Cytology - Part III

The Prokaryotic Cell

Bacteria and other prokaryotes are monerans, a diverse kingdom of small usually single-celled organisms, whose origins can be traced to the most ancient forms of life on earth.

If you’re not a prokaryote like the bacteria you must be a eukaryotic organism such as the protista, fungi, plants, or animals.

How life came to be divided into these two fundamental groups is the subject of the endosymbiotic theory.(3)

The main parts of the prokaryotic cell are diagramed below in figure 3.


Click on Fig. 3 above to see an electron micrograph of Bacillus megaterium.

Prokaryotes like most organisms fall into two general categories based on how they obtain their energy. For example:

  • some are autotrophs: Examples: chemosynthetic or photosynthetic. both obtain their food by abiotic means relying on energy of inorganic molecules or sunlight. These organisms are classified as producers.
  • others are heterotrophs: Examples: decomposers or parasites (disease causing microorganisms. Organisms which rely on the metabolism of other living things are called consumers


Cell size for prokaryotes, with the exception of the cyanobacteria, is about 10 times less than representative eukaryotic cells, or from 1 to 10 micrometers.

The cell wall of prokaryotic cells is made of proteoglycans, molecules which are a combination of carbohydrates and protein. What are some functions of proteoglycans?

In addition to the cell wall, many bacteria produce a sheath while others have a slimy protective capsule.

Within the cytoplasm of prokaryotes, surrounded by a single cell membrane, all the metabolic processes such as protein synthesis, respiration, replication, etc. take place.

The genetic material of bacteria, a single circular chromosome, is found in an unbounded area of the cytoplasm called the nucleoid.

Many bacteria have additional genes located in small circular molecules of DNA called plasmids. Plasmids and the genes they contain may be easily transferred from one bacteria to another, possibly even between species.

The flagellum found in most aquatic and soil bacteria provide movement by the unique process of spinning on an axis like a propeller. See Figure 4 below.

Many bacteria have pili. There are two varieties of pili.

Modified July 8, 2005