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What is it? and why is it important?
A membrane Potential exists when there is a charge gradient separated by a selectively permeable membrane. What does that mean you ask?
To understand membrane potential you have to first understand a few things. The most important thing being the fact that ions are charged particles. Ions are atoms that have either lost or gained electrons resulting in the formation of an atom that is now either positively, or negatively charged.
A membrane potential exists when there is the potential for electricity to flow (voltage) across the plasma membrane. In order for this potential electrical flow (current) to exist, one side of the membrane must be relatively positive and one side of the membrane must be relatively negative. When I say relatively I mean that there is a relative difference in charge between the interior side and the exterior side of the plasma membrane. To illustrate what potential electrical flow is I want you to imagine something. Imagine the plasma membrane is a concrete wall that ions cannot cross. One side of the wall is packed with negatively charged ions, and the other side packed with positively charged ions. If we all of a sudden opened a hole in this wall that only allowed the positive ions to pass, the positive ions would rush across the wall towards their negative counterparts (electrical flow/current).
Do you see now, how a membrane dividing charged particles demonstrates a membrane potential? By opening channels the membrane can allow current to flow.
Okay so now we have a problem. Cells in the body at rest are said to have a resting membrane potential. Meaning that at any given time there is a difference in charge on either side of the plasma membrane. But how can the the cell’s membrane maintain a potential if ions are allowed to diffuse freely across the membrane?
The answer is Active Transport. Remember that in the human body the intracellular concentration of ionic potassium is about ten times greater in the intracellular fluid than in the extracellular fluid. Since potassium is able to passively diffuse out of the cell through potassium channels it must be actively pumped back into the cell in order to maintain this concentration gradient. This is accomplished through the use of active transport, specifically via the Sodium Potassium Pump. This pump maintains the dramatic sodium and potassium concentration gradients we see between the intracellular and extracellular fluid.
Okay. Now you understand membrane potential and the relative concentrations of potassium and sodium inside and outside of cells (and how these concentrations are maintained).
Next you need to learn how this resting membrane potential in our cells is established
The resting membrane potential in human cells ranges from between -50 to -100 milivolts. The negative sign means that the inside of the membrane is negative compared to the outside of the membrane.
Remember that within our cells K+ (potassium) is found in very high concentrations. The positively charged K+ ions are balanced out by a large number of negatively charged proteins. The extracellular fluid has a very high sodium (Na+) concentration balanced out by equally large amounts of negatively charged chloride ions (Cl-)
Resting membrane potential is primarily established because of K+ diffusion. Lets take a look at how this works.
At rest K+ diffuses down its steep concentration gradient out of the cell into the extracellular fluid through leakage channels. This efflux of positive potassium ions results in the interior side of the membrane becoming relatively negative compared to the outside of the membrane (remember: it is a concentration gradient that drives this step)
As potassium leaves the cell, negatively charged proteins are dragged along with it, however they cannot escape. Eventually the pull of the negative protein’s charge from within the cell membrane actually begins to pull back some of the escaping K+ ions.
Eventually enough K+ will flow out of the cell down its concentration gradient that the cell membrane potential will be -90 millivolts. This means that the interior of the membrane is -90 millivolts more negatively charged than the exterior of the membrane. At this point the electrical pull of the negative interior (which draws potassium back into the cell) will be exactly balanced out by the concentration gradient forcing K+ out of the cell. This means that the net flow of K+ into and out of the cell will now be equal.
Almost done! Na+ also plays a small roll in establishing resting membrane potential in many cells. The plasma membrane is less permeable to Na+ so it plays a much smaller role than K+. All I want you to know is that a relatively small amount of sodium ions flow into the cell but but are pumped back out again by the Na+ K+ pump. The resulting affect is that the membrane potential moves from approx. -90 millivolts to -70 millivolts.
Voila!! we now have a resting membrane potential of -70 millivolts!
Let’s do a very basic recap
Selective diffusion of K+ (and to a lesser extent Na+) ions establishes the resting membrane potential. Then the Na+ K+ pumps maintain the strong ionic gradients necessary to maintain this potential.
If K+ was allowed to diffuse freely and was not pumped back into the cell actively, then its concentration would eventually equalize on either side of the membrane and the membrane potential would disappear. (bummer)
Now you understand resting membrane potential. Well hopefully you can begin to understand it at least! If you have any questions just leave a comment and I will do my best to explain.
It is important to remember that this charge separation exists at the cell membrane only (not the entire cell). If we averaged all of the charged particles within the cell, and then averaged all of the charged particles outside of the cell…. These two averages would be exactly equal. This is important! Let me know if you have a question.
Only read the following article if you fully understand what I’ve been talking about previously. The material in the article will not be covered until much later in our class.
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