Mitochondria are rod-shaped organelles are basically considered the power producers of the cell, it converts oxygen and nutrients into adenosine triphosphate or ATP, which is the chemical energy,also known as "currency" of the cell which powers the metabolic actions of the cell. This process is called aerobic respiration and it is the reason animals breathe oxygen. Cellular repiration happens in the mitochodrion. The 3 phases of cellular respiration are Krebs Cycle, Electron Transport and Glycolysis (Fermentation). Glycolysis takes place in the cytoplasm while the Krebs cycle and electron transport take place in the mitochondria.