The *ribosomes of prokaryotes and eukaryotes are similar in that both types consist of two different-sized subunits. Eukaryotic ribosomes are somewhat larger than those of prokaryotes, but the structure of prokaryotic ribosomes is better understood. Chemically, ribosomes consist of a special type of RNA called ribosomal RNA (rRNA). Ribosomes also contain more than 50 different protein molecules, which fit together noncovalently, employing forces for protein interaction such as hydrophobic interactions (see Figure 3.12). Additionally, some of the proteins bind noncovalently to the rRNA.

In prokaryotic cells, ribosomes generally float freely in the cytoplasm. In eukaryotic cells they are found in multiple places: floating freely in the cytoplasm or attached to the surface of the endoplasmic reticulum (a membrane-bound organelle), inside the mitochondria, and inside the chloroplasts in plant cells. In each of these locations, the ribosomes are molecular factories where proteins are synthesized. Although they seem small in comparison with the cells that contain them, by molecular standards ribosomes are huge complexes (about 25 nm in diameter), made up of several dozen different molecules.

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Because they lack membranes and are not a compartment, ribosomes are not classified as organelles. Rather, like the cytoskeleton they are called cell structures.