As you saw in Key Concept 1.1, biologists classify all living things into three domains: Archaea, Bacteria, and Eukarya. The organisms in Archaea and Bacteria are collectively called prokaryotes, and they have in common a prokaryotic cell organization. A prokaryotic cell does not typically have membrane-enclosed internal compartments; in particular, it does not have a nucleus. The first cells were probably similar in organization to those of modern prokaryotes.

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Eukaryotic cell organization is found in members of the domain Eukarya (eukaryotes), which includes the protists, plants, fungi, and animals. In contrast to prokaryotic cells, eukaryotic cells contain membrane-enclosed compartments called organelles. The most noticeable organelle is the cell nucleus, where most of the cell’s DNA is located and where gene expression begins:

Just as a cell is an enclosed compartment, separating its contents from the surrounding environment, so each organelle provides a compartment that separates molecules and biochemical reactions from the rest of the cell. This “division of labor” provides possibilities for regulation and efficiency that were important in the evolution of complex organisms and helps explain the complexity of eukaryotic cells relative to prokaryotic cells.