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OCR A Biology A Level Cell Membrane Question

A student investigated the movement of substances through the cell surface membrane of
yeast cells using an indicator.
The student was supplied with a suspension of yeast cells in a slightly alkaline solution.
The indicator used is yellow in alkaline conditions but turns red in acidic conditions.
The student mixed the indicator with the yeast suspension and labelled the tube containing
this suspension A. The suspension was red/pink in colour.
(i) The student took a small sample from tube A and centrifuged this sample.
After centrifuging, the student observed that the liquid portion was colourless but the
cells at the bottom were red/pink.
Suggest the mechanism by which the indicator enters the cells and suggest the
component of the membrane involved

The answer is active transport and carrier protein but I have no idea why?
Can anyone explain it to me please?
Reply 1
Original post by lollypenguin
A student investigated the movement of substances through the cell surface membrane of
yeast cells using an indicator.
The student was supplied with a suspension of yeast cells in a slightly alkaline solution.
The indicator used is yellow in alkaline conditions but turns red in acidic conditions.
The student mixed the indicator with the yeast suspension and labelled the tube containing
this suspension A. The suspension was red/pink in colour.
(i) The student took a small sample from tube A and centrifuged this sample.
After centrifuging, the student observed that the liquid portion was colourless but the
cells at the bottom were red/pink.
Suggest the mechanism by which the indicator enters the cells and suggest the
component of the membrane involved

The answer is active transport and carrier protein but I have no idea why?
Can anyone explain it to me please?

This is why I hate the new exams, if you misinterpret the question it can leave you blank. It says that the solution was alkaline, so we assume alkaline solution was the high concentration, yet the cells are pink, from the question it says that the indicator turns red if acid is detected. Since there is a high concentration of alkaline in the solution, there is a low concentration of acid in the solution. This means that acid moved from low concentration which is the liquid into the cells, and active transport is moving against the concentration gradient (From low concentration to high concentration), so active transport is the answer.
As for carrier proteins, there is no mention of ions or charged particles involved, so it can't be channel proteins, therefore it is carrier proteins.
I had to read the question 3 times before finally getting it, these examiners sure know how to throw you off.
ahhh thank you so much; its just worded so weird! i did think it was something to do with the pH and acidity etc but couldn't get it !
thank you :smile:
Original post by The RAR
This is why I hate the new exams, if you misinterpret the question it can leave you blank. It says that the solution was alkaline, so we assume alkaline solution was the high concentration, yet the cells are pink, from the question it says that the indicator turns red if acid is detected. Since there is a high concentration of alkaline in the solution, there is a low concentration of acid in the solution. This means that acid moved from low concentration which is the liquid into the cells, and active transport is moving against the concentration gradient (From low concentration to high concentration), so active transport is the answer.
As for carrier proteins, there is no mention of ions or charged particles involved, so it can't be channel proteins, therefore it is carrier proteins.
I had to read the question 3 times before finally getting it, these examiners sure know how to throw you off.
Reply 3
Original post by lollypenguin
A student investigated the movement of substances through the cell surface membrane of
yeast cells using an indicator.
The student was supplied with a suspension of yeast cells in a slightly alkaline solution.
The indicator used is yellow in alkaline conditions but turns red in acidic conditions.
The student mixed the indicator with the yeast suspension and labelled the tube containing
this suspension A. The suspension was red/pink in colour.
(i) The student took a small sample from tube A and centrifuged this sample.
After centrifuging, the student observed that the liquid portion was colourless but the
cells at the bottom were red/pink.
Suggest the mechanism by which the indicator enters the cells and suggest the
component of the membrane involved

The answer is active transport and carrier protein but I have no idea why?
Can anyone explain it to me please?


This is almost 3 years late but for anyone coming on to this thread the answer has nothing to do with acidic/alkaline concentration. The only thing you have to realise for this question is that ALL the indicator left the liquid and went into the cell because the liquid turned colorless. This means at some point there had to have been a lower conc. of indicator in the solution than in the cell. Since this is true, the indicator went from low to high conc. at some point so it's active transport. Thus, it has to be carrier proteins because channel proteins don't do active transport.
(edited 9 months ago)

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