Illustration of the ultrastructure of a typical human cell. The cell nucleus is spherical (coloured purple), its outer membrane pitted with pores. Two nucleoli (red) are visible within the nucleus. Surrounding the nucleus, and continuous with the nuclear membrane, is the endoplasmic reticulum (ER, mauve). Some parts of the ER are studded with ribosomes (red), sites of active protein synthesis. Also featured is the Golgi Body (yellow-green, bottom), associated with the storage & subsequent transport of proteins produced by the ER and several green, cigar- shaped mitochondria, the sites of energy synthesis within the cell.