Rough endoplasmic reticulum and mitochondria. Transmission electron micrograph of mitochondria (grey circles) and rough endoplasmic reticulum in a mouse liver cell. Mitochondria oxidize sugars and fats to supply the cell with energy. The are bound by a double membrane and contain numerous folded membranes called cristae, where respiration takes place. The endoplasmic reticulum (ER) is a network of membranes that divides the cell into compartments. Rough ER is studded with protein- manufacturing bodies called ribosomes (small black particles). Magnification x57,000 at 10x8 inch size.