|MadSci Network: Cell Biology|
Martin - helluva question! Makes a change from 'Why is the sky blue?' although probably far fewer people will want to read this answer than that one: Note: I referred mostly to Chang (1981) ‘Physical Chemistry with Applications to Biological Systems’ (2nd ed), Macmillan. I’m not sure I was able to dust of my knowledge of this topic enough to make sense of the question, never mind to compose an adequate answer. Please get back to MadSci if you need more detail. Answer: “Macroscopic electroneutrality” I would presume (not having met this phrase before) to mean that the cells and their immediate surroundings remain neutral. There are species of electric eel and rays where this is obviously not the case, but these are exceptional. Evidently, the interior of the cell is not neutral with respect to its immediate surroundings or there would be no potentials to speak of. The membrane cannot be a simple capacitor because it is permeable to charge. The potential is generated by charge separation (anions from cations) across the membrane. With a capacitor the charges derive from elsewhere in a circuit. I’m not sure what you mean by ‘infinitesimal differences’: A voltage is a voltage – the potential difference between two points. It doesn’t make sense in this case to add them up as if you have a battery of cells. Chang gives the potassium concentrations as 400 mM inside an axon and 15 mM outside. This would imply a membrane potential of -84mV i.e. if the inside is -84mV relative to the outside, then that would account for the lack of diffusion of potassium ions from the inside. Because the potential is only -75mV potassium will diffuse out of the cell, and pumps are needed to maintain the 400/15 concentration ratio. The -75mV potential can be calculated (more-or-less) using the Goldman- Hodgkin-Katz equation taking sodium ion concentrations as: 20mM inside and 440 mM outside, and a 100-fold greater permeability of the membrane to potassium than to sodium. The equation works in practice – I’m not sure why you have a problem with its derivation. The ‘arbitrary’ cancellation of ion fluxes is anything but an arbitrary assumption. If there was any net ion flux we would observe a change in resting potential, wouldn’t we? Of course, the ions are flowing into and out of the cell in dynamic equilibrium, but the influx and outflow of each type of ion is equal. Ewen McLaughlin
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