Ribosomes are complex molecular machines found within cells that serve as the site of biological protein synthesis. They were discovered in the mid-1950s by George Emil Palade using an electron microscope and are largely made up of specialized RNA and proteins. Ribosomes come in two subunits of different sizes, a large subunit and a small subunit, and link amino acids together according to messenger RNA instructions to form polypeptide chains and synthesize proteins.