Eukarya

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Eukaryotes

Archaea High Pressure Lovers

Some bacteria are heat lovers

Three of the bacterial groups that may have branched out earliest during bacterial evolution are all thermophiles (heat lovers), as are the most ancient of the archaea. This observation supports the hypothesis that the first living organisms were thermophiles that appeared in an environment much hotter than those that predominate today.

The Proteobacteria are a large and diverse group

By far the largest group of bacteria, in terms of numbers of described species, is the proteobacteria, sometimes referred to as the purple bacteria. Among the proteobac-teria are many species of Gram-negative, bacterio-chlorophyll-containing, sulfur-using photoautotrophs. However, the proteobacteria also include dramatically diverse bacteria that bear no resemblance to those species in phe-notype. The mitochondria of eukaryotes were derived from proteobacteria by endosymbiosis.

No characteristic demonstrates the diversity of the proteobacteria more clearly than their metabolic pathways (Figure 27.9). The common ancestor of all the proteobacteria was probably a photoautotroph. Early in evolution, two groups of proteobacteria lost their ability to photosynthesize and have been chemoheterotrophs ever since. The other three groups still have photoautotrophic members, but in each group, some evolutionary lines have abandoned photoau-totrophy and taken up other modes of nutrition. There are chemolithotrophs and chemoheterotrophs in all three groups. Why? One possibility is that each of the trends in Figure 27.9 was an evolutionary response to selective pressures encountered as these bacteria colonized new habitats that presented new challenges and opportunities.

Among the proteobacteria are some nitrogen-fixing genera, such as Rhizobium (see Figure 37.7), and other bacteria that contribute to the global nitrogen and sulfur cycles. E. coli, one of the most studied organisms on Earth, is a proteobacterium. So, too, are many of the most famous human pathogens, such as Yersinia pestis, Vibrio cholerae, and Salmonella typhimurium, all mentioned in our discussion of pathogens above.

Fungi cause most plant diseases, and viruses cause others, but about 200 plant diseases are of bacterial origin. Crown gall, with its characteristic tumors (Figure 27.10), is one of the most striking. The causal agent of crown gall is Agrobacterium tumefaciens, which harbors a plasmid used in recombinant DNA studies as a vehicle for inserting genes into new plant hosts (see Chapter 16).

Agrobacterium Tumefaciens Crown Gall

27.9 The Evolution of Metabolism in the Proteobacteria The common ancestor of all proteobacteria was probably a photoautotroph. As they encountered new environments, groups 1 and 2 lost the ability to photosynthesize; in the other three groups,some evolutionary lines became chemolithotrophs or chemoheterotrophs.

27.9 The Evolution of Metabolism in the Proteobacteria The common ancestor of all proteobacteria was probably a photoautotroph. As they encountered new environments, groups 1 and 2 lost the ability to photosynthesize; in the other three groups,some evolutionary lines became chemolithotrophs or chemoheterotrophs.

Crown Gall Tumor

Crown gall

27.10 A Crown Gall This colorful tumor growing on the stem of a geranium plant is caused by the Gram-negative bacillus Agrobacterium tumefaciens.

Crown gall

27.10 A Crown Gall This colorful tumor growing on the stem of a geranium plant is caused by the Gram-negative bacillus Agrobacterium tumefaciens.

Cyanobacteria are important photoautotrophs

Cyanobacteria, sometimes called blue-green bacteria because of their pigmentation, are photoautotrophs that require only water, nitrogen gas, oxygen, a few mineral elements, light, and carbon dioxide to survive. They use chlorophyll a for photosynthesis and release oxygen gas; many species also fix nitrogen. Their photosynthesis was the basis of the "oxygen revolution" that transformed Earth's atmosphere.

Cyanobacteria carry out the same type of photosynthesis that is characteristic of eukaryotic photosynthesizers. They contain elaborate and highly organized internal membrane systems called photosynthetic lamellae, or thylakoids. The chloroplasts of photosynthetic eukaryotes are derived from an endosymbiotic cyanobacterium.

Cyanobacteria may live free as single cells or associate in colonies. Depending on the species and on growth conditions, colonies of cyanobacteria may range from flat sheets one cell thick to filaments to spherical balls of cells.

Some filamentous colonies of cyanobacteria differentiate into three cell types: vegetative cells, spores, and heterocysts (Figure 27.11). Vegetative cells photosynthesize, spores are resting cells that can eventually develop into new filaments, and heterocysts are cells specialized for nitrogen fixation. All of the known cyanobacteria with heterocysts fix nitrogen. Heterocysts also have a role in reproduction: When filaments break apart to reproduce, the heterocyst may serve as a breaking point.

Spirochetes look like corkscrews

Spirochetes are Gram-negative, motile, chemoheterotrophic bacteria characterized by unique structures called axial filaments, which are modified flagella running through the periplasm (see Figure 27.4a). The cell body is a long cylinder coiled into a spiral (Figure 27.12). The axial filaments begin at either end of the cell and overlap in the middle, and there are typical basal bodies where they are attached to the cell wall. The basal bodies rotate, as they do in other prokaryotic flagella. Many spirochetes live in humans as parasites; a few are pathogens, including those that cause syphilis and Lyme disease. Others live free in mud or water.

Heterocyst

Heterocyst Vegetative cells

Spore

27.11 Cyanobacteria (a) Anabaena is a genus of cyanobacteria that form filamentous colonies containing three cell types. (b) A thin neck attaches a heterocyst to each of two vegetative cells in a filament. (c) Cyanobacteria appear in enormous numbers in some environments. This California pond has experienced eutrophication; phosphorus and other nutrients generated by human activity have accumulated in the pond, feeding an immense green mat (commonly referred to as "pond scum") that is made up of several species of free-living cyanobacteria.

Heterocyst Vegetative cells

Spore

Heterocysts
0.6 |m

Treponema pallidum

Habitat Treponema Pallidum

Treponema pallidum

200 nm

27.12 A Spirochete This corkscrew-shaped bacterium causes syphilis in humans.

200 nm

27.12 A Spirochete This corkscrew-shaped bacterium causes syphilis in humans.

Elementary bodies are taken into a eukaryotic cell by phagocytosis

Elementary bodies are taken into a eukaryotic cell by phagocytosis

2^ .where they develop into thin-walled reticulate bodies, which grow and divide.

Spirochete Matting

2^ .where they develop into thin-walled reticulate bodies, which grow and divide.

Hi Reticulate bodies reorganize into elementary bodies, which are liberated by the rupture of the host cell.

f<PJ

Chlamydia psittaci

Hi Reticulate bodies reorganize into elementary bodies, which are liberated by the rupture of the host cell.

27.13 Chlamydias Change Form during Their Life Cycle

Elementary bodies and reticulate bodies are the two major phases of the chlamydia life cycle.

Chlamydias

Chlamydias are extremely small

Chlamydias are among the smallest bacteria (0.2-1.5 |im in diameter). They can live only as parasites within the cells of other organisms. These tiny Gram-negative cocci are unique prokary-otes because of their complex life cycle, which involves two different forms of cells, elementary bodies and reticulate bodies (Figure 27.13). In humans, various strains of chlamydias cause eye infections (especially trachoma), sexually transmitted diseases, and some forms of pneumonia.

Most firmicutes are Gram-positive

The firmicutes are sometimes referred to as the Gram-positive bacteria, but some firmi cutes are Gram-negative, and some have no cell wall at all. Nonetheless, the firmicutes constitute a clade.

Some firmicutes produce endospores (Figure 27.14)—heat-resistant resting structures—when a key nutrient such as nitrogen or carbon becomes scarce. The bacterium

replicates its DNA and encapsulates one copy, along with some of its cytoplasm, in a tough cell wall heavily thickened with peptidoglycan and surrounded by a spore coat. The parent cell then breaks down, releasing the endospore. En-dospore production is not a reproductive process; the en-dospore merely replaces the parent cell. The endospore,

Endospore

Clostridium tetani

Tetanus Endospore

27.14 The Endospore: A Structure for Waiting Out Bad Times

This firmicute, which causes tetanus, produces endospores as resistant resting structures.

however, can survive harsh environmental conditions that would kill the parent cell, such as high or low temperatures or drought, because it is dormant—its normal activity is suspended. Later, if it encounters favorable conditions, the en-dospore becomes metabolically active and divides, forming new cells like the parent. Some endospores can be reactivated after more than a thousand years of dormancy. There are credible claims of reactivation of Bacillus endospores after millions of years—and even one claim, of uncertain validity, of more than a billion years!

Members of this endospore-forming group of firmicutes include the many species of Clostridium and Bacillus. The toxins produced by C. botulinum are among the most poisonous ever discovered; the lethal dose for humans is about one-millionth of a gram (1 pg). B. anthracis, as noted above, is the anthrax pathogen.

The genus Staphylococcus—the staphylococci—includes fir-micutes that are abundant on the human body surface; they are responsible for boils and many other skin problems (Figure 27.15). S. aureus is the best-known human pathogen in this genus; it is found in 20 to 40 percent of normal adults (and in 50 to 70 percent of hospitalized adults). It can cause respiratory, intestinal, and wound infections in addition to skin diseases.

Actinomycetes are firmicutes that develop an elaborately branched system of filaments (Figure 27.16). These bacteria closely resemble the filamentous growth habit of fungi at a reduced scale. Some actinomycetes reproduce by forming chains of spores at the tips of the filaments. In species that do not form spores, the branched, filamentous growth ceases and the structure breaks up into typical cocci or bacilli, which then reproduce by fission.

Actinomycete

Actinomyces sp.

27.16 Filaments of an Actinomycete The branching filaments seen in this scanning electron micrograph are typical of actino-mycetes, a medically important bacterial group.

Actinomyces sp.

27.16 Filaments of an Actinomycete The branching filaments seen in this scanning electron micrograph are typical of actino-mycetes, a medically important bacterial group.

The actinomycetes include several medically important bacteria. Mycobacterium tuberculosis causes tuberculosis. Strep-tomyces produces streptomycin as well as hundreds of other antibiotics. We derive most of our antibiotics from members of the actinomycetes.

Another interesting group of firmicutes, the mycoplas-mas, lack cell walls, although some have a stiffening material outside the plasma membrane. Some of them are the smallest cellular creatures ever discovered—they are even smaller than chlamydias (Figure 27.17). The smallest my-coplasmas capable of multiplication have a diameter of about 0.2 pm. They are small in another crucial sense as well: They

Plasma Membranes Gram Positive

27.15 Gram-Positive Firmicutes "Grape clusters" are the usual arrangement of Gram-positive staphylococci.

27.17 The Tiniest Living Cells Containing only about one-fifth as much DNA as E. coli, mycoplasmas are the smallest known bacteria.

27.15 Gram-Positive Firmicutes "Grape clusters" are the usual arrangement of Gram-positive staphylococci.

27.17 The Tiniest Living Cells Containing only about one-fifth as much DNA as E. coli, mycoplasmas are the smallest known bacteria.

Some archaea have long-chain hydrocarbons with glycerol at both ends, spanning the membrane and resulting in a lipid monolayer.

Other archaeal hydrocarbons fit the same membrane template as do the fatty acids of bacteria and eukaryotes, resulting in a lipid bilayer.

Some archaea have long-chain hydrocarbons with glycerol at both ends, spanning the membrane and resulting in a lipid monolayer.

Other archaeal hydrocarbons fit the same membrane template as do the fatty acids of bacteria and eukaryotes, resulting in a lipid bilayer.

Archaeal Lipid Monolayer
27.18 Membrane Architecture in Archaea The long-chain hydrocarbons of may archaeal membranes are branched, and may have glycerol at both ends.This lipid monolayer structure (on the left) still fits into a biological membrane, however. In fact, all three domains have similar membrane structures.

have less than half as much DNA as do most other prokaryotes—but they still can grow autonomously. It has been speculated that the amount of DNA in a mycoplasma may be the minimum amount required to encode the essential properties of a living cell.

We have discussed five clades of bacteria in some detail, but other bacterial clades are well known, and there may be dozens more waiting to be discovered. This conservative estimate is based on the fact that many bacteria and ar-chaea have never been cultured in the laboratory.

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  • Sophia
    Do eukarya have plasma membranes?
    7 years ago

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