Muscle cells, sperm cells, and kidney cells have a large number of mitochondria because they have high energy demands. Mitochondria are the powerhouses of the cell, responsible for producing the energy currency of the cell known as ATP through a process called cellular respiration. Muscle cells need a lot of energy to contract and move, sperm cells need energy to swim and fertilize an egg, and kidney cells need energy to filter and reabsorb ions and molecules in the blood. Having more mitochondria allows these cells to produce more ATP and meet their high energy demands.