Mitochondria. Coloured transmission electron micrograph of mitochondria (circles). A mitochondrion is a membrane-bound organelle that exists in eukaryotic cells. It converts energy in the form of glucose into ATP. ATP is then used by cells to provide usable energy to drive reactions - for instance to produce kinetic energy in muscles. Mitochondria have a double membrane and their own DNA. Magnification: x50,000 when printed 10 centimetres wide.