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I've found several sources that state that overall, the cytosol of a cell is electrically neutral. The extracellular fluid is also purportedly electrically neutral. How can that be when we have membrane potentials?

Also in that regard, how do the ion transporters that set up the membrane potential know when to stop pumping ions to get a certain potential set up?

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Membrane potentials are caused by different concentrations of ions on opposite sides of membranes in micro-environments along the membrane. The membrane can be, overall, electrically and chemically neutral with small areas that have differences in electrochemical gradients.

For cells to produce these potentials they have proteins embedded in the lipid membrane. Ion pumps are the main players in creating potentials and gradients; through active transport (the use of ATP) the protein moves ions from one side to the other. When the concentration of that area of membrane is at one concentration the proteins are held in one conformation (active or inactive) and when the gradient is changed or meets a threshold the protein changes conformation and becomes activated or inactivated. So the actual electrochemical gradient dictates when the proteins are transporting.

There are also other proteins/messengers that are produced which can interact with the transmembrane proteins that keep them in one configuration or another to facilitate the production of an action potential. Once again, when this specific concentration is met the signaling molecule/transporter will become inactivate or activated.

This is just stuff I recall from some classes. Honestly, wikipedia is a fine general knowledge source. And more detailed than my explanation. http://en.wikipedia.org/wiki/Membrane_potential#Ions_and_the_forces_driving_their_motion

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Thanks for your answer! I didn't know about microenvironments and just assumed it meant the whole cell would be at these different voltages. Regarding second question, are you saying that the sodium-potassium pump would be regulated by both sodium and potassium concentrations? –  Gorge Homless Jun 4 '12 at 21:04
    
Good response. If you look at a neuron, you will see that it has a lot of surface area and its easy for a small portion of it to have a strong potential while the rest of the cell would hardly feel the difference. –  shigeta Jun 5 '12 at 4:10
    
@GorgeHomless Basically. Proteins are effected by what environment they are in, ie. temp, pH, salt, etc. These factors affect the bonding that makes proteins a certain conformation. So when the concentration is at a specific level, meaning the environment has changed, the bonding and interactions of the amino acid side chains will be altered; the protein will change shape. So, when sodium or potassium or other ions are in high concentration they interact with the protein & enviro, disrupting or enhancing bonds, making the shape change. Hopefully that answers the question and makes sense :) –  Elisabeth Jun 5 '12 at 15:35

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