Biological membranes are like the vigilant guards of cells, separating the inside from the outside world. Picture them as dynamic barriers, selectively letting in essential molecules while keeping out the rest.
(b) 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.
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