MR 2

Question 1

Which molecules MAY NOT pass through biological membranes unaided?

Answer 1

Large uncharged polar molecules and ionic molecules

Eg Ions, glucose, sucrose.

Question 2

Distinguish between facilitated diffusion, passive diffusion and active transport.

Answer 2

Passive
- depends on membrane permeability and concentration gradient

Facilitated
- certain proteins can ‘ping pong’ and increase the permeability of ions and polar molecules

Active transport

• allows the transport of ions/molecules AGAINST a concentration / electrical gradient
• requires energy - either directly or indirectly from ATP hydrolysis

Question 3

Describe the direction of flow of the 4 main ions

Answer 3

Na+ is higher outside cell so the flow is inwards
Ca2+ is higher outside the cell so the flow is inwards
Cl- is higher outside the cells so the flow is inwards

K+ is the only ion to have a higher concentration inside the cell so therefore the flow of K+ is OUTWARDS

Question 4

Explain what is meant by the term ‘uniporter’ and give an example of one such protein.

Answer 4

Uniporter is an example of an ATP dependent membrane transport protein.
It can only exchange one ion at a time.
An example is the PMCA transporter.

Question 5

Explain what is meant by the term ‘co transporter’ and give an example of one such protein.

Answer 5

This is a protein that can transport more than one type of molecule at a time, and they travel in opposite directions.
An example is the Na/K ATPase.
This transports 3 Na out, and 2 K in.
Made of 2 subunits:
- alpha does the transporting
- beta brings the protein to the surface.

Question 6

Briefly describe the characteristics of the PMCA

Answer 6

• uniporter
• pumps Ca2+ OUT
• high affinity
• low capacity
• binds Ca2+ at low concentrations

Question 7

Briefly describe the characteristics of the NCX.

What does NCX even stand for?

Answer 7

NCX = Sodium - Calcium - Exchanger

• anti porter
• low affinity
• high capacity
• works well in high [Ca2+]
• sodium in and calcium out

Question 8

Briefly describe the characteristics of the NHX

Answer 8

This is the Na/H exchanger
When H+ is too high (acidic) this exchanges H+ out and Na+ in.
The increase of [Na+]i is corrected by the Na/K ATPase.

Question 9

What type of transporter is the Na/K ATPase?

Answer 9

Co transporter.

Moves ions in opposite directions.

Question 10

Compare and contrast the SERCA and PMCA

Answer 10

Similarities:

• both high affinity
• both low capacity
• both involve in Ca2+ homeostasis
• both are examples of primary active transporters

Differences:

• PMCA pumps Ca2+ out the cell
• SERCA pumps Ca2+ into the sarcoplasmic/endoplasmic reticulum.

Question 11

Describe the control of cellular pH

Answer 11

NHE involved

• electro neutral as its 1:1 both positive.
• inhibited by amiloride. This causes increase in [Na]o so water is not reabsorbed. This occurs in the DCT of the kidney.

Question 12

What membrane transporter is responsible for setting up the high extracellular concentration of sodium?

Answer 12

Na/K ATPase.

Question 13

Which are the osmotically active ions?

Answer 13

Na
K
Cl

Question 14

Describe the flow of ions that could occur if a cell wants to resist swelling.

Answer 14

Swelling = higher fluid inside.
Want to prevent higher fluid insides
Efflux of osmotically ions so water follows
Therefore this resists cell swelling.

Question 15

Describe the flow of ions that could occur if a cell wants to resist shrinkage.

Answer 15

Shrinkage = less water 
The cell needs more water
Influx of osmotically active ions 
Water follows
Shrinkage aborted!

Question 16

Which molecules may move across biological membranes unaided?

Answer 16

Hydrophobic and small uncharged polar molecules

Eg - O2, H20, CO2, glycerol, N2