Ribosomes (from ribonucleic aci and "Greek: soma (meaning body)") are complexes of RNA and protein that are found in all cells. Prokaryotic ribosomes from archaea and bacteria are smaller than most of the ribosomes from eukaryotes such as plants and animals. However, the ribosomes in the mitochondrion of eukaryotic cells resemble those in bacteria, reflecting the evolutionary origin of this organelle.