Sodium / Potassium Pump - ATPase backwards?

[ilovemcat]

Okay, so I'm trying to fully understand the concept of Recent Membrane Potential. Reviewing Princeton Review, they explain that the RMP is established soley by the Na+/K+ pump.

Here's what they had to say:
In an artificial cell with Na+/K+ ATPase pumps and normal cellular concentrations of ATP and ADP + Pi but no potassium leak channels, the RMP potential turned out to be -10 mV. (An artificial cell with just K+ leak channels wouldn't flow because the ion concentration would be the same on both sides.) By adding K+ leak channels, the RMP at normal cellular level is -70 mV.


Okay so that makes sense, but then they ask "What would happen if there were much more ADP + Pi than ATP in the artificial cell?

A: T he pump would run backwards. All active transporters are reversible.


Can anyone elaborate on this because I'm having difficulty understanding what they mean.

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[RogueUnicorn]

if there is a sufficient amount of ADP + Pi, it will use the pump to form ATP, which also reverses the ion transport. remember, everything is an equilibrium, so this is just leChat's principle.

 

[ilovemcat]

if there is a sufficient amount of ADP + Pi, it will use the pump to form ATP, which also reverses the ion transport. remember, everything is an equilibrium, so this is just leChat's principle.

So basically after ATP is used up, there would be a high intracelleular concentration of K+ and a high extracellular concentration of Na+. In order for the intracellular concentration of ADP + Pi to be converted to ATP, you would need to utilize a proton gradient which in this case would be supplied by the previously established Na+ gradient. So Na+ would flow back in and K+ back out. (I guess we disregard the fact there are leak channels for potassium since those work slowly and it'd take a while before concentrations became equal again.)

Does this explanation sound right?

 

[RogueUnicorn]

no no you're thinking ATP synthetase which is a different deal. proton gradient is not what drives it, but as you point out, the established Na and K gradients do drive it.

 

[Phantastic]

What he said ^^

(I guess we disregard the fact there are leak channels for potassium since those work slowly and it'd take a while before concentrations became equal again.)

Your artificial cell being used has no leak channels. If it did, the K+ would leak back into the extracellular space and no electrochemical gradient would exist to drive the ATP synthesis.

Just look at the gradient as being established (energy has already been used to create it), but with no more ATP, the most thermodynamically favorable movement of ions is back through the pump, moving with the concentration gradients, synthesizing ATP. Although this is not the main pathway for ATP synthesis (oxidative phosphorylation is), it is essentially the same concept: using an electrochemical gradient to drive the synthesis of ATP. (remember, this electrochemical gradient was established by the breakdown of ATP itself...)

 

[peppinos]

Hi,
I guess that maybe in red blood cell this pump could work backward.

It si interesting to notice that in red blood cells the membrane potential is -12 mV.
Then, in these cells there is not oxidative phosphorylation, and the glycolysis has a shunt (alternative pathway) at the level of the phosphoglycerate kinase (PGK), the Rapoport-Luebering shunt (RLS).

The greater the flux through RLS (with respect to PGK), the lower the level of the atpase, i.e. they are anticorrelated,
till to negative values, at least theoretically.

Through RLS the 2,3 biphosphoglycerate is produced
that binds to hemoglobin lowering its affinity for oxygen, that is thus released.
This is what happens in vessel.
Thus, the ATPase could work backward after a squeezing of the cell after passing into vessel, to restore its original form.

If someone more expert than me on this
would like to comment i will be happy.
thanks a lot

 

[Rabolisk]

Anyone know why ilovemcat was banned?

 

[salim271]

Anyone know why ilovemcat was banned?

No idea... my guess would be because he loves mcat. Seriously, who loves mcat?

Also in reply to the OP, if your ATP stores are so low that your Na/K pump is running backwards, ur pretty much dead. Theoretically, it can run backwards just like ATP synthase can, but the physiological conditions for it are near impossible to obtain, you would have to be beyond starving. The body keeps ATP levels so much higher than ADP for reasons exactly like this, could you imagine if all of your Na/K pumps started running backwards? Glucose couldnt be driven into the cell by the SGLT1, the membranes of the cells would slowly depolarize, gradients would be screwed to hell, APs wouldnt be felt/fired... list goes on and on. My AnP teacher called the Na/K pump the most important transport carrier in the body, and frankly I agree wholeheartedly.

 

[Integrated MCAT Course]

The general article on potassium channels at Wikipedia is a real gem:

http://en.wikipedia.org/wiki/Potassium_channel