Mitochondria
The mitochondria is an organelle found inside most eukaryotic cells. They generate the energy that the cell needs, so they are sometimes compared to power plants (plant as not in the green shrubs, flowers or trees, but plant as in a building in which things are made). The mitochondria makes adenosine triphosphate (ATP) which the cell can use as an energy source. The synthesis of ATP is done through the citric acid cycle, which uses glucose, pyruvate, and NADH (produced outside of the mitochondria) as the input molecules. The citric acid cycle consists of a large number of enzyme, and is pretty complicated, but the end product is C02 and GTP, which is converted to ATP. The reason that the mitochondria is so important is because it is called the "powerhouse" of the cell. It is highly folded to allow many reactions to take place inside, mostly processes that use proteins and enzymes to produce ATP and acid in cellular respiration. Another important detail of the mitochondria is that it is believed to have once been a free living bacteria that was engulfed by an accent cell, as it reproduces on it's own and contains it's own DNA separate from the nucleus. This DNA is only passed down by women, and has been helpful in training ancient maternal lineages.