Mitochondria. Coloured scanning electron micrograph of a sectioned cell showing its nucleus (grey background) surrounded by many mitochondria (green) and the cell wall (red). Mitochondria are the principal organelles involved in cellular respiration in mammals; this is a process through which a cell derives the energy necessary for all its metabolic functions by breaking down organic molecules. Mitochondria undergo self-replication by a process which is similar to bacterial cell division. The image was made by using an egg cell from the ovary of a 14 week old foetus. Magnification: x6380 at 6x7cm size.