I'm very confused so bear with me please.

Electrogenic pumps are carrier proteins that generate voltage through the movement of ions, right ?

When is a voltage generated ? When there's a net movement of ions ?

Doesn't that mean that when any ion passes through a channel protein, voltage is generated ? And energy is released ? Doesn't that make them electrogenic too ?

We need energy to move ions against their concentration gradient but if energy is released when ions move then why do we need energy ?

Is it that a voltage is generated when an ion moves towards the env with the same charge ? So when the movement enhances the overall seperation of charge , a voltage is generated ? And energy is released ?

In other words, the movement of Na+ through the Na+ channel protein down a concentration gradient doesn't release energy because the Na+ is moving towards the -ve environment but it being pumped outside does ? See that doesn't make sense either, because we need energy to pump and ion.

I'm very very confused


2 Answers 2


The main thing the sodium/potassium pump does is to move sodium and potassium ions, Na+ and K+. It technically generates a bit of charge separation in doing this, but you can ignore it for now.

What is important is that because of the operation of this pump, you now have two sides of membrane with different ion concentrations. One side is high in sodium ions (outside the cell), the other side is high in potassium ions (inside the cell).

So far, that doesn't really do anything electrical, but it does take a lot of energy to get this arrangement: the whole time, the pump is moving sodium ions to where sodium ions are high, and moving potassium ions to where potassium ions are high. Moving things against their concentration gradient costs energy. The sodium/potassium pump gets this energy from ATP.

Now, imagine you open some pores in the membrane (we call them leak channels), that are a particular size that only lets potassium ions through. The sodium ions can't move, but the potassium ions can. A few potassium ions move over to the high sodium side, down the concentration gradient for potassium. This doesn't take any energy, it just happens passively. It also doesn't involve very many ions, just a few, so the concentrations don't change much.

However, potassium ions have charge! These are positive charges, flowing out of the cell: that makes the inside more negative.

Also, because they are positively charged, potassium ions would tend to move towards more negatively charged spaces. So, as the inside of the cell gets more negative, potassium won't leave as quickly. There is an equilibrium, where the forces pulling potassium ions out to where potassium concentration is low equals the forces pulling potassium ions in to where the voltage is negative.

The voltage where this equilibrium occurs is called the resting membrane potential, and for many cells is somewhere between -100 and -40 millivolts: that's the voltage generated across the membrane, indirectly, by the sodium/potassium pump.

Purves' Neuroscience is a good general reference textbook for this sort of thing. The Goldman equation is a mathematical way to figure out what the voltage will be if you know the concentrations and permeability.

  • $\begingroup$ I think I understand but 1 more thing, my book says "A solute that exists in different concentrations across a membrane can do work as it moves across that membrane by diffusion down its concentration gradient". Doesn't that mean that whenever passive diffusion occurs, energy is released ? $\endgroup$
    – Jaca
    Commented Nov 20, 2019 at 15:48
  • $\begingroup$ @Jaca Creating a concentration gradient costs energy. Much of that energy is converted into potential energy, similar to how carrying water up a hill. When water flows down the hill or ions flow down their concentration gradient, that potential energy can be used to do other things. $\endgroup$
    – Bryan Krause
    Commented Nov 20, 2019 at 15:59
  • $\begingroup$ So, your answer ,in short, is yes ? I get the water example, but I have a question, is this potential energy ( the result of movement against concentration gradients ) electrical when the movement of ions is also down their electrical gradient ? $\endgroup$
    – Jaca
    Commented Nov 20, 2019 at 16:12
  • $\begingroup$ @Jaca They both apply. In the example I described from potassium above, the potassium ions aren't really available to do any work. They have both a concentration gradient and an electrical gradient, and at equilibrium (for the single ion case) these are equal and in opposite directions. If you want to stretch the water example a bit, you could imagine the electrical gradient as a constant vacuum keeping the water at the top of the hill. The vacuum pulls water up, gravity is pulling down, but when they are in equilibrium the water doesn't go anywhere and can't do work. $\endgroup$
    – Bryan Krause
    Commented Nov 20, 2019 at 16:41
  • 1
    $\begingroup$ @Jaca "when an ion moves down both its concentration gradient and its electrical gradient it is available to do work" - Yes, always. "when it moves down one of these gradients it is most likely not available to do work" - Only if it is in equilibrium with the other gradient. You could easily have a concentration gradient that does work without any electrical gradient at all, for example. Really what you should consider is the net electrical+concentration gradient. $\endgroup$
    – Bryan Krause
    Commented Nov 20, 2019 at 17:44

Voltage is not generated through the charge movement as such, it the magnetic field that is generated directly through movement of charges, although such movement contributes to voltage generation.

Voltage by definition is a potential to do work (to move charges), it always involves 2 points the difference in electric potential between which is voltage. If you want to measure voltage in a selected area, you always measure it relative to another reference area (like, there always is a "reference electrode"), because you measure the difference in electric potential between these 2 areas.

So, voltage is generated through separation of opposite charges. Like, a negative -80-60 mV resting potential inside a cell is maintained relative to an area of positive charges outside.

"The Na+/K+-ATPase enzyme is active (i.e. it uses energy from ATP). For every ATP molecule that the pump uses, three sodium ions are exported and two potassium ions are imported; there is hence a net export of a single positive charge per pump cycle." (wiki) Thus it can be supposed that energy is consumed by Na+/K+ ion propagation, though the definition of "energy" varies by context, voltage means potential energy.

  • $\begingroup$ My book says " A transport protein that generates voltage across a membrane is called an electrogenic pump. The sodium-potassium pump appears to be the major electrogenic pump of animal cells. ." If that's the case then Na K pumps generate a voltage aka release energy then why do they need energy ? Also, do Na + protein channels generate a voltage ? Or is that because the motion is from a +ve environment to a -ve environment no seperation of charge is enhanced and no voltage is produced ? Sorry if it's a dumb q I'm only 17 and taking this for the 1st time $\endgroup$
    – Jaca
    Commented Nov 13, 2019 at 11:20
  • $\begingroup$ "Generate a voltage" is not equal "release energy". If you take a smartphone battery out and measure voltage, it will display a manufacturer's preset voltage, so the voltage would already be generated. But there will not be any "release of energy" unless the battery is (generally) plugged into a conductive circuit, where the potential energy provided by voltage will be transformed into any type of energy used by circuit elements - kinetic, light, mechanical, etc. $\endgroup$ Commented Nov 13, 2019 at 20:06

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