Illustration of the structure of an eukaryotic cell. The cell membrane surrounds the internal structures (organelles). At centre is the nucleus (purple), which contains the nucleolus (grey) and chromosomes. DNA (deoxyribonucleic acid) is transcribed to mRNA (messenger ribonucleic acid) in the nucleus. Nuclear pores (yellow) in the nucleus control the transport of molecules between the nucleus and the cytoplasm. Surrounding the nucleus are the rough and smooth endoplasmic reticulum (ER, pink). The rough ER is studded with ribosomes and is the site of active protein synthesis. Vesicles leave the rough ER, releasing their protein cargo into the Golgi apparatus (orange) where the proteins are stored before being re-packaged for transport. Mitochondria (blue) create energy for the cell. Lysosomes (yellow) contain enzymes for digesting foreign particles. Microtubules (not seen) move vesicles and granules and are part of the cytoskeleton.