The organelle responsible for cellular respiration is the mitochondrion. It is often called the powerhouse of the cell because it produces most of the cell's ATP (adenosine triphosphate), which is the primary energy currency. Cellular respiration processes such as pyruvate oxidation and the citric acid cycle occur in the mitochondrial matrix, while the electron transport chain and ATP synthesis take place across the inner mitochondrial membrane's cristae. Glycolysis, the initial step of cellular respiration, happens in the cytoplasm but most of the process occurs inside the mitochondria.