Of all groups of organisms, prokaryotes show the greatest diversity of biochemical reactions—that is, metabolism. One aspect of metabolism involves nutrition: the use of substances from the environment for energy and for synthesizing new materials. Prokaryotes can be classified into two nutritional categories, based on the substances they require to build new materials for growth.

An organism that builds new molecules from the inorganic compound carbon dioxide is called an autotroph. An organism that requires organic compounds such as sugars to build new molecules for growth is called a heterotroph. Prokaryotes can also be classified into two groups based on their ultimate source of energy. Phototrophs get their energy from sunlight. Chemotrophs get their energy from particular chemicals in the environment. By combining these, we get four fundamental nutritional categories.

Many prokaryotes are photoautotrophs. They carry out photosynthesis and build new molecules from carbon dioxide, like most green plants. The oldest known fossils, dating to the dawn of life on Earth, have forms indistinguishable from modern photoautotrophic eubacteria. Organisms like these are believed responsible for creating an atmosphere rich in oxygen from one that originally had none.

Chemoautotrophs require only carbon dioxide to build new molecules, and obtain energy by oxidizing inorganic substances. Entire ecosystems supported by chemoautotrophic bacteria have been discovered near volcanic vents on the ocean floor.

Most prokaryotes are chemoheterotrophs: they obtain both energy and raw building materials from organic substances. This category includes decomposers, which break down dead organisms and other organic wastes. It also includes parasitic bacteria, which derive their nourishment from other living organisms.

A few bacteria are photoheterotrophs: they can obtain energy from light, but they must consume organic substances as a source of raw materials for growth.

Another set of metabolic categories for classifying prokaryotes is based on their interaction with oxygen. Many prokaryotes are aerobes: they require molecular oxygen in order to grow. Some prokaryotes can’t tolerate oxygen; it’s poisonous to them. These are obligate anaerobes. Their habitats may be deep in the soil, or in the digestive tracts of animals. Between these two extremes are the facultative anaerobes. These prokaryotes can use or at least tolerate oxygen when it is present, although it isn’t required for their growth.

A prokaryote that’s in an environment containing the proper nutrients is very likely to reproduce! Prokaryotic cells reproduce by binary fission, an asexual process in which one parent cell divides into two daughter cells. Binary fission in prokaryotes is much simpler than mitosis, which occurs in eukaryotes.

The chromosome, which is attached to the plasma membrane, replicates. The new copy of the chromosome is attached at a point slightly away from the attachment site of the parent chromosome. As the plasma membrane grows, it separates the chromosomes. A furrow of membrane and cell wall separates the parent cell into two daughter cells, each with one copy of the chromosome.

It’s not unusual for many prokaryotes to remain attached in filaments or clusters after reproducing. Prokaryotes have a tremendous ability to multiply. Some species can divide every 20 minutes if conditions are favorable.

With a reproductive process resulting in exact copies of the chromosome, the most important source of genetic variation in prokaryotes is mutation. However, it is also possible for prokaryotic cells to exchange genetic material. Prokaryotes can pick up DNA from their surroundings in a process called transformation. DNA can be transferred actively via conjugation. In this process, two bacterial cells come in contact, and one synthesizes a tube through which DNA can travel. In the process called transduction, a virus carries DNA from one prokaryotic cell to another.

Many prokaryotic cells are capable of motion. Most commonly, motile bacteria employ one or more flagella to move about. Prokaryotic flagella are quite different from those of eukaryotes. Most of the flagellum is a somewhat rigid helical filament composed of protein. The filament is anchored in the cell’s membrane and wall by a complex structure, which can rotate the filament like a whip mounted in a wheel.

Many bacteria are known for the diseases they cause in humans. A short list of some important ones might include plague, cholera, tuberculosis, tetanus, diphtheria, syphilis, and Lyme disease. Some bacteria cause their symptoms by releasing poisonous substances called toxins, which damage the cells and tissues of their host. But it’s important to understand that only a small number of bacterial species cause disease. Many are beneficial to humans.

Some are important sources of medicinal drugs, like the antibiotics streptomycin and tetracycline used to fight bacterial disease. Prokaryotes have an important role in industry and technology, where humans have taken advantage of their metabolic properties. For example, they’re used in sewage treatment plants to break down wastes.

Species like Escherichia coli (E. coli for short) are workhorses in genetic engineering.

For instance, they’re used to produce and manipulate large quantities of particular DNA molecules. The DNA, which might have come from a plant or animal, can then be sequenced, to learn more about the species from which it’s derived. Genetically engineered DNA might also be used to produce new protein molecules with special properties, like medicines, or industrial enzymes.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education