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| Prokaryotes |
Prokaryotes are one of two types of cell that form living organisms. Prokaryotic cells lack a nucleus and other organelles found in eukaryotic cells.
Prokaryotes include the unicellular life-forms found in two of the three domains of life, Archaea and Bacteria, whereas all protists, algae, fungi, plants, and animals are eukaryotic organisms, together forming the domain Eukarya.
There are architecturally two distinct types of cells of living organisms: prokaryotic cells and eukaryotic cells. The defining difference between these two types of cells is that prokaryotic cells lack any of the internal membrane-bound structures (organelles) found in eukaryotic cells, such as a nucleus, mitochondria, chloroplasts, endoplasmic reticulum, and Golgi apparatus.
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Bacterial and archaeal cells are prokaryotes, while plants, animals, fungi, algae, and protozoa (protists) are composed of eukaryotic cells.
Structure
Although prokaryotic cells do not contain membrane-bound organelles, they do have a highly complex organization and structure. Like all cells, prokaryotes are surrounded by a cytoplasmic membrane.
This membrane is composed of proteins and lipids and is semipermeable. This semipermeable layer regulates the flow of material into and out of the cell.
For most prokaryotes, the cell membrane is surrounded by a cell wall. The cell wall of almost every bacterial cell contains peptidoglycan, a cross-linked structure consisting of chains of sugar molecules, with the chains attached to one another through bridges composed of amino acids.
This cell wall protects the bacterial cell from osmotic shock. Some bacterial cells also have an outer membrane linked to the peptidoglycan layer by lipoproteins.
The outermembrane is a lipid bilayer that contains sugars and lipids and is known as lipopolysaccharide (LPS). LPS is often called endotoxin because this molecule can induce fever, shock, and death in animals.
Archaeal cells may have cell walls composed of pseudo peptidoglycan, which is very similar to the peptidogly can layer found in bacterial cells, or they may have cell walls composed of protein, polysaccharides, or other chemicals. Some bacteria and archaea lack cell walls entirely.
Some prokaryotic cells have structures external to the cell wall. These structures include capsules, slime layers, and S layers. Capsules are usually composed of polysaccharides, although some cells have proteinaceous capsules.
Capsules are protective layers that are particularly important in allowing disease-causing bacteria to evade attack by mammalian immune systems.
Slime layers are composed of polysaccharides and resemble less organized capsules. Slime layers help bacteria attach to surfaces, prevent dessication, and assist in trapping nutrients near the cell. S layers are crystalline protein layers of unknown function.
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Many prokaryotic cells are motile due to the presence of flagella. Some bacteria have flagella attached only at one or both ends of the cell. These flagella are known as polar flagella.
Other bacteria have flagella all around the cell, an arrangement known as peritrichous. Each flagellumis an inflexible, helical structure composed of molecules of the protein flagellin. Flagella rotate like propellers, causing bacteria to move in a corkscrew fashion.
Some prokaryotes produce spores, specialized structures that are extremely resistant to heat, cold, and desiccation. Spores are metabolically inert and can survive for extended periods, possibly for thousands of years. Spores form within prokaryotic cells when environmental conditions become unfavorable for survival.
Once the spore is formed, the cell that produced the spore breaks open, releasing the spore. When the spore finds itself in favorable growth conditions, it germinates by swelling, breaking out of the spore coat, and resuming metabolic function.
Reproduction
Prokaryotic cells reproduce by binary fission. The cell cycle of prokaryotes has three parts: elongation, DNA synthesis, and cell division. During elongation, the cell synthesizes and secretes cytoplasmic membrane and cell wall material.
Prokaryotes usually possess a single, double-stranded, circular DNA chromosome attached to the cytoplasmic membrane at one point. DNA synthesis, which occurs continuously in actively growing cells, results in two complete copies of the chromosome, each attached to the cytoplasmic membrane.
As new membrane material is inserted into the cytoplasmic membrane during elongation, the two chromosomes are swept away from one another. During cell division, a septum forms in the center of the cell which eventually divides the cell into two daughter cells.
Binary fission is a type of asexual reproduction. Each of the daughter cells is identical to the parent cell, and there is no exchange of genetic material. Some prokaryotes, however, do engage in genetic recombination through a process called conjugation.
Conjugation requires the presence of extrachromosomal pieces of DNA called plasmids. These plasmids are small, circular DNA molecules found in the cytoplasm of many prokaryotic cells. Some of these plasmids contain genes that encode a special structure, the F-pilus.
The F-pilus is a proteinaceous rod that extends from the surface of cells. Cells that have an F-pilus are donor cells and can attach, via the F-pilus, to recipient cells which lack an F-pilus.
Following attachment, the F-pilus contracts, drawing the donor and recipient close together. The donor then transfers DNA to the recipient. Although conjugation results in transfer of genes from one cell to another, it is not itself a method of reproduction.
Metabolism Prokaryotes are metabolically diverse. Two basic nutritional pathways are found: autotrophy and heterotrophy. Autotrophic prokaryotes are capable of synthesizing their own energy-yielding compounds from simple inorganic compounds such as carbon dioxide and water.
Some prokaryotic autotrophs, the cyanobacteria and the green and purple bacteria, utilize the energy from sunlight, in a process known as photosynthesis, to construct food molecules.
It has been hypothesized that chloroplasts in plant cells evolved from cyanobacteria that were engulfed by a eukaryotic cell more than one billion years ago. Other autotrophs extract energy from metabolizing inorganic compounds such as hydrogen sulfide, iron sulfide, and ammonia.
Heterotrophic prokaryotes obtain energy from the metabolism of organic compounds. Various prokaryotes are capable of metabolizing a wide variety of organic molecules, including sugars, lipids, proteins, petroleum products, antibiotics, and methanol.
Heterotrophs can metabolize food molecules using one of three methods: fermentation, aerobic respiration, and anaerobic respiration.
Fermentation and anaerobic respiration do not require the presence of oxygen, while aerobic respiration does require oxygen. Fermentation often results in metabolic end products that include acids, carbon dioxide, alcohol, or a combination of these.
The anaerobic respiration process is similar to aerobic respiration, except that molecules such as nitrate, sulfate, and iron are used instead of oxygen. The end products of aerobic respiration are carbon dioxide and water; for anaerobic respiration they are nitrite, hydrogen sulfide, or other reduced compounds.
Roles in the Global Ecosystem
Prokaryotes play important roles in the decay of organic matter as well as in three vital cycles of nature: the carbon, sulfur, and nitrogen cycles.
The major categories of biological macro molecules (carbohydrates, lipids, proteins, and nucleic acids) are all carbon-containing compounds. Photosynthetic organisms, including photosynthetic prokaryotes, take carbon dioxide and convert it into carbohydrates.
Those carbohydrates can be used for energy and biosynthesis by the photosynthetic organisms as well as by heterotrophs, which consume the photosynthetic organisms. Both heterotrophs and autotrophs also metabolize carbon-containing molecules, releasing carbon dioxide back into the atmosphere.
Sulfur is a component of certain amino acids found in proteins. As decomposers, prokaryotes decompose proteins deposited in water and soil by dead organisms and release the sulfur from sulfur containing amino acids, often in the form of hydrogen sulfide.
Some prokaryotes convert hydrogen sulfide to sulfates during their metabolism. The sulfates can then be taken up by plants, where they are reincorporated into sulfur-containing amino acids.
Nitrogen is an essential element in nucleic acids and proteins. Some prokaryotes, particularly soil microbes, digest proteins and release ammonia. Denitrification occurs when groups of symbiotic prokaryotes metabolize ammonia, first to nitrites, then to nitrates, then to atmospheric nitrogen.
Nitrogen fixation occurs when nitrogen-fixing prokaryotes in the soil trap atmospheric nitrogen and convert it to ammonia that can be used by plants to synthesize new proteins and amino acids.
Disease
Infectious disease is a disturbance in normal organismal function caused by an infecting agent. Although most prokaryotes do not cause disease, some bacteria are capable of parasitizing a host and disrupting normal function.
Prokaryotes capable of producing disease in plants are widely distributed and cause a number of diseases, including wilts, rots, blights, and galls. Some of these diseases are caused by soil-dwelling prokaryotes, while others are seed borne or are caused by obligate parasites, unable to survive outside plant tissue.
Commercial Uses
Prokaryotes are easily manipulated and therefore are useful for many commercial applications. Prokaryotes have been used for centuries in the production of food. Yogurt, sauerkraut, poi, kim chee, dry and semi dry sausages, and vinegar are all examples of bacterially produced foods.
Genetic engineering is more easily accomplished in prokaryotes than in eukaryotes. Prokaryotes now produce human insulin, antibiotics, plant hormones, and industrial solvents.
Prokaryotes have been engineered to protect plants from frost damage, while plants have been genetically engineered, using bacterial vectors, to develop resistance to herbicides and to produce toxins that destroy insect pests.
