Ribosomes are where proteins are made. Cells that need to produce more proteins have a high ribosome count, such as a human liver cell, which may have a few million ribosomes. Ribosomes can either be free, such as the one in the illustration at right, or bound to endoplasmic reticulum or the nucleus. Some also exist within the mitochondria. Most of the proteins produced by free ribosomes function in the cytosol, such as enzymes that catalyze metabolic processes. Bound ribosomes typically create proteins to be included into membranes, for packaging within certain organelles, or to be sent out from the cell. Cells, such as pancreatic cells, that specialize in secretion generally have many bound proteins. Bound and free ribosomes are structurally identical and interchangeable, and cells can adjust the numbers of each type when necessary.

Structurally, ribosomes appear to have three creases on their surface: one holds mRNA (messenger) and two hold tRNA (transfer). Enzymes bound to the ribosome attach amino acids, in the appropriate sequence, to the protein that is being created by the ribosome.