Bacteria and mollicutes are prokaryotes. These are generally single-celled microorganisms whose genetic material (DNA) is not bound by a membrane and, therefore, is not organized into a nucleus. Their cells consist of cytoplasm containing DNA and small (70S) ribosomes. The cytoplasm in mollicutes is surrounded by a cell membrane only, but in bacteria, it is surrounded by a cell membrane and a cell wall. The cells of all other organisms (eukaryotes) contain membrane-bound organelles (nuclei, mitochondria, and—in plants only—chloroplasts). Eukaryotes also have two types of ribosomes, larger ones (80S) in the cytoplasm and smaller ones (70S) in mitochondria and chloroplasts. In fact, the organelles of eukaryotic cells and prokaryotes have much in common. For example, some of the antibiotics that affect bacteria often inhibit the functions of mitochondria or chloroplasts but do not interfere with the other functions of eukaryotic plant cells.