Mitochondria are found in all cells, but they are particularly densely packed in cells that need the most energy or that regulate energy storage, such as muscle cells, fat cells, and certain brain cells. They are wrapped around the tails of sperm, to provide them enough swimming energy to reach an egg. They divide within the cell, but when it’s the cell’s turn to reproduce, mitochondria are only split between the two daughter cells. In other words, they have no autonomous life; they can live only within cells.
