Thursday, October 21, 2010

The Sodium-Potassium Pump

When trying to think of a topic to start with, the Sodium-Potassium Pump came to mind almost immediately.  Maybe because I am getting ready to teach about the Action Potential in A&P I.  Perhaps because I am teaching next about the Urinary system in A&P II.  In any case, it seems like a good place to start, because is a single protein that affects so many functions and processes across a number of different systems.

 
Remember, as you are looking at textbook figures and viewing animations, that the chemical symbol for sodium ions is Na+, and the chemical symbol for potassium ions is K+.

 
The sodium-potassium pump is an active transporter.  It is a protein found in the membrane of many cells, including excitable cells like muscle fibers and neurons.  It moves 3 sodium out of the cell and 2 potassium into the cell, and requires the use (hydrolysis) of ATP for the energy to move those ions against their concentration gradients.

 
See animation here and here.  And of course, there is this video from YouTube.

 
The sodium-potassium pump creates a concentration gradient, or chemical gradient, for sodium, and one for potassium.  Moving 3 sodium out of the cell on every turn ends up creating a large concentration of sodium on the outside of the cell.  Moving 2 potassium into the cell on every turn ends up creating a large concentration of potassium on the inside of the cell.

 
In addition to creating these two concentration gradients, the sodium-potassium pump also creates an electrical gradient.  Moving three positive charges out of the cell, but only moving two positive charges into the cell, causes more positive charges to accumulate on the outside of the membrane.

 
Because of this, we often talk about the "electrochemical" gradients for sodium and potassium. 

 
So in a normal cell, such as a muscle cell, that has a sodium-potassium pump, you will find the following:
  • More sodium ions outside the cell
  • More potassium ions inside the cell
  • More positive charges outside the cell

 
And all of that is created by the sodium-potassium pump.

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