Bacteria is the domain of prokaryotic organisms that are considered to be true bacteria, and Archaea is the domain of prokaryotic organisms able to live in extreme environments. Eukarya differs from the prokaryotic domains in basic characteristics of cellular organization, biochemistry, and molecular biology. Further, unlike the prokaryotic organisms, many of the Eukarya are truly multicellular. Eukaryotic cells,which are structurally more complex than prokaryotic ones, have many of their cellular functions segregated into semiautonomous, membrane-bound cell regions, called organelles. The principal organelle is the nucleus, which contains the geneticmaterial, deoxyribonucleic acid (DNA). In prokaryotic organisms, in contrast, the DNA is not segregated from the rest of the cell. Other distinguishing organelles in eukaryotic cells include the mitochondria. These are the sites of respiration, in which energy is generated by breaking down food, in the presence of oxygen, into water and carbon dioxide. The plants and the algae have additional organelles, the plastids.
