Prokaryote classification and diversity (article) | Khan Academy
Archaea and Bacteria are small, relatively simple cells surrounded by a Eukaryotic cells are more complex than prokaryotes, and the DNA is linear and found. Explain the evolutionary relationship between prokaryotes and eukaryotic organelles. The early eukaryote cell evolved more than a billion years ago. See how Evolution in Cells is connected to other aspects of biology. case study in the evolutionary relationship between prokaryotic and eukaryotic cells.
Representative species include Nitrosomas, which oxidize ammonia into nitrate, and Spirillum minus, which causes rat bite fever. A micrograph of spiral-shaped Spirillum minus is shown.
Gamma Proteobacteria include many are beneficial symbionts that populate the human gut, as well as familiar human pathogens. Some species from this subgroup oxidize sulfur compounds. Representative species include Escherichia coli, normally beneficial microbe of the human gut, but some strains cause disease; Salmonella, certain strains of which cause food poisoning, and typhoid fever; Yersinia pestis—the causative agent of Bubonic plague; Psuedomonas aeruganosa— causes lung infections; Vibrio cholera, the causative agent of cholera, and Chromatium—sulfur producing bacteria bacteria that oxidize sulfur, producing H2S.
Micrograph shows rod-shaped Vibrio cholera, which are about 1 micron long. Some species of delta Proteobacteria generate a spore-forming fruiting body in adverse conditions.
Others reduce sulfate and sulfur. Representative species include Myxobacteria, which generate spore-forming fruiting bodies in adverse conditions and Desulfovibrio vulgaris, an aneorobic, sulfur-reducing bacterium.
Micrograph shows a bent rod-shaped Desulfovibrio vulgaris bacterium with a long flagellum.
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Epsilon Proteobacteria includes many species that inhabit the digestive tract of animals as symbionts or pathogens. Bacteria from this group have been found in deep-sea hydrothermal vents and cold seep habitats.
The next phylum described is chlamydias. All members of this group are obligate intracellular parasites of animal cells. Cells walls lack peptidoglycan. Micrograph shows a pap smear of cells infected with Chlamydia trachomatis. Chlamydia infection is the most common sexually transmitted disease and can lead to blindness.
All members of the phylum Spirochetes have spiral-shaped cells.
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Most are free-living anaerobes, but some are pathogenic. Flagella run lengthwise in the periplasmic space between the inner and outer membrane. Representative species include Treponema pallidum, the causative agent of syphilis and Borrelia burgdorferi, the causative agent of Lyme disease Micrograph shows corkscrew-shaped Trepanema pallidum, about 1 micron across.
Bacteria in the phylum Cyanobacteria, also known as blue-green algae, obtain their energy through photosynthesis. They are ubiquitous, found in terrestrial, marine, and freshwater environments. Eukaryotic chloroplasts are thought be derived from bacteria in this group. Micrograph shows a long, thin rod-shaped species called Phormidium.Evolution From Prokaryotes to Eukaryotes
Gram-positive Bacteria have a thick cell wall and lack an outer membrane. Soil-dwelling members of this subgroup decompose organic matter. Some species cause disease. Representative species include Bacillus anthracis, which causes anthrax; Clostridium botulinum, which causes botulism; Clostridium difficile, which causes diarrhea during antibiotic therapy; Streptomyces, from which many antibiotics, including streptomyocin, are derived; and Mycoplasmas, the smallest known bacteria, which lack a cell wall.
Some are free-living, and some are pathogenic. Micrograph shows Clostridium difficile, which are rod-shaped and about 3 microns long. Chlamydias are pathogens that live inside host cells, while cyanobacteria are photosynthesizers that make much of Earth's oxygen.
Spirochetes include both harmless bacteria and harmful ones, like the Borrelia burgdorferi that cause Lyme disease. The same is true of Gram-positive bacteria, which range from probiotic bacteria in yogurt to the Bacillus anthracis that cause anthrax. Cell walls lack peptidoglycan.
Chlamydia trachomatis, common sexually transmitted disease that can lead to blindness. Most members of this species, which has spiral-shaped cells, are free-living anaerobes, but some are pathogenic. Treponema pallidum, causative agent of syphilis, and Borrelia burgdorferi, causative agent of Lyme disease. Treponema pallidum, a corkscrew-shaped bacterium. In spite of these differences, the same basic molecular mechanisms govern the lives of both prokaryotes and eukaryotes, indicating that all present-day cells are descended from a single primordial ancestor.
How did this first cell develop?
And how did the complexity and diversity exhibited by present-day cells evolve? Prokaryotic and Eukaryotic Cells. The First Cell It appears that life first emerged at least 3. How life originated and how the first cell came into being are matters of speculation, since these events cannot be reproduced in the laboratory.
Nonetheless, several types of experiments provide important evidence bearing on some steps of the process.
The scale indicates the approximate times at which some of the major events in the evolution of cells are thought to have occurred. It was first suggested in the s that simple organic molecules could form and spontaneously polymerize into macromolecules under the conditions thought to exist in primitive Earth's atmosphere. At the time life arose, the atmosphere of Earth is thought to have contained little or no free oxygen, instead consisting principally of CO2 and N2 in addition to smaller amounts of gases such as H2, H2S, and CO.
Such an atmosphere provides reducing conditions in which organic molecules, given a source of energy such as sunlight or electrical discharge, can form spontaneously. The spontaneous formation of organic molecules was first demonstrated experimentally in the s, when Stanley Miller then a graduate student showed that the discharge of electric sparks into a mixture of H2, CH4, and NH3, in the presence of water, led to the formation of a variety of organic molecules, including several amino acids Figure 1.
Although Miller's experiments did not precisely reproduce the conditions of primitive Earth, they clearly demonstrated the plausibility of the spontaneous synthesis of organic molecules, providing the basic materials from which the first living organisms arose. Water vapor was refluxed through an atmosphere consisting of CH4, NH3, and H2, into which electric sparks were discharged. Analysis of the reaction products revealed the formation of a variety of organic molecules, more The next step in evolution was the formation of macromolecules.
The monomeric building blocks of macromolecules have been demonstrated to polymerize spontaneously under plausible prebiotic conditions. Heating dry mixtures of amino acids, for example, results in their polymerization to form polypeptides.
But the critical characteristic of the macromolecule from which life evolved must have been the ability to replicate itself.
Only a macromolecule capable of directing the synthesis of new copies of itself would have been capable of reproduction and further evolution. Of the two major classes of informational macromolecules in present-day cells nucleic acids and proteinsonly the nucleic acids are capable of directing their own self-replication. Nucleic acids can serve as templates for their own synthesis as a result of specific base pairing between complementary nucleotides Figure 1.
A critical step in understanding molecular evolution was thus reached in the early s, when it was discovered in the laboratories of Sid Altman and Tom Cech that RNA is capable of catalyzing a number of chemical reactions, including the polymerization of nucleotides.
RNA is thus uniquely able both to serve as a template for and to catalyze its own replication. Consequently, RNA is generally believed to have been the initial genetic system, and an early stage of chemical evolution is thought to have been based on self-replicating RNA molecules—a period of evolution known as the RNA world. Complementary pairing between nucleotides adenine [A] with uracil [U] and guanine [G] with cytosine [C] allows one strand of RNA to serve as a template for the synthesis of a new strand with the complementary sequence.
The first cell is presumed to have arisen by the enclosure of self-replicating RNA in a membrane composed of phospholipids Figure 1. As discussed in detail in the next chapter, phospholipids are the basic components of all present-day biological membranes, including the plasma membranes of both prokaryotic and eukaryotic cells. The key characteristic of the phospholipids that form membranes is that they are amphipathic molecules, meaning that one portion of the molecule is soluble in water and another portion is not.
Phospholipids have long, water-insoluble hydrophobic hydrocarbon chains joined to water-soluble hydrophilic head groups that contain phosphate. When placed in water, phospholipids spontaneously aggregate into a bilayer with their phosphate-containing head groups on the outside in contact with water and their hydrocarbon tails in the interior in contact with each other.
Such a phospholipid bilayer forms a stable barrier between two aqueous compartments—for example, separating the interior of the cell from its external environment.
Prokaryote classification and diversity
The first cell is thought to have arisen by the enclosure of self-replicating RNA and associated molecules in a membrane composed of phospholipids. Each phospholipid molecule has two long hydrophobic more The enclosure of self-replicating RNA and associated molecules in a phospholipid membrane would thus have maintained them as a unit, capable of self-reproduction and further evolution. RNA-directed protein synthesis may already have evolved by this time, in which case the first cell would have consisted of self-replicating RNA and its encoded proteins.
The Evolution of Metabolism Because cells originated in a sea of organic molecules, they were able to obtain food and energy directly from their environment. But such a situation is self-limiting, so cells needed to evolve their own mechanisms for generating energy and synthesizing the molecules necessary for their replication. The generation and controlled utilization of metabolic energy is central to all cell activities, and the principal pathways of energy metabolism discussed in detail in Chapter 2 are highly conserved in present-day cells.
The mechanisms used by cells for the generation of ATP are thought to have evolved in three stages, corresponding to the evolution of glycolysisphotosynthesisand oxidative metabolism Figure 1. The development of these metabolic pathways changed Earth's atmosphere, thereby altering the course of further evolution.
Glycolysis is the anaerobic breakdown of glucose to lactic acid. Photosynthesis utilizes energy from sunlight to drive the synthesis of glucose from CO2 and H2O, with the release of O2 as a by-product. The O2 released by more In the initially anaerobic atmosphere of Earth, the first energy-generating reactions presumably involved the breakdown of organic molecules in the absence of oxygen.
These reactions are likely to have been a form of present-day glycolysis —the anaerobic breakdown of glucose to lactic acid, with the net energy gain of two molecules of ATP. In addition to using ATP as their source of intracellular chemical energy, all present-day cells carry out glycolysisconsistent with the notion that these reactions arose very early in evolution.
Glycolysis provided a mechanism by which the energy in preformed organic molecules e. The development of photosynthesis is generally thought to have been the next major evolutionary step, which allowed the cell to harness energy from sunlight and provided independence from the utilization of preformed organic molecules. The first photosynthetic bacteria, which evolved more than 3 billion years ago, probably utilized H2S to convert CO2 to organic molecules—a pathway of photosynthesis still used by some bacteria.
The use of H2O as a donor of electrons and hydrogen for the conversion of CO2 to organic compounds evolved later and had the important consequence of changing Earth's atmosphere. The use of H2O in photosynthetic reactions produces the by-product free O2; this mechanism is thought to have been responsible for making O2 abundant in Earth's atmosphere. The release of O2 as a consequence of photosynthesis changed the environment in which cells evolved and is commonly thought to have led to the development of oxidative metabolism.
Alternatively, oxidative metabolism may have evolved before photosynthesis, with the increase in atmospheric O2 then providing a strong selective advantage for organisms capable of using O2 in energy-producing reactions.
In either case, O2 is a highly reactive molecule, and oxidative metabolism, utilizing this reactivity, has provided a mechanism for generating energy from organic molecules that is much more efficient than anaerobic glycolysis. For example, the complete oxidative breakdown of glucose to CO2 and H2O yields energy equivalent to that of 36 to 38 molecules of ATP, in contrast to the 2 ATP molecules formed by anaerobic glycolysis.
With few exceptions, present-day cells use oxidative reactions as their principal source of energy. Present-Day Prokaryotes Present-day prokaryotes, which include all the various types of bacteria, are divided into two groups—the archaebacteria and the eubacteria —which diverged early in evolution. Some archaebacteria live in extreme environments, which are unusual today but may have been prevalent in primitive Earth.
The eubacteria include the common forms of present-day bacteria—a large group of organisms that live in a wide range of environments, including soil, water, and other organisms e. Their DNA contents range from about 0. The largest and most complex prokaryotes are the cyanobacteriabacteria in which photosynthesis evolved. The structure of a typical prokaryotic cell is illustrated by Escherichia coli E.