"Mitochondria" are the membrane-bound structures that produce energy for cells. The mitochondria are thought to be descended from a once free-living cell that set up a symbiotic relationship with a host cell.
Mitochondria are organelles, membrane-bound structures that perform specific functions within a cell. Cells vary in the number of mitochondria they have, depending on their energy needs. If a cell needs more energy to be produced, then the mitochondria make more of themselves by dividing in two.
A normal animal cell may have between 1,000 to 2,000 mitochondria. Muscle and fat cells have many mitochondria, as they have high energy needs and must store energy. Respiration is the process by which mitochondria produce energy in the form of a molecule called ATP.