Transmission electron micrograph showing smooth endoplasmic reticulum with large mitochondria. Magnification 30,000x Endoplasmic reticulum, a network of tubular membranes within the cytoplasm of the cell, occurring either with a smooth surface (smooth endoplasmic reticulum) or studded with ribosomes (rough endoplasmic reticulum), involved in the transport of materials. Mitochondrion, an organelle in the cytoplasm of cells that functions in energy production.