Mitochondria. Coloured transmission electron micrograph (TEM) of a section through mammalian tissue culture cells showing mitochondria. The mitochondria (red/yellow) have double outer membranes and numerous internal membranes called cristae (red), where respiration takes place. Mitochondria provide cells with energy by oxidising sugars and fats. They are found in almost all cells, have their own DNA (deoxyribonucleic acid) and replicate autonomously. They are thought to have originated from parasitic bacteria that invaded primitive cells and became symbiotic. Magnification: x4000 at 35mm size.