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Which molecules MAY NOT pass through biological membranes unaided?

Large uncharged polar molecules and ionic molecules
Eg Ions, glucose, sucrose.


Distinguish between facilitated diffusion, passive diffusion and active transport.

- depends on membrane permeability and concentration gradient

- 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


Describe the direction of flow of the 4 main ions

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


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

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.


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

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.


Briefly describe the characteristics of the PMCA

-pumps Ca2+ OUT
-high affinity
-low capacity
-binds Ca2+ at low concentrations


Briefly describe the characteristics of the NCX.
What does NCX even stand for?

NCX = Sodium - Calcium - Exchanger
-anti porter
-low affinity
-high capacity
-works well in high [Ca2+]
-sodium in and calcium out


Briefly describe the characteristics of the NHX

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.


What type of transporter is the Na/K ATPase?

Co transporter.
Moves ions in opposite directions.


Compare and contrast the SERCA and PMCA

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

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


Describe the control of cellular pH

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.


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

Na/K ATPase.


Which are the osmotically active ions?



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

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


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

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


Which molecules may move across biological membranes unaided?

Hydrophobic and small uncharged polar molecules
Eg - O2, H20, CO2, glycerol, N2