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. The reactions in respiration occur at an elevated rate and the enzymes involved are in an organised system. Many are tightly bound together or found in enzyme complexes or in ordered sequences within the membrane of mitochondria. Intermediate products within the reaction sequence can therefore be 'handed' form one enzyme to the other.