These are spherical or rod-shaped structure surrounded present in both plant and animal cells. These are surrounded by double membrane containing matrix. The outer membrane is smooth and the inner membrane is folded into finger-like projection called cristae. Large numbers of knob-like structures are present in the inner membrane called oxysomes. The head of the oxysomes contains the enzymes of ATP synthesis. Electron transport occurs in inner membrane. ATP are synthesized and stored in mitochondria. Hence mitochondria are called as power house of cells.