
3. After glucose is released, transporter returns to its original conformation.
4. Continues until glucose conc gradient is diminished
5. To continue this, the cell also removes its own glucose inside by converting it into another molecule, e.g. glucose 6 phosphate.

Question 15

How to increase transport of glucose?

Answer 15

Increase the number of transporters on the cell surface to overcome saturation

Question 16

How is glucose transported into cells through SGLUT-1 and 2

Answer 16

From the gut into the bloodstream, when Na+ is present, glucose is transported into the cell through SGLUT-1 through co-transport, as sodium is in a higher conc outside of the cell. (symporter)

Question 17

How is a high concentration of sodium maintained outside of the cell?

Answer 17

Sodium removed from cell by sodium potassium ATPase (active transport)

Question 18

How to counter dehydration from diarrhoea

Answer 18

Re-hydration therapy - 1litre of water, 1 tsp of salt, 8tsp sugar. Sodium and glucose are absorbed by the gut, glucose then used as an energy source. Sodium will draw in water and rehydrate the individual.

Question 19

Describe the 3 features of compartmentalisation

Answer 19

1. Intracellular membranes create local environments and separate reactions
2. Enables electrochemical gradients to be established
3. Brings enzymes and reactants together and/or keep them apart

Question 20

Describe the compartmentalisation of the mitochondria

Answer 20

The mitochondria outer membrane is a barrier, but the inner membrane is to generate ATP. The inner membrane has more proteins to generate a H+ gradient and then ATP, as the gradient dissipates. The impermeability also allows it to form ATP

Question 21

Describe macromolecule degradation

Answer 21

Lysosome peroxisome - Proton pump to pump H+ ions into the cells, (requires hydrolysis of ATP) to maintain lower pH

Peroxisome contains many enzymes, pH similar to cytosol

Question 22

How do these proteins get to the correct compartment using protein signal sequences?

Answer 22

Small sequence (2-3 amino acids) directs particular protein to particular organelle. E.g. cytosol protein, add small nuclear localising sequence, protein then moves to nucleus.