20 April 2015

#84 Question 7

Beetroot cells contain a red pigment that cannot normally escape from the cells through the cell surface membrane. A student carried out an investigation into the effect of temperature on the permeability of the cell surface membrane of beetroot cells.

She measured permeability by using a colorimeter to measure the absorbance of green light by the solutions in which samples of beetroot had been immersed. The greater the absorbance, the more red pigment had leaked out of the beetroot cells. The graph above shows her results.

(a) With reference to the graph, describe the effect of temperature on the absorbance of light in the colorimeter.
(3 marks)
(b) With reference to the structure of cell membranes, explain the effects you have described in (a). 
(4 marks)
(Total 7 marks)

Candidate A

(a) Between 0 and 30 the absorbance goes up very slightly . ü Above 40°C it goes up
very quickly. û Then it starts to level out at about 70°C .ü

* The student has correctly identified the three main regions of the graph, stating where changes in gradient occur. However, he or she has used the term 'quickly', which is not correct because the graph does not show anything about time. He or she could also have gained a third mark by quoting some figures from the graph. 2/3

(b) High temperatures damage the proteins in the membrane . ü They become denatured, so they leave holes ü in the membrane that the beetroot pigment can get through.

~ This is a good answer as far as it goes, clearly expressed. However, it is not enough to store a full set of marks. 2/4

Candidate B

(a) The general trend is that the higher the temperature, the greater the absorbance. Between 0 and 30°C, the absorbance increases very slightlyü from about 16 to 18 arbitrary units. Above 40°C it increases much more steeply, ü  levelling off at about 70°C .ü The maximum absorbance is 98 arbitrary units.

*A good answer. However, although the candidate has quoted some figures from the graph, he or she has not manipulated them in any way - for example, he or she could have calculated the increase in absorbance between 0 and 30°C.3/3

(b) As temperature increases, the phospholipids and protein molecules in the membrane move about faster ü  and with more energy. This leaves gaps in the membrane, so the beetroot pigment molecules can get through ü  and escape from the cell. The protein molecules start to lose their shape at high temperatures ü because their hydrogen bonds break, ü so the protein pores get wider ü which increases permeability.

*An excellent answer. 4/4

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