Counter transport , Active of facilitated?

[SuperOrchestra]

I just did a kaplan passage in regard to Bicarb production in the kidney.
In the passage there is a diagram of a tubule epithelial cell, on the right side is the lumen, on the left side extracellular fluid.
There is a sodium potassium pump, pumping 3 Na+ into the extracellular fluid for two potassiums inside the tubule cell. This establishes a gradient, that sodium ions in the tubule lumen take advantage of to enter the epithelial cell..... Now the question.

It asks me by what mechanism is the sodium transported back into the tubule cell from the tubule lumen. Now since it takes advantage of a concentration gradient established by a Na/K pump which requires ATP, I put active transport, but they say is facilitated diffusion.

I thought if energy is expended at all, whether primarily or secondarily it was considered active, is this not the case?

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[RySerr21]

I just did a kaplan passage in regard to Bicarb production in the kidney.
In the passage there is a diagram of a tubule epithelial cell, on the right side is the lumen, on the left side extracellular fluid.
There is a sodium potassium pump, pumping 3 Na+ into the extracellular fluid for two potassiums inside the tubule cell. This establishes a gradient, that sodium ions in the tubule lumen take advantage of to enter the epithelial cell..... Now the question.

It asks me by what mechanism is the sodium transported back into the tubule cell from the tubule lumen. Now since it takes advantage of a concentration gradient established by a Na/K pump which requires ATP, I put active transport, but they say is facilitated diffusion.

I thought if energy is expended at all, whether primarily or secondarily it was considered active, is this not the case?

i think for secondary active transport it needs to involve two substances. For example solute A is actively transported across a membrane against its concentration gradient, and as it flows back across, solute B comes with it. That is secondary active transport. PLEASE someone double check what i just said in case it is wrong, its amazing how quickly you forget things after you dont need them!

As far as the question they asked, stricly following the sodium (ONE solute) the sodium is actively pumped across the membrane and then it passively fuses back across. Therefore, facilitated diffusion.

that is my understanding. i apologize if it is incorrect, hopefully someoen here calls me out on it if it is, and hopefully this helps you understand.

 

[Lukkie]

ions like sodium can not simply pass through the cell membrane (simple diffusion). they need a channel to go through. for this question it helps to know basic ion concentrations inside a cell vs outside.

sodium - higher outside
potassium - higher inside

this is why you need ACTIVE transport to move sodium outside. if you just opened a channel that let sodium through, the sodium would enter the cell due to the concentration gradient (there is more Na outside than inside the cell, so Na would rush in)

i think your problem is you did not read the question properly. it is not asking 'what established the concentration gradient'. the question is 'now that you have this concentration gradient, how does sodium get back in the cell?'

it does not require ANY energy since there is already a concentration gradient. hence, facilitated diffusion. facilitated diffusion is passive.

 

[SuperOrchestra]

I understood the question. Its a problem of semantics. I have had a problem in EK in the past where it did consider counter/co transport active because the concentration gradient that was being used was established in a prior step using ATP. So I'm just concerned in what specific instances I can consider something active or facilitated in examples like this. Obviously in a vacuum if you look at sodium diffusing along concentration gradient it will be facilitated.

In this instance the sodium being pumped out is going to the extracellular fluid and the sodium entering is coming from the tubule lumen. So its different than say a leaky potassium channel during repolarization in a neuron.

Its a little tricky, I really hope the MCATs wouldn't have such an ambigious question.

 

[Lukkie]

its not ambiguous at all!

"what mechanism is the sodium transported back into the tubule cell from the tubule lumen"

 

[GoldShadow]

Agreed with Lukkie.

 

[SuperOrchestra]

alright alright, i'm just being stubborn. This scenario is an example of secondary active transport though? That's whats holding me up I think.

 

[Adnama]

I just did a kaplan passage in regard to Bicarb production in the kidney.
In the passage there is a diagram of a tubule epithelial cell, on the right side is the lumen, on the left side extracellular fluid.
There is a sodium potassium pump, pumping 3 Na+ into the extracellular fluid for two potassiums inside the tubule cell. This establishes a gradient, that sodium ions in the tubule lumen take advantage of to enter the epithelial cell..... Now the question.

It asks me by what mechanism is the sodium transported back into the tubule cell from the tubule lumen. Now since it takes advantage of a concentration gradient established by a Na/K pump which requires ATP, I put active transport, but they say is facilitated diffusion.

I thought if energy is expended at all, whether primarily or secondarily it was considered active, is this not the case?

I am not sure I understand the question but if Na+ is going back into the cell then I thought it means there is the opening of the gated channels - b/c of an of action potential. This is facilitated diffusion.

You are right - energy expended means active transport - if it is primary or secondary.

 

[nfg05]

alright alright, i'm just being stubborn. This scenario is an example of secondary active transport though? That's whats holding me up I think.

It's not secondary active transport. Ryserr was correct when he said that secondary active transport must involve two substances:

1) the one being moved AGAINST its concentration gradient
2) the one whose concentration gradient is being harnessed to transport substance 1 across the membrane.

In this case, you have a sodium concentration gradient which COULD be used to transport something else back into the tubule cell, but since another substance is never mentioned, secondary active transport isn't going on.

 

[SuperOrchestra]

Sorry, I left information from the question out; as the Sodium travels with its gradient into the cell from the lumen, a hydrogen ion is expelled into the lumen

So for the two above points
1) the one being moved AGAINST its concentration gradient
- This would be the sodium ion
2) the one whose concentration gradient is being harnessed to transport substance 1 across the membrane.
- And hydrogen ion harnesses the sodiums concentration gradient

BUT, the question stills says facilitated, maybe because it only asks about the sodium ion. But then the hydrogen ion is leaving the cell via secondary active transport?

 

[Lukkie]

if the question asked 'how is the hydrogen being transported out of the cell' yes, secondary active might be the right answer. but right now, ALL WE CARE ABOUT IS SODIUM.