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.