Why is pyruvate is not oxidized to CO2 and H20 under anaerobic conditions?(2 marks)
The three stages of cellular respiration – pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation – require oxygen in order to occur. Glycolysis can take place without oxygen in a process called fermentation. There is another way that the NADH molecule can be re-oxidized.
Under anaerobic conditions, pyruvate has a different fate. Instead of entering mitochondria, the cytosolic enzyme lactate dehydrogenase converts pyruvate to lactate. Although lactate itself is not utilized by the cell as a direct energy source, this reaction also allows for the regeneration of NAD+ from NADH. NAD+ is an oxidizing cofactor necessary to maintain the flow of glucose through glycolysis. Glycolysis produces 2 ATP per glucose molecule, and thus provides a direct means of producing energy in the absence of oxygen. This process of breaking down glucose in the absence of oxygen is aptly named anaerobic glycolysis.
Anaerobic conditions in yeast convert pyruvate to carbon dioxide and ethanol. This occurs with the help of the enzyme pyruvate decarboxylase which removes a carbon dioxide molecule from the pyruvate to yield an acetaldehyde.