Part (a) is a simplified 3D diagram of an animal cell. The cell is bounded by a thin, flexible cell membrane. Inside is the nucleus, which is also membrane-bound and contains the DNA. Also present are membrane-bound components called organelles, and many tiny structures called ribosomes. The fluid interior of the cell is called the cytosol. The ribosomes are either free in the cytosol or attached to the outside of membranous sacs. Part (b) is a simplified 3D diagram of a bacterium. A thick cell wall surrounds the cell membrane. There is no nucleus nor are there any interior membranes; the ‘naked’ DNA and all the ribosomes are free in the cytosol.