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confusing A level biology question

this is from the 2021 aqa paper 2 question 2.1

I feel like the mark scheme is back to front here. I'm fine with the first 2 points but I'm really confused why they are saying for the 3rd that Sodium ions move out of the neurone as they said in mark point 1 that they were already in high concentration outside the neurone. I feel like it would make sense if mark point 3 was before mark point 1 as if sodium was being activley transported out then the outside of the neurone would have a higher conentration.

https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2021/november/AQA-74022-QP-NOV21.PDF

https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2021/november/AQA-74022-MS-NOV21.PDF
Hi,
The crucial difference between the permeabilities of the membrane to sodium [low permeability] and to potassium [high permeability] is the INITIAL factor that generates the negative potential on the inside of the membrane relative to the outside [this movement is brought about by simple diffusion [as sodium is principally an extracellular cation [so tends to move in], while potassium is principally intracellular [so tends to move out]]]. [1st 2 points in mak scheme]

The 3rd point in the mark scheme i.e. movement of 3 Na+ out of the cell in exchange for 2 K+ into the cell [so net inward negative] is an active process that needs ATP [sodium-potassium pump or Na+, K+, ATP-ase] AND COMES LATER [in a sense tho in miilliseconds] so MAINTAINS the negative potential inside after it is INITIATED by the first 2 points in the mark scheme.

I hope this makes things easier for you!
M [specialist biology tutor]
Reply 2
Original post by macpatgh-Sheldon
Hi,
The crucial difference between the permeabilities of the membrane to sodium [low permeability] and to potassium [high permeability] is the INITIAL factor that generates the negative potential on the inside of the membrane relative to the outside [this movement is brought about by simple diffusion [as sodium is principally an extracellular cation [so tends to move in], while potassium is principally intracellular [so tends to move out]]]. [1st 2 points in mak scheme]
The 3rd point in the mark scheme i.e. movement of 3 Na+ out of the cell in exchange for 2 K+ into the cell [so net inward negative] is an active process that needs ATP [sodium-potassium pump or Na+, K+, ATP-ase] AND COMES LATER [in a sense tho in miilliseconds] so MAINTAINS the negative potential inside after it is INITIATED by the first 2 points in the mark scheme.
I hope this makes things easier for you!
M [specialist biology tutor]

So do you mean by cell the neurone? I'm fine with the first 2 marking points but with the 3rd point I thought Na+ is already outside of the neurone as more Na+ is found outside the membrane while more K+ is found inside the membrane. I was just confused on why we say Na+ is actively transported out when I thought it was already out and K+ was already in
Original post by ax4
So do you mean by cell the neurone? I'm fine with the first 2 marking points but with the 3rd point I thought Na+ is already outside of the neurone as more Na+ is found outside the membrane while more K+ is found inside the membrane. I was just confused on why we say Na+ is actively transported out when I thought it was already out and K+ was already in

This process is more known and discussed for a neurone, yes, but does occur in other cells, too.
Yes again you are right that more Na+ is outside and K+ inside the cell, but we need movement AGAINST the chemical gradient i.e. using energy in the form of ATP to maintain the -ve resting potential intracellularly
OTHERWISE:
if Na+ moves in ALONG the conc gradient, then the resting potential generated by the 1st 2 points in the MS will be nullified hence NOT MAINTAINED.

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