Illustration of a human cell. The cell membrane surrounds the internal structures (organelles). At centre is the nucleus (blue), which contains the nucleolus and chromosomes. DNA (deoxyribonucleic acid) is transcribed to mRNA (messenger ribonucleic acid) in the nucleus. Surrounding the nucleus are the rough (pink, left) and smooth (blue) endoplasmic reticulum (ER). The rough ER is studded with ribosomes (dark purple spheres), the sites of active protein synthesis. Vesicles leave the rough ER, releasing their protein cargo into the Golgi apparatus where the proteins are stored before being re-packaged for transport. Mitochondria (green) create energy for the cell. Lysosomes contain enzymes for digesting foreign particles. Microtubules move vesicles and granules and are part of the cytoskeleton.