Human cell, illustration. The cell membrane (brown) surrounds the internal structures (organelles). At centre is the nucleus (surrounded by transparent membrane), which contains the nucleolus (orange) and chromosomes (X-shaped). DNA (deoxyribonucleic acid, double helices) is transcribed to mRNA (messenger ribonucleic acid, single helices) in the nucleus. Surrounding the nucleus are the rough (green) and smooth (brown) endoplasmic reticulum (ER). The rough ER is studded with ribosomes (green spheres), the sites of active protein synthesis. Vesicles (blue spheres) leave the rough ER, releasing their protein cargo into the Golgi apparatus (yellow, at bottom) where the proteins are stored before being re-packaged for transport. Mitochondria (transparent, one at bottom centre) create energy for the cell. Lysosomes (purple spheres) contain enzymes for digesting foreign particles. Microtubules (blue rods) move vesicles and granules and are part of the cytoskeleton.