If oxygen is not available, or is short supply, anaerobic respiration takes place, and glucose is only partially broken down, generally to lactate or ethanol. The two forms of anaerobic respiration, known as lactic fermentation and alcoholic fermentation, are exogonic but release significantly less energy than aerobic respiration. The chemical process of oxidation is defined as the addition of oxygen, the removal of hydrogen or the loss of electrons. Two steps in this process release ATP, so as a result there is a net gain of two ATP molecules and two NADH for each glucose molecule broken down. During glycolosis oxygen is not required.
Water is used up and hydrogen is made available for the electron transport chain.
With the enzyme citrate synthetase CoA transfers the acetyl group to oxaloacetate to make a molecule of citrate. The remaining steps of the Krebs cycle breakdown the 6-carbon citrate until oxloacetate is reformed. Some steps use up water, others release carbon dioxide. ATP is made. The action of citrate sythetase Electron Transport Chain and Mitochondria Chemical reactions take place only when molecules collide.