Talk:Cell potential

100 mV is not the same as 0.01 V. Which is the real value?

The article is pretty general and I don't know about the resting cell potential of other types of cells. My guess is that a general figure is not worth much. I took out the contradictory 0.01 V and 100 mV figures and gave an example for which I do know the resting cell potential. Delta G 23:04, 2 May 2004 (UTC)

I changed some phrasing to clarify about impulses and action potentials and removed the section about the rate of action potential firing being limited by metabolism. If this were true, we wouldn't be able to hear, run or see! The article about action potential is truly excellent that the poor explanations here really show up in bold relief. I think more changes are required - my corrections are just a sticking plaster. Aplested 15:16, 22 Sep 2004 (UTC)

One question

... resting cells are negative because positively charged potassium ions, which are more concentrated inside than outside, are allowed to leak out.

Why positive ions make negative potential? --Daxue | Talk 08:34, 8 Jan 2005 (UTC)

• It's a couple of months since you asked this question, but just in case you still need an answer I will attempt to provide one...

When making any measurement you need to know what you mean by zero—this may seem obvious, but let me explain. If I measure the weight of something, to have zero weight means to have nothing of that thing, but what about when I want to measure the height of a mountain? Wikipedia tells me that the height of the top of Mount Everest is 8,850 metres - but 8,850 metres relative to what? By convention people have agreed that, when measuring the height of points on land, the mean height of sea level is zero, and that all heights should be measured relative to this. This is a quite arbitary convention, people could have chosen to measure height relative to the centre of the earth, the lowest point on the surface, or even the highest point on the surface. So what we really mean when we say that Everest is 8,850 m high is that the vertical difference between the top of Everest and mean sea level is 8,850 m. In this respect voltage is very similar to height—when we say that a battery is a 12 V battery, what we mean is that the difference between the voltages at the positive and negative poles of that battery is 12V. In terms of a cell, the membrane potential is defined as being the voltage change across the cell membrane—we need to measure the voltage inside the cell and the voltage outside the cell and calculate the difference between them. So how do we choose whether the zero point is the inside or the outside of the cell. The answer is one of practicality. When I measure the resting membrane potential of a cell I start off with that cell in a dish surrounded by an extracellular soution that is basically saline. I need two electrodes, one is going to make the measurement and the other is going to be my reference. So I start off with both my electrodes outside the cell in the extracellular solution and I measure the voltage difference between them. Because both electrodes are outside the cell, I know that the difference in voltage between my electrodes at this point is zero—so I can set the read-out on my amplifier to be zero. Next I put my recording electrode inside the cell and once again measure the voltage difference between them. The practical result of this method of measuring membrane potential is that I have defined the membrane potential to be the voltage inside the cell minus the voltage outside the cell—because I zeroed my amplifier with both electrodes outside the cell this is the zero point.

Now I have given that background I can give the answer to your question. As the positively charged potassium ions move out of the cell we end up with a situation where there is a higher concentration of positive ions outside the cell than there are inside. This means that the outside of the cell will have a more positive voltage than the inside but, as I described above we measure the voltage inside the cell relative to the voltage outside it. If we subtract a more positive number from a less positive number the result is a negative number, so when we measure the membrane potential the read-out is negative. JeremyA 02:22, 16 Mar 2005 (UTC)

Cell =/= membrane

Cell potential and membrane potential should not be merged. Cell potential can refer to a chemical cell and can have nothing to do with biology.