Transmission Electron Micrograph (TEM) showing mitochondria and rough endoplasmic reticulum. The mitochondrion (plural mitochondria) is a double membrane-bound organelle found in all eukaryotic organisms. Mitochondria generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. Unless specifically stained, they are not visible. In addition to supplying cellular energy, mitochondria are involved in other tasks, such as signaling, cellular differentiation, and cell death, as well as maintaining control of the cell cycle and cell growth. The endoplasmic reticulum (ER) is a type of organelle in eukaryotic cells that forms an interconnected network of flattened, membrane-enclosed sacs or tube-like structures known as cisternae. The surface of the rough endoplasmic reticulum (often abbreviated RER or Rough ER) (also called granular endoplasmic reticulum) is studded with protein-manufacturing ribosomes. Magnification unknown.