Answer: Mitochondria is considered as the power house of the cell as it helps in producing large amount of energy in the form of ATP (adenosine triphosphate) that is used in driving various cellular activities.
Large amount of ATP is produced through Aerobic cellular respiration, which is a process of oxidation of food (glucose) through a series of three steps-
1) Glycolysis (occurs in cytoplasm where glucose breakdown to form 2 pyruvate molecules and release energy as ATP and NADH)
2) Kreb cycle (occurs in the mitochondrial matrix) - pool of energy (ATP, NADH, FADH2) is generated by oxidation of acetyl coenzyme A ( produced from pyruvate).
3) Electron transport chain (occurs in the inner mitochondrial membrane) also called oxidative phosphorylation. It involves the transport of electrons (from electron carriers NADH, FADH2) through a series of protein complexes, which ultimately generates large amount of ATP.
Thus, mitochondria is described as the engine of the cell.