Turn over 33 11 Yeast, Saccharomyces cerevisiae, is a single-celled organism used in brewing and baking. The photograph shows yeast cells seen using a light microscope. (Source: © Kateryna Kon/Shutterstock) 34 A student counted the number of yeast cells, at 90-minute intervals, growing in a liquid culture at 30°C. The graph shows the results. 12 11 10 9 8 7 6 5 4 3 2 1 Time / hours Number of yeast cells × 102 per cm3 0 200 100 0 400 300 Turn over 35 (a) (i) Calculate the percentage increase in the number of yeast cells from 2 hours to 6 hours. (2) Answer .............................................................. % (ii) Predict the number of cells per cm3 of the culture at 12 hours. Assume that the culture continues to grow at the same rate. (1) Answer .............................................................. × 102 per cm3 (b) Describe two precautions that would prevent the yeast culture becoming contaminated with bacteria. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 36 (c) The student counted the yeast cells using a haemocytometer. This is a slide with chambers containing a known volume of liquid. The yeast cells are counted when viewed using a light microscope. Any yeast cells totally within the counting square should be counted, along with those which overlap the top or left-hand side of the square. (i) The volume of liquid in the counting square shown is 0.004 mm3 . (Source: © Joseph Elsbernd/Flickr) Calculate the number of yeast cells per mm3 . Give your answer in standard form. (2) Answer .............................................................. per mm3 (ii) Identify two causes of inaccuracy in this method of counting cells. 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Turn over 37 (d) The table shows the number of cells counted using a haemocytometer. Time / hours Number of cells 1 2 3 4 5 Mean Standard deviation (SD) 0.0 1 2 2 1 2 1.6 0.5 1.5 2 3 3 3 2 2.6 0.5 3.0 4 9 9 3 7 6.4 4.5 13 10 12 9 16 12.0 2.7 6.0 43 31 18 14 15 24.2 12.5 (i) The student calculated the standard deviations using the following formula. SD x x n 2 1 Calculate the standard deviation for the sample at 3 hours. (3) Answer .............................................................. 38 (ii) The student concluded that the yeast population doubles every 90 minutes. Comment on the validity of this conclusion. 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(Total for Question 11 = 15 marks) TOTAL FOR PAPER = 120 MARKS 39 BLANK PAGE 40 BLANK PAGE