Transmission electron micrograph showing mitochondrion, nucleus and nuclear pores. No magnification given. Mitochondrion, an organelle in the cytoplasm of cells that functions in energy production. Nucleus, a specialized, usually spherical mass of protoplasm encased in a double membrane, and found in most living eukaryotic cells, directing their growth, metabolism, and reproduction, and functioning in the transmission of genetic characters. Nuclear pores, any of numerous structures with complex openings in a nuclear membrane which allow passage of molecules between the nucleus and surrounding cytoplasm; any of the numerous perforations in a nuclear membrane which allow materials to flow in and out.