The structure that is responsible for producing new molecules is called a ribosome. Ribosomes are large, complex structures found in the cells of all living organisms. They are composed of two subunits, one large and one small, that come together to form a functional ribosome. Ribosomes are responsible for protein synthesis, which is the process by which cells create new proteins. This process involves reading the genetic code of a molecule of messenger RNA (mRNA) and using this information to assemble a chain of amino acids into a protein.