Mitochondria generate energy for cells from food items the cells take in or create. They convert the chemical energy from compounds into a form accessible by the cells. This energy is ATP, a specific molecule that all cells use to store and transport energy.
Mitochondria are found only in cells with nuclei. A typical animal cell has between 1,000 and 2,000 mitochondria. Mitochondria specifically perform respiration, which uses oxygen to release energy from molecules and bind it into a new form. Respiration is the most efficient way of producing energy available to most cells. Cells that have frequent need to use energy aerobically produce more mitochondria.