Introduction

Prokaryotes include the domains Bacteria and Archaea. 

Prokaryotes are single-celled organisms. They are the smallest, simplest organisms. They are abundant in the air, water, soil, and on most objects.

Structure

Prokaryotes are small (0.5 - 1.5 microns).

A plasma membrane surrounds the cell. As in eukaryotes, the plasma membrane is involved in the movement of materials into and out of the cell.

Prokaryotes do not have membrane-bound organelles such as a nucleus, mitochondria, chloroplasts, golgi apparatus, or endoplasmic reticulum.

Many enzymes such as those needed in cellular respiration are attached to the plasma membrane. The plasma membrane may be folded and extend into the cell, and function in cellular respiration. In eukaryotes, the enzymes needed for cellular respiration are located within the mitochondrion. 

Photosynthetic prokaryotes do have membraneous vesicles where photosynthetic pigments (chlorophyll molecules) are located. These structures are called thylakoids.

Ribosomes are the only cytoplasmic organelles. They are smaller than eukaryote ribosomes.

The nucleoid is a region where the circular chromosome (DNA) is located.

Plasmids are accessory rings of DNA. Some biotechnology techniques involve the use of plasmids as vectors to insert foreign DNA into the bacteria. For example, human genes are inserted into bacteria by first splicing them into a plasmid. The plasmid is then taken up by a bacterium.

The cell wall prevents bursting or shrinking when the osmotic concentration changes.

The cell is surrounded by a capsule (attached) and/or by a loose gelatinous sheath (slime layer).

Some move by means of flagella. The flagellum contains a hook and a basal body. It rotates 360° to propel the cell.

Many prokaryotes adhere to surfaces by short hair-like structures called fimbriae.

Prokaryotes have a single circular chromosome that may be 500 times the length of the cell. It is attached to the inside of the plasma membrane.

The chromosome lacks the associated proteins found in eukaryotes. It consists of only DNA.

Binary Fission

Prokaryotes reproduce by binary fission.

Binary fission differs from mitosis in that a spindle is not utilized. The cell does not go through the stages of mitosis. The spindle apparatus evolved later in eukaryotes.

During cell division, the DNA replicates. The replicating DNA molecules attach to the plasma membrane. As the cell elongates, the chromosomes are pulled apart.

When cell is approximately twice its original length, the plasma membrane grows inward and a cell wall forms between the two cells.

Two more chromosomes may start to develop before the first two are separated from each other.

Sexual reproduction in Eukaryotes combines genes from two different individuals and thus promotes variation. Prokaryotes do not reproduce sexually but the processes listed below promote genetic recombination.

Conjugation – A cell with DNA called F factor is able to replicate and transfer a copy if it's DNA to another cell without F factor through a tube called a sex pilus.  F factor may exist as a plasmid or become integrated into the chromosome. If it is integrated into the chromosome, it may also transfer part of the chromosome. The sex pilus usually breaks before the entire DNA from the donor cell is transferred. The DNA that is transferred is used to replace similar genes in the recipient cell. Enzymes destroy the replaced genes.

Transformation - occurs when a bacterium picks up fragments of DNA released by dead bacteria or secreted by live bacteria.

Transduction is when bacteriophages (viruses) carry portions of bacterial DNA from one cell to another.

Mutation is a major source of variation in prokaryotes. A rapid mutation rate coupled with rapid reproduction promotes variation.

Some bacteria form endospores when environmental conditions become unfavorable.

Endospores are DNA and a portion of cytoplasm encased in a tough cell wall. They are resistant to extremes in temperature, drying, and harsh chemicals.

Prokaryotic Diversity

Classification

Traditionally, staining techniques, cell shape, mode of nutrition, and mode of cellular respiration have been used to classify prokaryotes but these techniques may not reveal evolutionary relatioinships. These characteristics are useful, however, for identifying certain kinds of prokaryotes.

Living organisms require organic compounds for food. Organic refers to molecules that contain carbon and hydrogen. Examples of organic nutrients are carbohydrates (sugars, starches), lipids, and proteins.

Autotrophs are organisms that make their own organic food. Heterotrophs consume food that is already present in the environment. For example, plants are autotrophs because they make their own food by photosynthesis. Animals are heterotrophs because they obtain their food by eating it.

There are two kinds of Autotrophic prokaryotes. Those that make organic food using energy from sunlight are photosynthetic. Autotrophs that make organic food using energy from inorganic chemicals are chemosynthetic.

Photoautotrophic Prokaryotes

The first photosynthetic prokaryotes to evolve did not produce oxygen.

Cyanobacteria evolved later with the same kinds of chlorophyll found in plants. During photosynthesis, water (H₂O) molecules are split and O₂ molecules are released. The balanced equation is below.

Energy + 6CO₂ + 6H₂O ® C₆H₁₂O₆ + 6O₂

The green sulfur bacteria and purple sulfur bacteria do not split water during photosynthesis. Instead, they split H₂S; oxygen is therefore not released.

Photosynthetic prokaryotes have extensions of the plasma membrane called thylakoids. Many of the molecules needed in the reactions of photosynthesis are found within the thylakoid membrane.

Chemosynthetic Prokaryotes

Chemosynthetic prokaryotes obtain energy to make their organic food by oxidizing high-energy inorganic compounds (hydrogen gas, ammonia, nitrites, and sulfides) instead of consuming organic nutrients or using sunlight.

Many chemosynthetic prokaryotes are anaerobic; they are often found deep in the sediments of lakes and swamps.

Chemosynthetic prokaryotes form the basis of the food chain for some communities 2.5 km beneath the sea. Energy for these communities comes from hydrothermal (volcanic) vents. The hot water pouring out of these vents contains high concentrations of inorganic minerals such as sulfides that can be used as an energy source by the prokaryotes..

Heterotrophic prokaryotes feed on organic matter by secreting enzymes and absorbing the digested material.

Most heterotrophic prokaryotes are aerobic.

Feeding

Three types of heterotrophs are described below. Each category is determined by the feeding mode.

Saprotrophic organisms are decomposers. They play a critical role in recycling (releasing) nutrients that are tied up in the bodies of dead organisms. Most heterotrophic prokaryotes fall into this category.

Parasites are organisms that live in close association with another species and one species benefits at the expense of the other. Usually, the smaller species resides within a larger species and derives its food from the larger organism. Normally, the larger organism is not killed.

Mutualistic organisms are those that live in close association with another species and both species benefit as a result of the association. For example, some nitrogen-fixing bacteria live in nodules on the roots of plants. They convert atmospheric nitrogen (N₂) to a form that is usable by plants. Plants provide the bacteria with carbohydrates.

Obligate anaerobes are unable to grow in the presence of oxygen.

Facultative anaerobes can grow with or without oxygen.

Aerobic organisms require oxygen. Most bacteria are aerobic.

Staining

Different staining techniques can be used to help classify prokaryotes into different groups. For example, gram staining can be used to distinguish between gram positive and gram negative bacteria based on the thickness of the cell wall.

The shape of a cell is used to help classify bacteria. Round cells are called cocci (sing. coccus), rod-shaped cells are bacilli (bacillus), and rigid, spiral-shaped cells are spirilla (spirillum). Flexible, spiral-shaped bacteria are spirochetes.

Below: Cocci X 400. Click on the photograph to view an enlargement, then click "Back" to return.

Below: Bacilli X 1000. Click on the photograph to view an enlargement, then click "Back" to return.

Below: Spirilla X 400. Click on the photograph to view an enlargement, then click "Back" to return.

Cyanobacteria (sometimes called blue-green algae) are gram-negative bacteria that can photosynthesize and some can fix atmospheric nitrogen. 

The only organisms capable of fixing nitrogen are bacteria, and this is primarily done by the cyanobacteria. The fixation of nitrogen by cyanobacteria may have allowed plants to invade the land during the Paleozoic.

Like plants, cyanobacteria have the photosynthetic pigment chlorophyll A and they use water as an electron donor during photosynthesis. When water molecules are split, oxygen is liberated. This process resulted in oxygen accumulating in the earth’s early atmosphere.

Unicellular, filamentous, and colonial species of cyanobacteria are common. Anabaena and Oscillatoria (below are filamentous forms.

	Anabaena X 1000 (live) The large cell in the center is a heterocyst. It functions in nitrogen fixation. Other cells in the filament are photosynthetic. Click on the photograph to view an enlargement.
	Oscillatoria (live)
	Oscillatoria X 400 stained.
	Gloeocapsa X 400 (stained). Gloeocapsa is a unicellular cyanobacteria. The gelatinous material surrounding each cell causes the cells to stick together.

Some Cyanobacteria form symbiotic associations with fungi forming structures called lichens. 

Evolutionary Relationships

Analysis of molecules found within the cells suggests that eukaryotes are more closely related to archaea than to bacteria.

Major Groups of Archaea

Three major groups of archaea are found in extreme habitats.

Methanogens are found in anaerobic environments such as marshes and in the intestinal tracts of animals. They produce methane as a result of cellular respiration.

Halophiles are found in environments with high salt concentration such as the great salt lake or soil with a high salt concentration.

Thermophiles live in hot environments such as hot springs and hydrothermal vents.