Kaplan Sectional Test 7 Question

[Dr Gerrard]

56. Which of the following processes is NOT ATP-
dependent?
A. Exocytosis of synaptic vesicles at a nerve
terminal
B. Movement of urea across a cell membrane
C. Movement of Ca2+ into a muscle cell
D. Export of Na+ from a neuron

The answer is B. I cannot argue that, I just didn't really think about it.

I would have chosen that at first, but then I chose D.

Anyways, I thought that when Na+ leaves a neuron, a lot of it had already entered in, which was the cause of the action potential. Thus, there was a large concentration gradient, with much less sodium outside the neuron. If this is the case, ATP would not be required to get rid of it. Correct?

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[Dr Gerrard]

Does the sodium potassium pump really need ATP to transport ions down their concentration gradient?

 

[G1SG2]

56. Which of the following processes is NOT ATP-
dependent?
A. Exocytosis of synaptic vesicles at a nerve
terminal
B. Movement of urea across a cell membrane
C. Movement of Ca2+ into a muscle cell
D. Export of Na+ from a neuron

The answer is B. I cannot argue that, I just didn't really think about it.

I would have chosen that at first, but then I chose D.

Anyways, I thought that when Na+ leaves a neuron, a lot of it had already entered in, which was the cause of the action potential. Thus, there was a large concentration gradient, with much less sodium outside the neuron. If this is the case, ATP would not be required to get rid of it. Correct?

Does the sodium potassium pump really need ATP to transport ions down their concentration gradient?

Urea is small, uncharged, and can cross the membrane via diffusion independent of ATP.

But the answer choice D does not mention depolarization, so there's no reason for you to think a lot of the sodium went back in already. The question is asking about export of Na+ from a neuron, which is an active process because the process goes against the concentration gradient. Only ATP dependent transport/active transport can accomplish the movement of a molecule or ion against its concentration gradient.

 

[Dr Gerrard]

So after an action potential, after the large influx of sodium ions, is ATP needed to get rid of this Na?

Other than that, yeah, I shouldn't have assumed that about the neuron.

Also, if anyone else has access to this, I have a question about number 61. It is an experimental passage based question, so I don't think I would be able to explain it on here.

My reasoning for that was that Since fraction B eluted at the smallest pH, it had the smallest positive charge to start out with, so after it was eluted, this positive charge was turned into a negative charge. The larger the positive charge that started, the more positive charge turned into negative charge, so the later elutes, which had a higher starting positive charge, had to have a higher negative charge at the end of the experiment.

That is why I said B had the smallest negative charge, so it would stay closest to the cathode, when compared to D and E, which had larger negative charges, since they started out with a greater positive charge.

 

[G1SG2]

So after an action potential, after the large influx of sodium ions, is ATP needed to get rid of this Na?

Other than that, yeah, I shouldn't have assumed that about the neuron.

Also, if anyone else has access to this, I have a question about number 61. It is an experimental passage based question, so I don't think I would be able to explain it on here.

My reasoning for that was that Since fraction B eluted at the smallest pH, it had the smallest positive charge to start out with, so after it was eluted, this positive charge was turned into a negative charge. The larger the positive charge that started, the more positive charge turned into negative charge, so the later elutes, which had a higher starting positive charge, had to have a higher negative charge at the end of the experiment.

That is why I said B had the smallest negative charge, so it would stay closest to the cathode, when compared to D and E, which had larger negative charges, since they started out with a greater positive charge.

Not too sure. I would think that at first, the sodium ions would just diffuse out via facilitated diffusion, but as the concentration gradient builds, active transport would be required.