Microbiology: Introduction to Prokaryotes, Eukaryotes

Microorganisms and all other living organisms are classified as prokaryotes or eukaryotes. Prokaryotes and eukaryotes are distinguished on the basis of their cellular characteristics. For example, prokaryotic cells lack a nucleus and other memorane-bound structures known as organelles, while eukaryotic cells have both a nucleus and organelles (Figure 1 ).

Figure 1

The important cellular features of (a) a prokaryotic cell (a bacterium) and (b) a eukaryotic cell.

Prokaryotic and eukaryotic cells are similar in several ways. Both types of cells are enclosed by cell membranes (plasma membranes), and both use DNA for their genetic information.

Prokaryotes include several kinds of microorganisms, such as bacteria and cyanobacteria. Eukaryotes include such microorganisms as fungi, protozoa, and simple algae. Viruses are considered neither prokaryotes nor eukaryotes because they lack the characteristics of living things, except the ability to replicate (which they accomplish only in living cells).