The simplest organisms are the bacteria. Their cells are called prokaryotic (or procaryotic) because no membrane-enclosed nucleus is present. Cells of all other organisms contain nuclei separated from the cytoplasm by membranes. They are called eukaryotic.
While viruses (Chapter 5) are sometimes regarded as living beings, these amazing parasitic objects are not complete organisms and have little or no metabolism of their own. The smallest bacteria are the mycoplasmas. They do not have the rigid cell wall characteristic of
most bacteria. For this reason they are easily deformedand often pass through filters designed to stop bacteria. They are nutritionally fussy and are usually, if not always, parasitic. Some live harmlessly in mucous membranes of people, but others cause diseases.
For example, Mycoplasma pneumoniae is responsible for primary atypical pneumonia.
Cells of mycoplasmas sometimes grow as filaments but are often spherical and as small as 0.3 micrometer (µm) in diameter. Their outer surface consists of a thin cell membrane about 8 nanometers (nm) thick. This membrane encloses the cytoplasm, a fluid material
containing many dissolved substances as well as submicroscopic particles. At the center of each cell is a single, highly folded molecule of DNA, which constitutes the bacterial chromosome. Besides the DNA there may be, in a small spherical mycoplasma, about
1000 particles ~20 nm in diameter, the ribosomes. These ribosomes are the centers of protein synthesis. Included in the cytoplasm are many different kinds of proteins, but there is room for a total of only about 50,000 protein molecules. Several types of RNA as
well as many smaller molecules are also present. Although we don’t know what minimum quantities of proteins, DNA, and other materials are needed to make a living cell, it is clear that they must all fit into the tiny cell of the mycoplasma.