Mitochondria are the largest organelles in cytoplasm. They are spherical, oval, or rod-like bodies, about 0.5 μm – 1.0 μm in diametre and up to 7 μm in length. The structure of mitochondria is shown in Figure 1.4. Mitochondria are considered to be the power houses of the cell because energy is released from oxidation that takes place in these organelles. The energy released is stored in the form of ATP. Mitochondrion is covered by a smooth outer membrane and an inner membrane. The inner core is known as the matrix. The inner membrane is folded to form cristae. Matrix contains enzymes concerned with oxidative phosphorylation and the metabolism of carbohydrate, lipids, and amino acids. The outer membrane controls membrane permeability.