Baker's yeast, Saccharomyces cerevisiae, has two metabolic pathways for the production of ATP, aerobic respiration and fermentation. Oxygen is not used in fermentation. These pathways are summarised in Fig. 1.1. [Figure 1.1] Some students were researching details of the respiration of yeast. In their research, the students found information about the Crabtree effect. This effect occurs when yeast is kept in high concentrations of glucose. The yeast gains sufficient ATP from substrate-linked phosphorylation during glycolysis. Yeast does not carry out the Krebs cycle or oxidative phosphorylation and therefore does not use oxygen. The students wanted to investigate the effect of temperature on the activity of yeast and decided to use yeast solutions in which the yeast obtained all of its ATP by fermentation. • The students made a suspension of yeast from 1g of dried yeast and 25cm³ of water. • The suspension was left for 2 hours at 20°C. • After 2 hours, 25cm³ of glucose solution was added to the yeast suspension and the mixture was stirred. • Some of the mixture was added immediately to the apparatus shown in Fig. 1.2. The students measured the volume of carbon dioxide produced by yeast fermentation.

1. (a)(i)State the independent variable and the dependent variable in this investigation. independent variable dependent variable [2]
2. (a)(ii)Explain why the students stirred the mixture of yeast and glucose before adding it to the tube in the apparatus in Fig. 1.2.[1]
3. (a)(iii)Describe a method using the apparatus, set up as shown in Fig. 1.2, that the students could use to find the temperature at which yeast carries out fermentation at its maximum rate. Your method should be set out in a logical order and be detailed enough for another person to follow. You should not include details of how to make the yeast suspension or how to set up the apparatus.[7]
4. (a)(iv)Complete the sketch graph to predict the results that you would expect from the method that you have given in part (iii). Include axes labels with units in your answer.[2]
5. (b)Samples taken from the flasks kept in the experimental conditions were put into the apparatus shown in Fig. 1.2 to measure the volume of carbon dioxide produced by the two yeast species. The results are shown in Fig. 1.3. The final volume of carbon dioxide produced by the sample of yeast from flask 1 at 30 minutes is shown as a maximum volume of 100% in Fig. 1.3. All other volumes were calculated as a proportion of this maximum. This allows a direct comparison to be made between the two species. State and explain the evidence in Fig. 1.3 that supports the idea that C. tropicalis does not show the Crabtree effect.[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.