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Section 3.2 Respiration 1994 1 (b) 1(b) Two flasks contained yeast cells growing in equal volumes of a sugar solution. Flask A received a constant air supply whereas flask B did not have any air supply at all. The graph below shows the changes in the number of yeast cells with time:

1(b) (i) Calculate the difference in the number of yeast cells at hour 20 between the two flasks. Give a reason to explain why there is such a difference. (5 marks) (ii) Suggest two reasons for the different rates of reproduction of yeast in flask A at hour 8 and hour 16. (3 marks) (iii) Draw and label a simple experimental set-up to show whether heat is released by the yeast cells in flask A. (4 marks) Answer 1. (b) (i) The difference in no. of yeast cells between flasks A and b at hour 20 = 675-150(1) = 525 (1) Any one of the following sets (1+1+1) EITHER In flask A, the yeast cells undergo aerobic respiration whereas in flask B, the yeast cells undergo anaerobic respiration (1) Anaerobic respiration releases less energy (1) For cell division/growth of the yeast (1) OR In flask A, the yeast cells undergo aerobic respiration whereas in flask B, the yeast cells undergo anaerobic respiration (1) Anaerobic respiration produces ethanol/alcohol (1) Which inhibits the cell division/growth of the yeast (1) (ii) The rate of reproduction of the yeast at hour 8 is much faster than that at hour 16(1) Reasons: At hour 16, nutrients in the solution may become exhausted (1) At hour 16, there may be accumulation of toxic wastes (1) Both cause a decline in the reproduction rate of yeast (iii)


No marks for the whole set-up if it is not workable (e.g. using dry yeast, with an oil layer) insulating layer and stopper (1) thermometer (the bulb must be immersed in the mixture) (1) correct connection of tubes (1) clear, well-labelled and accurate diagram (D) (1) Marker Report (b) (i) Many candidates failed to read correctly from the graph the number of yeast cells at hour 20 for flask A. This was mainly due to careless calculation from the scale of the axis give. In explaining the different results of flasks A and B, many candidates just described the conditions in one flask instead of comparing the conditions of the two flasks to account for the different results. Some candidates mistakenly thought that, under anaerobic conditions, yeast could not carry out respiration and would die due to lack of an energy supply. (ii) The candidates in general could relate the slope of the curve to the reproductive rate of the yeast. Most candidates, however, did not point out that toxic waste would accumulate in the solution after a period of time. (iii) This question assessed students’ ability in experimental design and their knowledge of practical skills. The rather poor performance on this question reflects a general weakness in this area. Very few candidates could present a design that satisfied the condition required: presence of an insulating layer, a thermometer immersed in the yeast and glucose mixture, and tubes for aeration. In general, the quality of drawing was poor; clear, well-labeled and accurate diagrams were very rare. 1997 4 b (b) Yeast is commonly used in the making of bread. bread-making :

The diagrams below show the steps in

(i) (ii) (iii)

Explain what happens to the volume of the dough after keeping it at 30oC for 1 hour. Give a reason why the volume of the dough will not change any more after it has been kept in the oven for 10 minutes. Suggest another industrial application of yeast. 2/8


If the bread is left in a warm and humid place for several days, black dots will be found on the bread surface. Make a labeled drawing to show some of these black dots and their associated structures when observed under a microscope. The volume of the dough increases …………………………………..1 because the yeast carries out anaerobic respiration/alcoholic fermentation ……………………………………………….………….1 which produces carbon dioxide that raises the dough...
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