Of all the cells and organisms in the world, bacteria and archaebacteria are the: (1) smallest, (2) simplest, (3) most numerous, (4) most metabolically diverse, and (5) oldest. Although for many decades scientists classified archaebacteria in the same category as bacteria, most now recognize archaebacteria to be an evolutionary lineage as distinct from the bacteria (also called eubacteria, or true bacteria) as they are from the complex organisms.

The typical eubacterial or archaebacterial cell is about 10 times smaller than the typical cell of complex organisms, in each dimension; therefore the typical bacterium has a volume a thousand times smaller than that of a typical cell of a complex organism. There are exceptions. The largest known bacterium is Thiomargarita namibiensis, which is one-twenty-fifth of an inch (1 mm) in diameter, which is larger than many complex cells.

Eubacteria and archaebacteria are prokaryotic, which means they do not have a nucleus (Greek, karyo). Human, plant, fungus, and protist cells are eukaryotic because they have a nucleus.

Most eubacteria and archaebacteria have:

- a cell wall. In eubacteria, the wall consists mostly of a combination of protein and polysaccharide called peptidoglycan. Because the Gram stain indicates the presence of peptidoglycan, the Gram positive bacteria are those with a thick cell wall, and the Gram negative bacteria have thinner cell walls. Archaebacteria do not have peptidoglycan walls.

- a cell membrane, which regulates the molecules that can enter and leave the cell. The cell membranes of bacteria resemble those of eukaryotic cells, but the cell membranes of archaebacteria have a different chemical composition from those of all other cells.

- cytoplasm, the liquid portion in which the molecules are dissolved

- ribosomes, which make proteins. Bacteria and archaebacteria have smaller ribosomes than those of eukaryotic cells.

Although bacteria and archaebacteria do not have the complex structures found in the eukaryotic cell, they do carry out most of the same chemical processes. Prokaryotes do not have nuclei, but they do have DNA and use it in a manner similar to eukaryotic cells.

Prokaryotes do not have mitochondria, but many bacteria carry out the same reactions that mitochondria do. Some bacteria have numerous internal membranes and others have simple flagella that allow them to swim. Most bacteria function as single cells, although some can form multicellular aggregates. The shapes of bacteria are relatively simple: round ones are called cocci, rod-shaped ones are called bacilli, and spiral ones are called spirilla (if they have a central axis, they are spirochetes).

Bacteria and archaebacteria are the most numerous cells in the world. A typical spoonful of soil can contain billions of bacteria. . .

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