What is the purpose of the sodium postssium pump?
I know how it works but my book fails to say what the point of it is.

Thanks!

It's creates a concentration gradient across the cell membrane so that it can be used to create a membrane potential in the cell (esecially in neurons.)

It also creates a membrane potential as 3 sodiums are pumped for every two potassiums.

The concentration gradient and the potential energy from this can be used a bit like charging a battery. Positive charge builds up outside of the cell.

Look up neurons and that should explain the main reasons for it.

Thanks so much...
Just another quick question... what's an example of a voltage gradient? Is an action potential (same as membrane potential?) a voltage gradient?

Thanks again for any help!

A voltage gradient is a difference in electral potential over the cell membrane.

An action potential is something a little bit different, have a read of this site, I think you will find it helpful

Nerve Impulses (http://www.biologymad.com/NervousSystem/nerveimpulses.htm)

The action potential is the change in the voltage gradient, so cannot be used as an example.

If I were asked to give an example of a voltage gradient I would quote the K, Ca and Na concentrations and say outside of the cell is more positive. I would then say the mV that you can record inside of the resting cell (~-70mV) and outside* (using a voltage clamp to measure it- talking about experimental procedures always gives you extra marks).

*I've seen a few of these use whatever your lecture notes say.

Definitions

Voltage Gradient
Sect. 1, Ch. 1: Definition of Voltage Gradient (http://www.lib.mcg.edu/edu/eshuphysio/program/section1/1ch1/s1ch1_12.htm)

Membrane Potential- what results form the voltage gradient
Membrane potentials (http://www.cvphysiology.com/Arrhythmias/A007.htm)

Okay thanks!

You've just saved part of my biology grade! :-)