Ribosomes are responsible for protein synthesis in a cell by translating messenger RNA, or mRNA, into amino acid chains. Ribosomes exist in the cell's fluid component and are bound to the rough endoplasmic reticulum.
Two subunits comprise a ribosome: a small subunit and a large subunit. The subunits come together during protein synthesis when the ribosome attaches to a strand of RNA. The small subunit translates, or reads, the RNA strand, determining the necessary amino acids.
A second variety of RNA that is known as transfer RNA or tRNA gathers amino acids and brings them to the ribosome. The tRNA enters the small ribosomal subunit and binds to mRNA along with its amino acids. Then the large ribosomal subunit begins linking the amino acids together to form a polypeptide chain. The polypeptide chain folds itself into the three-dimensional structure that determines its function. The amino acid sequence as well as various environmental factors determine the three-dimensional shape. This folding is essential to proper protein function, and misfolded proteins can have a serious, even lethal, effect upon an organism.
Ribosomes exist in the fluid component, or cytosol, of a cell. These free ribosomes produce proteins that function within the cytosol. Bound proteins exist on the cell's rough endoplasmic reticulum, and these ribosomes create proteins for the cell membrane or for dispersal to other cells.