Ribosomes, one of the most complex aspects of the molecular machine, are the site of protein synthesis in a cell. They consist of a large and small subunit composed of ribosomal RNA and protein. Ribosomes differ from most other organelles because they are not bound by a membrane; however, compared with membrane-bound organelles, ribosomes are tiny structures. The number of ribosomes differs depending on the type of cells. For example, a bacterial cell will typically have a few thousand ribosomes, while a human liver cell contains several million ribosomes. Actively growing mammalian cells contain five to ten million ribosomes—all that have to be synthesized each time the cell divides.