General Biology of the Prokaryotes

There are many, many prokaryotes around us—everywhere. Although most are so small that we cannot see them with the naked eye, the prokaryotes are the most successful of all creatures on Earth, if success is measured by numbers of individuals. The bacteria in one person's intestinal tract, for example, outnumber all the humans who have ever lived, and even the total number of human cells in their host's body. Some of these bacteria form a thick lining along the intestinal wall. Bacteria and archaea in the oceans number more than 3 x 1028. This stunning number is perhaps 100 million times as great as the number of stars in the visible universe.

Although small, prokaryotes play many critical roles in the biosphere, interacting in one way or another with every other living thing. In this section, we'll see that some prokaryotes perform key steps in the cycling of nitrogen, sulfur, and carbon. Other prokaryotes trap energy from the sun or from inorganic chemical sources, and some help animals digest their food. The members of the two prokaryotic domains outdo all other groups in metabolic diversity. Eukary-otes, in contrast, are much more diverse in size and shape, but their metabolism is much less diverse. In fact, much of the energy metabolism of eukaryotes is carried out in or-ganelles—mitochondria and chloroplasts—that are descended from bacteria.

Prokaryotes are found in every conceivable habitat on the planet, from the coldest to the hottest, from the most acidic to the most alkaline, and to the saltiest. Some live where oxygen is abundant and others where there is no oxygen at all. They have established themselves at the bottom of the seas, in rocks more than 2 km into Earth's solid crust, and inside other organisms, large and small. Their effects on our environment are diverse and profound. What do these tiny but widespread organisms look like?

(a) Enterococcus sp.

(b) Escherichia coli

(c) Leptospira interrogans

(a) Enterococcus sp.

(b) Escherichia coli

(c) Leptospira interrogans

Spherical Cocci
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27.3 Shapes of Prokaryotic Cells (a) These spherical cocci of an acid-producing bacterium grow in the mammalian gut. (b) Rod-shaped E.coli are the most thoroughly studied of any bacteria— indeed, of almost any organism on Earth. (c) This spiral bacterium belongs to a genus of human pathogens that cause leptospirosis, an infection of the kidney and liver that is spread by contaminated water. The disease has historically been a problem for soldiers in crowded, transient campsites; this particular bacterial strain was isolated in 1915 from the blood of a soldier serving in World War I.

Prokaryotes and their associations take a few characteristic forms

Three shapes are particularly common among the prokaryotes: spheres, rods, and curved or spiral forms (Figure 27.3). A spherical prokaryote is called a coccus (plural, cocci). Cocci may live singly or may associate in two- or three-dimensional arrays as chains, plates, blocks, or clusters of cells. A rod-shaped prokaryote is called a bacillus (plural, bacilli). Spiral forms are the third main prokaryotic shape. Bacilli and spiral forms may be single or may form chains.

Prokaryotes are almost all unicellular, although some mul-ticellular ones are known. Associations such as chains do not signify multicellularity because each cell is fully viable and independent. These associations arise as cells adhere to one another after reproducing by fission. Associations in the form of chains are called filaments. Some filaments become enclosed within delicate tubular sheaths.

Prokaryotes lack nuclei, organelles, and a cytoskeleton

The architectures of prokaryotic and eukaryotic cells were compared in Chapter 4. The basic unit of archaea and bacteria is the prokaryotic cell (see Figure 4.5), which contains a full complement of genetic and protein-synthesizing systems, including DNA, RNA, and all the enzymes needed to transcribe and translate the genetic information into proteins. The prokaryotic cell also contains at least one system for generating the ATP it needs.

In what follows, bear in mind that most of what we know about the structure of prokaryotes comes from studies of bacteria. We still know relatively little about the diversity of archaea, although the pace of research on archaea is accelerating.

The prokaryotic cell differs from the eukaryotic cell in three important ways. First, the organization and replication of the genetic material differs. The DNA of the prokaryotic cell is not organized within a membrane-enclosed nucleus. DNA molecules in prokaryotes (both bacteria and archaea)

27.3 Shapes of Prokaryotic Cells (a) These spherical cocci of an acid-producing bacterium grow in the mammalian gut. (b) Rod-shaped E.coli are the most thoroughly studied of any bacteria— indeed, of almost any organism on Earth. (c) This spiral bacterium belongs to a genus of human pathogens that cause leptospirosis, an infection of the kidney and liver that is spread by contaminated water. The disease has historically been a problem for soldiers in crowded, transient campsites; this particular bacterial strain was isolated in 1915 from the blood of a soldier serving in World War I.

are usually circular; in the best-studied prokaryotes, there is a single chromosome, but there are often plasmids as well (see Chapter 13).

Second, prokaryotes have none of the membrane-enclosed cytoplasmic organelles that modern eukaryotes have—mitochondria, Golgi apparatus, and others. However, the cytoplasm of a prokaryotic cell may contain a variety of infold-ings of the plasma membrane and photosynthetic membrane systems not found in eukaryotes.

Third, prokaryotic cells lack a cytoskeleton, and, without the cytoskeletal proteins, they lack mitosis. Prokaryotic cells divide by their own elaborate method, fission, after replicating their DNA.

Prokaryotes have distinctive modes of locomotion

Although many prokaryotes cannot move, others are motile. These organisms move by one of several means. Some spiral bacteria, called spirochetes, use a corkscrew-like motion made possible by modified flagella, called axial filaments, running along the axis of the cell beneath the outer membrane (Figure 27.4a). Many cyanobacteria and a few other bacteria use various poorly understood gliding mechanisms, including rolling. Various aquatic prokaryotes, including some cyanobacteria, can move slowly up and down in the water by adjusting the amount of gas in gas vesicles (Figure 27.4b). By far the most common type of locomotion in prokaryotes, however, is that driven by flagella.

(a) Internal fibrils

(a) Internal fibrils

Gas Vesicles
27.4 Structures Associated with Prokaryote Motility (a) A spirochete from the gut of a termite, seen in cross section, shows the axial filaments used to produce a corkscrew-like motion. (b) Gas vesicles in a cyanobacterium, visualized by the freeze-fracture technique.

Bacterial flagella are slender filaments that extend singly or in tufts from one or both ends of the cell or are randomly distributed all around it (Figure 27.5). Abacterial flagellum consists of a single fibril made of the protein flagellin, projecting from the cell surface, plus a hook and basal body responsible for motion (see Figure 4.6). In contrast, the flagel-lum of eukaryotes is enclosed by the plasma membrane and usually contains a circle of nine pairs of microtubules surrounding two central microtubules, all containing the protein tubulin, along with many other associated proteins. The prokaryotic flagellum rotates about its base, rather than beating as a eukaryotic flagellum or cilium does.

Prokaryotes have distinctive cell walls

Most prokaryotes have a thick and relatively stiff cell wall. This wall is quite different from the cell walls of plants and

Bacteria With Flagella
27.5 Some Bacteria Use Flagella for Locomotion Flagella propel this rod-shaped Salmonella.

algae, which contain cellulose and other polysaccharides, and from those of fungi, which contain chitin. Almost all bacteria have cell walls containing peptidoglycan (a polymer of amino sugars). Archaeal cell walls are of differing types, but most contain significant amounts of protein. One group of archaea has pseudopeptidoglycan in its wall; as you have probably already guessed from the prefix pseudo-, pseudopeptidogly-can is similar to, but distinct from, the peptidoglycan of bacteria. Peptidoglycan is a substance unique to bacteria; its absence from the walls of archaea is a key difference between the two prokaryotic domains.

In 1884 Hans Christian Gram, a Danish physician, developed a simple staining process that has lasted into our high-technology era as a useful tool for identifying bacteria. The Gram stain separates most types of bacteria into two distinct groups, Gram-positive and Gram-negative, on the basis of their staining (Figure 27.6). A smear of cells on a microscope slide is soaked in a violet dye and treated with iodine; it is then washed with alcohol and counterstained with safranine (a red dye). Gram-positive bacteria retain the violet dye and appear blue to purple (Figure 27.6a). The alcohol washes the violet stain out of Gram-negative cells; these cells then pick up the safranine counterstain and appear pink to red (Figure 27.6b). Gram-staining characteristics are useful in classifying some kinds of bacteria and are important in determining the identity of bacteria in an unknown sample.

For many bacteria, the Gram-staining results correlate roughly with the structure of the cell wall. Peptidoglycan forms a thick layer outside the plasma membrane of Gram-

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Gram-positive bacteria have a uniformly dense cell wall consisting primarily of peptidoglycan.

Gram-positive bacteria have a uniformly dense cell wall consisting primarily of peptidoglycan.

Cell wall-(peptidoglycan)

Plasma -membrane

40 nm

Cell wall-(peptidoglycan)

Plasma -membrane

40 nm

Outside of cell

Cytoplasm

-Periplasmic space

Gram-negative bacteria have a very thin peptidoglycan layer and an outer membrane.

Gram-negative bacteria have a very thin peptidoglycan layer and an outer membrane.

Peptidoglycan General Gram Negative

40 nm

27.6 The Gram Stain and the Bacterial Cell Wall When treated with Gram stain, the cell wall components of different bacteria react in one of two ways. (a) Gram-positive bacteria have a thick peptidoglycan cell wall that retains the violet dye and appears deep blue or purple. (b) Gram-negative bacteria have a thin peptidoglycan layer that does not retain the violet dye, but picks up the counterstain and appears pink-red.

40 nm

Periplasmic space

27.6 The Gram Stain and the Bacterial Cell Wall When treated with Gram stain, the cell wall components of different bacteria react in one of two ways. (a) Gram-positive bacteria have a thick peptidoglycan cell wall that retains the violet dye and appears deep blue or purple. (b) Gram-negative bacteria have a thin peptidoglycan layer that does not retain the violet dye, but picks up the counterstain and appears pink-red.

positive bacteria. The Gram-negative cell wall usually has only one-fifth as much peptidoglycan, and outside the peptidoglycan layer the cell is surrounded by a second, outer membrane quite distinct in chemical makeup from the plasma membrane (see Figure 27.6b). Between the inner (plasma) and outer membranes of Gram-negative bacteria is the periplasmic space. This space contains enzymes that are important in digesting some materials, transporting others, and detecting chemical gradients in the environment.

The consequences of the different features of prokaryotic cell walls are numerous and relate to the disease-causing characteristics of some prokaryotes. Indeed, the cell wall is a favorite target in medical combat against diseases that are caused by prokaryotes because it has no counterpart in eu-karyotic cells. Antibiotics such as penicillin and ampicillin, as well as other agents that specifically interfere with the synthesis of peptidoglycan-containing cell walls, tend to have little, if any, effect on the cells of humans and other eukaryotes.

Prokaryotes reproduce asexually, but genetic recombination does occur

Prokaryotes reproduce by fission, an asexual process. Recall, however, that there are also processes—transformation, conjugation, and transduction—that allow the exchange of genetic information between some prokaryotes quite apart from either sex or reproduction (see Chapter 13).

Some prokaryotes multiply very rapidly. One of the fastest is the bacterium Escherichia coli, which under optimal conditions has a generation time of about 20 minutes. The shortest known prokaryote generation times are about 10 minutes. Generation times of 1 to 3 hours are common for others; some extend to days. Bacteria living deep in Earth's crust may suspend their growth for more than a century without dividing and then multiply for a few days before suspending growth again. What kinds of metabolism support such a diversity of growth rates?

Prokaryotes have exploited many metabolic possibilities

The long evolutionary history of the bacteria and archaea, during which they have had time to explore a wide variety of habitats, has led to the extraordinary diversity of their metabolic "lifestyles"—their use or nonuse of oxygen, their energy sources, their sources of carbon atoms, and the materials they release as waste products.

anaerobic versus aerobic metabolism. Some prokaryotes can live only by anaerobic metabolism because molecular oxygen is poisonous to them. These oxygen-sensitive organisms are called obligate anaerobes.

Other prokaryotes can shift their metabolism between anaerobic and aerobic modes (see Chapter 7) and thus are called facultative anaerobes. Many facultative anaerobes alternate between anaerobic metabolism (such as fermentation) and cellular respiration as conditions dictate. Aerotolerant anaerobes cannot conduct cellular respiration, but are not damaged by oxygen when it is present.

At the other extreme from the obligate anaerobes, some prokaryotes are obligate aerobes, unable to survive for extended periods in the absence of oxygen. They require oxygen for cellular respiration.

nutritional categories. Biologists recognize four broad nutritional categories of organisms: photoautotrophs, pho-toheterotrophs, chemolithotrophs, and chemoheterotrophs. Prokaryotes are represented in all four groups (Table 27.2).

Photoautotrophs perform photosynthesis. They use light as their source of energy and carbon dioxide as their source of carbon. Like the photosynthetic eukaryotes, one group of photoautotrophic bacteria, the cyanobacteria, use chlorophyll a as their key photosynthetic pigment and produce oxygen as a by-product of noncyclic electron transport (see Chapter 8).

By contrast, the other photosynthetic bacteria use bacteri-ochlorophyll as their key photosynthetic pigment, and they do not release oxygen gas. Some of these photosynthesizers produce particles of pure sulfur instead because hydrogen sulfide (H2S), rather than H2O, is their electron donor for pho-tophosphorylation. Bacteriochlorophyll absorbs light of

27,2 How Organisms Obtain Their Energy and Carbon
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Responses

  • Nadine
    What is a peptidoglycan?
    3 years ago
  • ryan rosario
    Where are gas vesicles found in prokaryotes?
    2 years ago
  • KAARLO
    What are the general functions of the prokaryote membrane?
    2 years ago
  • stella myers
    What a cross section of a plasma membrane looks like?
    2 years ago
  • Folcard
    What does a cross section of a plasma membrane look luke?
    2 years ago
  • KIROS
    Does prokaryotes contain a plasma membrane?
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