Mitochondrion. Colored Transmission Electron Micrograph (TEM) of a mitochondrion (red) inside a cell, showing the double outer membrane and numerous internal membranes (cristae) where respiration takes place. The folded structure outside the mitochondrion (right) is rough endoplasmic reticulum, where proteins are manufactured. The green circles at left are secretory granules containing enzymes made by the cell. Mitochondria provide cells with energy by oxidizing sugar and fat. They are found in almost all cells have their own DNA, and replicate autonomously. This cell is from digestive enzyme-secreting (acinar) tissue from a bat's pancreas. Magnification unknown.