The next stage in respiratory metabolism is the citric acid cycle, also known as Kreb’s cycle. Enzymes of this cycle and the electron transportation chain are situated in an endogenous organelle, the mitochondrion. Under anaerobic conditions, mature, fully functional mitochondria are not present in Saccharomyces. Since respiration is entirely dependent on oxygen, the enzymes required are only synthesized when needed, so in this respect, the availability of oxygen is a control mechanism for gene expression. In yeast, the expression of these enzymes is also controlled by the concentrations of glucose and other fermentable sugars in the medium. The expression is also suppressed by high concentrations of glucose and this regulatory phenomenon is known as glucose repression or the Crabtree effect (Boulton and Quain, 2001c). When the substrate is not limiting (i.e. present in high concentrations), yeasts depend entirely on fermentation or substrate level phosphorylation for the production of ATP. If the sugar concentration becomes limiting, the yeast need to convert to the respiratory metabolism in order to produce sufficient energy for continued growth and metabolism.