Re: Why is the work of the sodium-potassium pump secondary active transport?

Hi Gemma,

      You're right - the pumps restore ionic concentrations by moving ions against their concentration gradients, and the secondary active transport could be one means of restoring that balance. I'll address some aspects of transport and ionic concentrations below.

      First, I should mention that there are a couple types of active transport : primary active transport and secondary active transport. The big distinction is that the primary active transport utilizes ATP, while secondary transport does not. Primary active transport can move two ions against their concentration gradients (such as the widely studied Na+/K+ ATPase). This pump contributes greatly to the restoration of the membrane potential following an action potential, moving 3 Na+ ions out for each 2 K+ ions it brings in.
      Secondary active transport doesn't use ATP, but uses the energy from moving one molecule down its concentration gradient to move another one against its gradient, such as allowing Na+ back into the cell to actively pump K+ out. However, I should say that a quick scan suggested that K+ isn't the favored molecule for secondary active transport. This form of transport is utilized more for the purpose of bringing glucose and amino acids into the cell (using Na+ as the 'energy source').

      One quick note about the relative ionic concentrations during and following an action potential. There are always more Na+ ions outside the cell and alwasy more K+ ions inside.
      While in the hyperpolarized state (i.e. at rest), there are many more K+ ions inside the axon/cell than outside, even with a -70 mV membrane potential! With the assistance of the pumps, K+ is being constantly sucked into the cell. However, at rest, the membrane is permeable to K+, but not to Na+. Thus K+ keeps flowing out down its concentration gradient, making the membrane hyperpolarized. (If you have learned about the Nernst equations , you can test it out - this equation shows that with a high intracellular concentration of a positive ion, the membrane potential is negative).
      Similarly, at the peak of the action potential, there are still many more Na+ ions outside the cell than inside. At the peak, the membrane potential heads up to ~+50 mV, the Nernst (reversal) potential for Na+. If the membrane can get close to this voltage, that means that there is still a relatively large imbalance of ions between the inside and the outside. Otherwise, the potential would stay at 0! You can check this for yourself. Plug a few different inside and outside concentrations into a single-ion Nernst equation. If the concentrations are equal, the Nernst equation gives a reversal potential of 0 mV.

      I hope this helps!
-Alex