TPR Bio question

[TheMightyBoosh]

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The movement of sodium during an action potential is an example of:

I. facilitated diffusion
II. simple diffusion
III. active transport

A. I only
B. II only
C. I and III only
D. I, II, and III

The solutions say the answer is A, but shouldn't the answer be C since sodium is always moving through the Na+/K+ ATPase?

 

[fizzgig]

at odd hours of the night i ignore typed questions and make up my own and then answer them apparently... edited to remove totally irrelevant answer.

 

[kehlsh]

I think the question is talking about the leak channel not the Na+/K+ pump.

Notice the word "during actional potential," it means it's going away from the resting potential. Namely, sodium efflux causes depolarization, diffused through the faciliated diffusion.

On the other hand, it is Na+/K+ pump that causes resting membrane potential.

Hope it helped,

Kehlsh

 

[TheMightyBoosh]

I think the question is talking about the leak channel not the Na+/K+ pump.

Notice the word "during actional potential," it means it's going away from the resting potential. Namely, sodium efflux causes depolarization, diffused through the faciliated diffusion.

On the other hand, it is Na+/K+ pump that causes resting membrane potential.

Hope it helped,

Kehlsh

Right, but the action potential includes "depolarization", "repolarization", and "hyperpolarization." So I agree that sodium efflux via facilitated diffusion through voltage gated sodium channels causes depolarization during the AP. But to get back to resting potential (and overshoot it) I'm pretty sure you have a combination of slow K+ channels opening and normal action of the Na+/K+ pump. I don't see any reason why the Na+/K+ pump would ever stop working during the AP.

 

[PiBond]

because the question is asking for the movement of sodium during an AP, it is focusing on the fact that we have an influx of sodium through voltage gated channels when the membrane potential hits threshold. because membrane channels are used in the movement of sodium across the membrane, it is facilitated diffusion. Simple diffusion is when no channels are used and the molecule is simply able to cross due to its chemical properties (ie steroids, small hydrophobic molecules).

Na+/K+ ATPase is always working in every cell in order to maintain osmotic balance. so although it is "working" during an AP, it is not specific to an AP and the question is specifically asking about Na+ during an AP.

 

[TheMightyBoosh]

Thanks for the response... you still don't have me convinced though. During an AP, you have influx of sodium via voltage gated channels that depolarize the cell, then close, and allow sodium to be pumped out via sodium-potassium pumps. So there has to be a net movement of Na+ in both directions before the cell gets back to resting potential.

 

[PiBond]

Thanks for the response... you still don't have me convinced though. During an AP, you have influx of sodium via voltage gated channels that depolarize the cell, then close, and allow sodium to be pumped out via sodium-potassium pumps. So there has to be a net movement of Na+ in both directions before the cell gets back to resting potential.

it's the huge efflux of potassium (via slow voltage gated K+ channels) that repolarizes the cell, not Na+. think of the Na+/K+ as maintenance in keeping our RMP around -70mV

 

[TheMightyBoosh]

Agree. But I'm more annoyed by the wording of the question stem here. Since the question does not specify what kind of movement its talking about, you have to assume that Na+ movement during all parts of the AP are fair game. Yes, it's the influx of K+ that quickly restores the membrane potential during repolarization, but technically you still have movement of Na+ out of the cell through active transport, even though it's contribution to restoring the membrane potential is not as big.

 

[PiBond]

Agree. But I'm more annoyed by the wording of the question stem here. Since the question does not specify what kind of movement its talking about, you have to assume that Na+ movement during all parts of the AP are fair game. Yes, it's the influx of K+ that quickly restores the membrane potential during repolarization, but technically you still have movement of Na+ out of the cell through active transport, even though it's contribution to restoring the membrane potential is not as big.

i can see how it's confusing, but the words "movement" and "action potential" lead us to the best answer A. if the question had been phrased: all of the following are observed in the transport of sodium in neurons...i think we could safely pick I and III.

i think it comes down to isolating the events of the AP and asking ourselves, how is the movement of sodium represented in the shape of the AP? we see that the influx of sodium causes the depolarization (upstroke) and this is caused by the activation of our voltage gated Na+ channels. while the Na+/K+ pump is still working in the cell, it's representation in terms of the shape of the AP is virtually non-existent. i've known many texts to exclude the return to RMP from part of the action potential

 

[golgiapparatus88]

Like kehlsh said, the key word was "during." If the question had asked, "what transports sodium AFTER an action potential" or "what keeps the membrane at resting potential" then the answer would have been facilitated and active.

 

[TheMightyBoosh]

i can see how it's confusing, but the words "movement" and "action potential" lead us to the best answer A. if the question had been phrased: all of the following are observed in the transport of sodium in neurons...i think we could safely pick I and III.

i think it comes down to isolating the events of the AP and asking ourselves, how is the movement of sodium represented in the shape of the AP? we see that the influx of sodium causes the depolarization (upstroke) and this is caused by the activation of our voltage gated Na+ channels. while the Na+/K+ pump is still working in the cell, it's representation in terms of the shape of the AP is virtually non-existent. i've known many texts to exclude the return to RMP from part of the action potential

This is what I read during content review in the TPRH Biological Sciences Review chapter on the nervous system:

"After depolarization, the membrane is repolarized, re-establishing the original resting membrane potential by the activity of several factors:

1. Voltage-gated sodium channels close very quickly after they open, shutting off the flow of sodium into the cell.

2. Voltage-gated potassium channels open more slowly than the voltage-gated sodium channels and stay open longer.

3. Potassium leak channels and the Na+/K+ ATPase continue to function, as they always do, and bring the membrane back to resting potential. These factors alone would repolarize the membrane potential even without the voltage-gated potassium channels, but it would take a lot longer."

After all that, I can let go of this question. But you can understand why it's easy to make a mistake on that problem after having read this portion of the text.

 

[Charles_Carmichael]

This is what I read during content review in the TPRH Biological Sciences Review chapter on the nervous system:

"After depolarization, the membrane is repolarized, re-establishing the original resting membrane potential by the activity of several factors:

1. Voltage-gated sodium channels close very quickly after they open, shutting off the flow of sodium into the cell.

2. Voltage-gated potassium channels open more slowly than the voltage-gated sodium channels and stay open longer.

3. Potassium leak channels and the Na+/K+ ATPase continue to function, as they always do, and bring the membrane back to resting potential. These factors alone would repolarize the membrane potential even without the voltage-gated potassium channels, but it would take a lot longer."

After all that, I can let go of this question. But you can understand why it's easy to make a mistake on that problem after having read this portion of the text.

The contribution of the Na+/K+ pump is negligible compared to the K+ efflux due to voltage-gated channels in the repolarization of the neuron. The most important job of the Na+/K+ pump is to maintain the concentration gradients for Na+ and K+, not to contribute to repolarization. The sentence after the bolded one is referring to this.

 

[MightyMoose]

The contribution of the Na+/K+ pump is negligible compared to the K+ efflux due to voltage-gated channels in the repolarization of the neuron. The most important job of the Na+/K+ pump is to maintain the concentration gradients for Na+ and K+, not to contribute to repolarization. The sentence after the bolded one is referring to this.

This is true, but it is also just a poorly worded question.

OP, you won't see a question that leaves this type of ambiguity on the actual MCAT. You just need to take all these practice questions with a grain of salt. Don't waste your study time fixating on a problem you already know. Move on to the next topic. I used to be the king of bogging during studying, but it won't get you anywhere for standardized exams.

 

[kehlsh]

Funny how EK 1001 had a question about this.
I missed it due to thinking like you at the first time.
It says "During the resting phase, a negative resting membrane is established with closed Na+ and Cl- channels and open ("leaky") K+ channels. During depolarization, NA+ channel open rapidly to make the cell membrane potential less negative. The sodium channels are ALWAYS closed unless the cell is being depolarized."