Cellular respiration begins with the glycolytic pathway. Both anaerobic respiration and aerobic respiration begin with glycolysis to obtain pyruvate, which can then undergo the citric acid cycle in aerobic respiration or fermentation in anaerobic respiration.
After the citric acid cycle in aerobic respiration, many electron transport chain reactions occur in the mitochondria of organisms. These reactions form large concentrations of adenosine triphosphate, which is used by the cell as energy. In anaerobic respiration, pyruvate can enter the fermentation pathway to produce large concentrations of adenosine triphosphate. Both aerobic and anaerobic respirations lead to adenosine triphosphate production to give the cell and the organism energy. Adenosine triphosphate is the main energy source for all living organisms.