The mitochondria of the cell make the ATP of the cell. They provide energy for the cell to keep functioning.
http://hyperphysics.phy-astr.gsu.edu/hbase/biology/mitochondria.html
http://micro.magnet.fsu.edu/cells/mitochondria/mitochondria.html
http://hyperphysics.phy-astr.gsu.edu/hbase/biology/mitochondria.html
http://micro.magnet.fsu.edu/cells/mitochondria/mitochondria.html