Na/K ATPase question...

[pineappletree]

So, according to kaplan, The Na/K ATPase pumps 3 Na+ out for every 2K+ in
The membrane of the neuron cell is also completely permeable to the K+ but not the Na+


So why is there a K+ gradient in the first place if the membrane is permeable to K+? Wouldn't it just diffuse down its concentration gradient?
Or is the charge gradient of greater + outside stronger than the force of the concentration gradient ----> driving the K+ into the cell??


Also, When the book says that the membrane is permeable to K+, is this via facilitated transport??

---

Members don't see this ad.

 

[loveoforganic]

If I remember right, there are potassium channels that can open and close... Potassium doesn't just freely flow out.

 

[Fifty 3rds]

yes. Think very small flow. Normally the Na/K ATPase will have it's most rapid activity after a depolarization. (I know, duh!)

The important thing to remember is that there are K leak channels, but the flow is very small. So, Imagine that your Na/K pumps have just repolarized a neuron by puming 3 Na out, and 2 K in until you see that -70 potential (or whatever it is). Once that occurs (and it is dynamic = constant movement of ions to maintain this) there are more K in the cell, more Na outside of the cell. The electronegative gradient, wants to bring positive charges into the cell. The Na gradient wants to bring charges into the cell, the K gradient wants to flow out of the cell.

This is why the cell is constantly doing work to maintain this gradient. If you get enough of a stimulus to depolarize and those gates fly open, you will see mass exodus of both cations flowing opposite directions of one another to achieve an equilibrium (and in doing so, you get a hyperpolarization event, because they are in such a "hurry" to even out)

Wow, long winded reply. Hope that makes sense.

 

[pineappletree]

So the facilitated diffusion of K+ is negligible compared to the activity of the ATPase...



Thank you!!!

 

[Phantastic]

The MCAT wants you to know that it is the action of the Na+/K+ ATPase Pump and the K+ Leak (slow) channels that maintain the RMP (resting membrane potential). When K+ leaks out (down its gradient), the cell becomes less positive on the inside.

Note that cardiac muscle can self-depolarize due to sodium leak channels which slowly but surely bring the RMP to the threshold voltage, triggering an action potential. This is how a heart could beat without nervous stimulation.

The action potential is caused by rapid depolarization due to voltage gated Na+ channels (fast influx, depolarization). Repolarization occurs thanks to voltage gated K+ channels (fast efflux, repolarization).

 

[Fifty 3rds]

Indeed. Thanks for the clarification/added info Phantastic.

 

[pineappletree]

I was also wondering
How does the repolarization via K+ allow the potential to return to normal?


I know that the K+ concentration difference is less than the Na+ difference. So I how can opening the K+ channels can have the SAME magnitude of effect as the Na+ channels?


(I already understand hyperpolarization)

 

[Fifty 3rds]

I know that the K+ concentration difference is less than the Na+ difference./QUOTE]

What does this mean? Could you clarify?

Following the chronological timeline of depolarization/repolarization, you get Na rushing in, then the Na gates close, polarizing the cell. K rushing out (down it chemical gradient), and then the K channels closing. Now that the cell is negative again, it gained positive charge, then lost positive charge, it's Na/K atpases get to work restoring the Electrochemical gradient and moving the Cations back to where they belong.

Does that make sense?

 

[Charles_Carmichael]

I was also wondering
How does the repolarization via K+ allow the potential to return to normal?

In simple terms, when a cell is depolarized, there's lots of positive stuff inside the cell. During repolarization K+ leaves the cell. So, you're losing positive stuff from inside the cell. This helps the cell get back to its resting membrane potential of around -70 mV.

 

[PiBond]

i'll throw my hat in the ring as well.

Na+/K+ ATPase helps to establish resting membrane potential and also maintain osmotic balance (aka keep water from coming in and lysing the cell). as the posters before me said, the cell also has potassium leak channels which explains why the resting membrane potential is close to the equilibrium potential of K+. thus, we have an overall net efflux of positive charge from the cell (net of +1 from ATPase, and the leaving of positive charge via our leak channels). therefore, relative to the outside our cell interior is negative.

action potentials use different channels called voltage gated sodium and potassium channels. they are both stimulated when a cell reaches "threshold" but the sodium channels are fast acting...hence our initial depolarization. potassium channels are slower and help to repolarize the cell via our net efflux of positive charge.

 

[pineappletree]

Yes I understand,
sorry, it was a stupid question on my part...


THank you!!!