A ribosome is a biological molecule made of RNA and proteins. The structure of a ribosome is complex, and it is responsible for making the millions of proteins that are needed by cells.
A ribosome may be located in many places within the cell. Some are in the cytosol, others are bound to cellular membranes. Membrane-bound ribosomes are responsible for the characteristic roughness of the endoplasmic reticulum when seen under a microscope.
A ribosome is made of two pieces (subunits). These two subunits are named according to their ability to sediment on a special gel. The bigger the number given to the subunit the bigger the molecule. A typical eukaryotic cell ribosome consists of two subunits named 60S (large subunit) and 40S (small).