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This difference in potential is called the ‘resting potential’. This potential difference is not very big, ranging from -20mV to -90 mV but, as we shall later see, very important! (mV = millivolt; one thousand of 1 Volt).
3. For example, the red blood cell (= erythrocyte) has a relatively ‘small’ resting potential; -20 mV while the muscle cell has a much higher resting potential: -90 mV!
4. By the way, we say ‘-20mV’ or ‘-90 mV’ because the inside is negative compared to the outside, extracellular, space.
To create a resting potential, the following four components are required:
The sodium-potassium pumps, continuously, potassium ions into the cell and sodium ions out of the cell. Therefore, the concentration of potassium will become higher inside the cell than outside the cell. This is called the ‘concentration gradient’ for potassium ions.
Then, because the potassium channels, in the resting membrane, are open, the potassium ions will flow (=diffuse) out of the cell (from high concentration inside to low concentration outside).
Once the potential gradient is equal (but opposite!) to the concentration gradient, then equilibrium will have been reached and both gradients remain stable.
The potential reached at this stage is called the ‘equilibrium potential’. Because this relates in this case to potassium, this is called the EK+.
(=Equilibrium potential for potassium)Students often come up with the following problem; does the concentration gradient not decrease when the potassium ions diffuse outside? The answer is; yes but very little. Charge and concentration are not the same. In fact, very little potassium diffusion is necessary to induce a large potential difference.
There is sometimes confusion about cause and effect! The sequence must be very clear:
So, this is the correct sequence:
Na-K pumps -> concentration gradient for K+ -> electrical gradient (resting potential).
As the Na-K pump needs energy (=ATP), anything that affects the energy supply, will affect the resting potential.
Another (simple) mind experiment: The Na-K pump also pumps sodium ions out of the cell. What then is the concentration gradient for sodium?
In the case of potassium ion, of which there many more inside the cell then outside, the equilibrium potential will be negative inside the cell, as discussed above.
3.
For sodium ions, of which many more ions are located outside the cell than inside, the equilibrium potential will be positive! (see Mind Game #3).
In this diagram, I have plotted the values of ENa+ (=+35mV) and EK- (=-90mV).
6.
And, between these two, a shadow of an action potential (see next page!).
Please note that the resting potential is not the same as the Equilibrium potential for potassium. In fact, the resting potential is slightly less negative than the EK-
This is because the cell membrane is not perfect; there are always some sodium channels open and therefore some sodium ions will leak into the cell making the inside less negative than the Equilibrium potential.
This was first developed by Walter Nernst, a German chemist who got a Nobel prize in 1920 for his work!
So, in the case of potassium ions, and given the usual concentration difference in a muscle cell, the Nernst formula would look like this:
And, in the case of sodium ions, the Nernst potential would look like this:
R = ideal gas constant; T = temperature in Kelvin; F = Faraday’s constant
Answer 1: resting potential becomes positive inside the cell
Answer 2: High sodium concentration outside and low inside the cell.
Answer 3: Sodium ions will flow into the cell, as they are attracted by the negative potential, taking positive charges with them; this will make the inside positive.
(Hint: this is actually how the action potential starts!)
