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# Membrane potentials - part 2

Continue to explore how a cell that is permeable to one ion can become charged (either positive or negative) if there is permeability and a concentration gradient. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

## Want to join the conversation?

• At you said you don't know what 'V' stands for. Isn't it referring to Voltage?
• Yup. Membrane potential refers to a potential difference, a difference in charge across the membrane. Another name for potential difference is voltage.
• I am curious about one thing. If the resting concentration of K+ inside the cell is 150mMol/L and is 5 mMols/L outside the cell, then why is the membrane potential negative? Shouldn't it be positive since there are more K+ inside than outside at any given time? Thanks!
• The K+ on the inside and outside are coupled with negatively charged proteins and therefore the cell and the outside are neutral. Now when the concentration gradient drives K+ outside, the cell only allows K+ to get outside as (certain) cell gates are only permeable to potassium (specifically) but their partners which are the larger (negative) protein molecules, can't get through the gates.
So , as the positively charged K+ cations move outside , the negatively charged protein molecules remain on the inside of the cell.
Therefore the cell ends up being more negatively charged than its surroundings and thus gets a NEGATIVE POTENTIAL!
• In the equation, you said the constant is 61.5. My teacher has been using 58. Which is correct?
• The constant Rishi is using is based on the Nernst Equation (google/wikipedia it), which can be calculated by ''(R*T)/(F)'' (assuming you are using the natural log, otherwise you add log10(2.72) in the denominator, which you have to in Rishi's case) While R & F are constants, the temperature, T in kelvin, will actually change your constant. By plugging in different values for temperature, you can find out that in fact, both Rishi and your teacher are correct, although at different temperatures. Rishi used 37 Celcius (310.15 Kelvin) for his calculations, your teacher used 20 Celcius (293.15 Kelvin) for his. At thus, Constant(310.15)=61.537mV, while Constant(293.15)=58,164mV.

TL;DR Rishi is right because he did the calculations at body temperature, which would be the most likely temperature conditions at a cell membrane of a live human being. Therefore 61.5mV is correct. This is why everyone should state their assumptions/conditions under which they answer their problem, to allow reproducibility of their data by other researchers.
• Literally nobody has been able to answer me this one question: if Potassium is always driven out by leak channels, why is the interior concentration of Potassium said to be higher than the exterior?
• Higher K+ charges are on the interior of the cell than the outside in the "starting" scenario, so the K+ flows out through leakage channels. The concentration gradient drives K+ out, because in the same starting scenario the interior of the cell is more positive than the outside.
This outflow leaves anions in the cell, which re-attracts the K cations. This all stabilizes net-net at about -92mV, the equilibrium potential for K+ when the cell is only permeable to K+.
• At you say that if there's a concentration gradient but no permeability, then there will be no membrane potential because there's no way for the ions to leave. Why would that mean there's no membrane potential if there'd still be a voltage difference (as indicated by the concentration gradient)?
• At , Rishi mentions the the voltage created through membrane potential of ions. If the equation for it is a constant x LN ([out]/[in]) , then CL-, which has a negative potential, should have a higher concentration inside the cell than its surroundings. Wouldn't this want CL- to flow out of the cell then due to concentration gradient?? (Rishi states it flows inwards)
• There is actually a higher concentration of CL outside the cell than inside the cell in the body, thus creating a "desire" for the ion to move from the higher concentration, outside, to a lower concentration, inside. Hope this helps!
(1 vote)
• at 8.20, when Dr. Rishi tells us the concentration gradients of each of the ions, he tells us about the direction of flow of ions. Does the sign of the membrane potential (positive or negative) give us the direction of flow of ions?
• It is a great video and just to let you know that the speakers says where the Vm come from as there is no letter V in the Membrane Potential....I would say that V is for Voltage which is potential and M is for membrane.
• Yeah, that's exactly where that comes from seeing as voltage is a measurement of the difference in potential between two points.
(1 vote)
• This video refers the equilibrium potentials being properties of the ions.

Being properties with defined values, that implies that these are the equilibrium potentials for these ions in all cells.

That being said, it seems that the equilibrium potential for any given ion is dependent on the concentration and charge of the anions present in the cell.

For the equilibrium potentials of ions to be properties (i.e. they are constant and ubiquitous) wouldn't the concentration and charges of the anions in cells need to be assumed to be constant and ubiquitous also?

That is, are we assuming that the concentration and charges of anions are constant for all cells?

If not, wouldn't we expect the equilibrium potentials of ions to vary, based on the anion concentration of the respective cell?

For example, the equilibrium potential of K+ is -92mV, which means that the concentration gradient and membrane potential are equal when the membrane potential is -92 mV, thus if we changed the concentration and/or charge of the anions inside the cell were different, then the equilibrium potential of K+ would be different, too.

Finally, it looks like the equilibrium potential equation is self-referencing.

The way it's described in this video implies that the equilibrium potential of an ion is a function of a constant multiplied by the log of the ratio of the concentration of the ion outside of the cell and inside the cell.

Yet, if the equilibrium potential is a property, being constant, does that not mean that those concentrations are a function of the equilibrium potential?

I hope those questions make sense.

Any help would be very much appreciated.
• I am afraid I am unable to follow all the questions here. The reference I am going to offer at Wikipedia does go into the chemistry of the membrane potential. You said "That is, are we assuming that the concentration and charges of anions are constant for all cells?" Well, we are assuming it is constant for all neurons, let us stay there. What I do not think is completely emphasized in this lecture is that there are experiments that show if only sodium gates were open, then the equilibrium for the cell would become +66mv because sodium would come into the cell because the anions are constant, they can't leave the cell and they are a negative charge so the sodium is attracted in and secondly sodium comes in due to passive movement by diffusion. So the cell becomes depolarized and it can not return to normal because the sodium potassium pump is disabled in this experimental example. If this happens the neuron dies. This can happen if someone eats a puffer fish, which has a toxin that has this precise effect. The second experimental scenario is if sodium gates are closed and only the potassium channels are open ( which naturally occurs with the poison of the black mamba) and if that happens, then the cell achieved a membrane potential of - 90mv. Potassium is a positive ion and it leaves the cell due to diffusion, while some return due to the anions inside the cell that attract them back, there is a net loss of potassium so the cell moves to - 90mv. But in a normal cell with all channels and pumps working, the sodium potassium pump turns on and returns 3 sodium to the outside of the cell and returns 2 potassium to the inside of the cell to a normal resting potential of -70mv. So in a normal cell the ions are returned to the "correct" side of the membrane and reestablish the resting membrane potential of -70mv. Threshold for the neuron is -55 mv. Given what I just said, how do you think that -55 mv is reached? Did you say that it must be due to the influx of sodium ions? If so you are right, sodium ions coming into the cell make it more positive. Sodium ions continue in to a normal cell and cause the cell to become + 30 mv and that change in the membrane potential signals the potassium gates to open and potassium ions leave the cell which moves it to a negative membrane potential or repolarize it. Those are the basics. Here are some additional links. You may have to copy and paste some of them as they often get corrupted in these replies. I will include a link to a biology textbook on line that you can read as well because you are an in-depth thinker and you are sure to have a bunch more questions. all the best!
In da club - Membranes & transport (video) | Khan Academy
Crash Course Hank Green discusses how things move in and out of a cell

Bozeman Biology

The Action Potential, Crash Course

Biology 2e - OpenStax https://openstax.org/details/books/biology-2e

Membrane potential - Wikipedia https://en.wikipedia.org/wiki/Membrane_potential