1、,CHAPTER 27 Prokaryotes and the Origins of Metabolic Diversity,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Section A: The World of Prokaryotes,1. Theyre (almost) everywhere! An overview of prokaryotic life 2. Bacteria and archaea are the two main branches of prokaryote ev
2、olution,Prokaryotes were the earliest organisms on Earth and evolved alone for 1.5 billion years. Today, prokaryotes still dominate the biosphere. Their collective biomass outweighs all eukaryotes combined by at least tenfold. More prokaryotes inhabit a handful of fertile soil or the mouth or skin o
3、f a human than the total number of people who have ever lived.,1. Theyre (almost) everywhere! An overview of prokaryotic life,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Prokarytes are wherever there is life and they thrive in habitats that are too cold, too hot, too salt
4、y, too acidic, or too alkaline for any eukaryote. The vivid reds, oranges, and yellows that paint these rocks are colonies of prokaryotes.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 27.1,We hear most about the minority of prokaryote species that cause serious illnes
5、s. During the 14th century, a bacterial disease known as bubonic plague spread across Europe and killed about 25% of the human population. Other types of diseases caused by bacteria include tuberculosis, cholera, many sexually transmissible diseases, and certain types of food poisoning.,Copyright 20
6、02 Pearson Education, Inc., publishing as Benjamin Cummings,However, more bacteria are benign or beneficial. Bacteria in our intestines produce important vitamins. Prokaryotes recycle carbon and other chemical elements between organic matter and the soil and atmosphere. Prokaryotes often live in clo
7、se association among themselves and with eukaryotes in symbiotic relationships. Mitochondria and chloroplasts evolved from prokaryotes that became residents in larger host cells.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Modern prokaryotes are diverse in structure and i
8、n metabolism. About 5,000 species of prokaryotes are known, but estimates of actual prokaryotic diversity range from about 400,000 to 4 million species.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Molecular evidence accumulated over the last two decades has led to the con
9、clusion that there are two major branches of prokaryote evolution, not a single kingdom as in the five-kingdom system. These two branches are the bacteria and the archaea. The archaea inhabit extreme environments and differ from bacteria in many key structural, biochemical, and physiological charact
10、eristics.,2. Bacteria and archaea are the two main branches of prokaryote evolution,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Current taxonomy recognizes two prokaryotic domains: domain Bacteria and domain Archaea. A domain is a taxonomic level above kingdom. The ration
11、ale for this decision is that bacteria and archaea diverged so early in the history of life and are so fundamentally different. At the same time, they are both structurally organized at the prokaryotic level.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 27.2,CHAPTER 2
12、7 Prokaryotes and the Origins of Metabolic Diversity,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Section B1: The Structure, Function, and Reproduction of Prokaryotes,1. Nearly all prokaryotes have a cell wall external to the plasma membrane 2. Many prokaryotes are motile,
13、Most prokaryotes are unicellular. Some species may aggregate transiently or form true colonies, even extending to division of labor between specialized cell types. The most common shapes among prokaryotes are spheres (cocci), rods (bacilli), and helices.,Introduction,Copyright 2002 Pearson Education
14、, Inc., publishing as Benjamin Cummings,Fig. 27.3,Most prokaryotes have diameters in the range of 1-5 um, compared to 10-100 um for most eukaryotic cells. However, the largest prokaryote discovered so far has a diameter of 0.75 mm. It is a sulfur-metabolizing marine bacterium from coastal sediments
15、off Namibia.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 26.4,In nearly all prokaryotes, a cell wall maintains the shape of the cell, affords physical protection, and prevents the cell from bursting in a hypotonic environment. Most bacterial cell walls contain peptid
16、oglycan, a polymer of modified sugars cross-linked by short polypeptides. The walls of archaea lack peptidoglycan.,1. Nearly all prokaryotes have a cell wall external to the plasma membrane,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,The Gram stain is a valuable tool for
17、identifying specific bacteria based on differences in their cell walls. Gram-positive bacteria have simpler cell walls, with large amounts of peptidoglycans.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 27.5a,Gram-negative bacteria have more complex cell walls and les
18、s peptidoglycan. An outer membrane on the cell wall contains lipopolysaccharides, carbohydrates bonded to lipids.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 27.5b,Among pathogenic bacteria, gram-negative species are generally more threatening than gram-positive spec
19、ies. The lipopolysaccharides on the walls are often toxic and the outer membrane protects the pathogens from the defenses of their hosts. Gram-negative bacteria are commonly more resistant than gram-positive species to antibiotics because the outer membrane impedes entry of antibiotics.,Copyright 20
20、02 Pearson Education, Inc., publishing as Benjamin Cummings,Many antibiotics, including penicillins, inhibit the synthesis of cross-links in peptidoglycans, preventing the formation of a functional wall, particularly in gram-positive species. These drugs are a very selective treatment because they c
21、ripple many species of bacteria without affecting humans and other eukaryotes, which do not synthesize peptidoglycans.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Many prokaryotes secrete another sticky protective layer, the capsule, outside the cell wall. Capsules adhere
22、 the cells to their substratum. They may increase resistance to host defenses. They glue together the cells of those prokaryotes that live as colonies.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Another way for prokaryotes to adhere to one another or to the substratum is
23、 by surface appendages called pili. Pili can fasten pathogenic bacteria to the mucous membranes of its host. Some pili are specialized for holding two prokaryote cells together long enough to transfer DNA during conjugation.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig
24、. 27.6,About half of all prokaryotes are capable of directional movement. The action of flagella, scattered over the entire surface or concentrated at one or both ends, is the most common method of movement. The flagella of prokaryotes differ in structure and function from those of eukaryotes.,2. Ma
25、ny prokaryotes are motile,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,In a prokaryotic flagellum, chains of a globular protein are wound in a tight spiral to form a filament, which is attached to another protein (the hook), which is inserted into the basal apparatus.Rotat
26、ion of the filament is driven by the diffusion of protons into the cell through the basal apparatus after the protons have been actively transported by proton pumps in the plasma membrane.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 27.7,A second motility mechanism i
27、s found in spirochetes, helical bacteria. Two or more helical filaments under the cell wall are attached to a basal motor attached to the cell. When the filaments rotate, the cell moves like a corkscrew. A third mechanism occurs in cells that secrete a jet of slimy threads that anchors the cells to
28、the substratum. The cell glides along at the growing end of threads.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,In a relatively uniform environment, a flagellated cell may wander randomly. In a heterogenous environment, many prokaryotes are capable of taxis, movement tow
29、ard or away from a stimulus. With chemotaxis, binding between receptor cells on the surface and specific substances results in movement toward the source (positive chemotaxis) or away (negative chemotaxis). Other prokaryotes can detect the presence of light (phototaxis) or magnetic fields.,Copyright
30、 2002 Pearson Education, Inc., publishing as Benjamin Cummings,CHAPTER 27 Prokaryotes and the Origins of Metabolic Diversity,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Section B2: The Structure, Function, and Reproduction of Prokaryotes,3. The cellular and genomic organi
31、zation of prokaryotes is fundamentally different from that of eukaryotes 4. Populations of eukaryotes grow and adapt rapidly,Prokaryotic cells lack a nucleus enclosed by membranes. The cells of prokaryotes also lack the other internal compartments bounded by membranes that are characteristic of euka
32、ryotes.,3. The cellular and genomic organization of prokaryotes is fundamentally different from that of eukaryotes,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Instead, prokaryotes used infolded regions of the plasma membrane to perform many metabolic functions, including
33、cellular respiration and photosynthesis.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 27.8,Prokaryotes have smaller, simpler genomes than eukaryotes. On average, a prokaryote has only about one-thousandth as much DNA as a eukaryote. Typically, the DNA is concentrated
34、as a snarl of fibers in the nucleoid region. The mass of fibers is actually the single prokaryotic chromosome, a double-stranded DNA molecule in the form of a ring. There is very little protein associated with the DNA.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Prokaryot
35、es may also have smaller rings of DNA, plasmids, that consist of only a few genes. Prokaryotes can survive in most environments without their plasmids because essential functions are programmed by the chromosomes. However, plasmids provide the cell genes for resistance to antibiotics, for metabolism
36、 of unusual nutrients, and other special contingencies. Plasmids replicate independently of the chromosome and can be transferred between partners during conjugation.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Although the general processes for DNA replication and transl
37、ation of mRNA into proteins are alike for eukaryotes and prokaryotes, some of the details differ. For example, the prokaryotic ribosomes are slightly smaller than the eukaryotic version and differs in its protein and RNA content. These differences are great enough that selective antibiotics, includi
38、ng tetracycline and chloramphenicol, can block protein synthesis in many prokaryotes but not in eukaryotes.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Prokaryotes reproduce only asexually via binary fission, synthesizing DNA almost continuously. A single cell in favorabl
39、e conditions will produce a colony of offspring.,4. Populations of prokaryotes grow and adapt rapidly,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 27.9,While lacking meiosis and sex as seen in eukarotes, prokaryotes have several mechanisms to combine genes between ind
40、ividuals. In transformation, a cell can absorb and integrate fragments of DNA from their environment. This allows considerable genetic transfer between prokaryotes, even across species lines. In conjugation, one cell directly transfers genes to another cell. In transduction, viruses transfer genes b
41、etween prokaryotes.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Lacking meiotic sex, mutation is the major source of genetic variation in prokaryotes. With generation times in minutes or hours, prokaryotic populations can adapt very rapidly to environmental changes, as na
42、tural selection screens new mutations and novel genomes from gene transfer.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,The word growth as applied to prokaryotes refers to multiplication of cells and population increases, rather than enlargement of individual cells. Condi
43、tions for optimal growth vary according to species. Variables include temperature, pH, salt concentrations, nutrient sources, among others.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,In the absence of limiting resources, growth of prokaryotes is effectively geometric. Th
44、e number of cells doubles each generation. Typical generation times range from 1-3 hours, but some species can double every 20 minutes in an optimal environment. Prokaryotic growth in the laboratory and in nature is usually checked at some point. The cells may exhaust some nutrient. Alternatively, t
45、he colony poisons itself with an accumulation of metabolic waste.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Prokaryote can also withstand harsh conditions. Some bacteria form resistant cells, endospores. In an endospore, a cell replicates its chromosome and surrounds on
46、e chromosome with a durable wall. While the outer cell may disinte- grate, an endospore, such as this anthrax endospore, dehy- drates, does not metabolize, and stays protected by a thick, protective wall.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 27.10,An endospore
47、 is resistant to all sorts of trauma. Endospores can survive lack of nutrients and water, extreme heat or cold, and most poisons. Sterilization in an autoclave kills even endospores by heating them to 120oC. Endospores may be dormant for centuries or more. When the environment becomes more hospitabl
48、e, the endospore absorbs water and resumes growth.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,In most environments, prokaryotes compete with other prokaryotes (and other microorganisms) for space and nutrients. Many microorganisms release antibiotics, chemicals that inhi
49、bit the growth of other microorganisms (including certain prokaryotes, protists, and fungi). Humans have learned to use some of these compounds to combat pathogenic bacteria.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,CHAPTER 27 Prokaryotes and the Origins of Metabolic Diversity,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Section C: Nutritional and Metabolic Diversity,1. Prokaryotes can be grouped into four categories according to how they obtain energy and carbon 2. Photosynthesis evolved early in prokaryotic life,
