B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 12
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Razin et al., Infect. Immun. 30:538–546,1980. With permission from the AmericanSociety for Microbiology; B, courtesy ofRoger Cole, in Medical Microbiology, 4thed. [S. Baron ed.]. Galveston: University ofTexas Medical Branch, 1996.)THE DIVERSITY OF GENOMES AND THE TREE OF LIFE11are called organotrophic (from the Greek word trophe, meaning “food”).
Othersderive their energy directly from the nonliving world. These primary energy converters fall into two classes: those that harvest the energy of sunlight, and thosethat capture their energy from energy-rich systems of inorganic chemicals in theenvironment (chemical systems that are far from chemical equilibrium). Organisms of the former class are called phototrophic (feeding on sunlight); those of thelatter are called lithotrophic (feeding on rock). Organotrophic organisms couldnot exist without these primary energy converters, which are the most plentifulform of life.Phototrophic organisms include many types of bacteria, as well as algae andplants, on which we—and virtually all the living things that we ordinarily seearound us—depend.
Phototrophic organisms have changed the whole chemistryof our environment: the oxygen in the Earth’s atmosphere is a by-product of theirbiosynthetic activities.Lithotrophic organisms are not such an obvious feature of our world, becausethey are microscopic and mostly live in habitats that humans do not frequent—deep in the ocean, buried in the Earth’s crust, or in various other inhospitableenvironments. But they are a major part of the living world, and they are especiallyimportant in any consideration of the history of life on Earth.Some lithotrophs get energy from aerobic reactions, which use molecular oxygen from the environment; since atmospheric O2 is ultimately the product of living organisms, these aerobic lithotrophs are, in a sense, feeding on the productsof past life.
There are, however, other lithotrophs that live anaerobically, in placeswhere little or no molecular oxygen is present. These are circumstances similarto those that existed in the early days of life on Earth, before oxygen had accumulated.The most dramatic of these sites are the hot hydrothermal vents on the floor ofthe Pacific and Atlantic Oceans. They are located where the ocean floor is spreading as new portions of the Earth’s crust form by a gradual upwelling of materialfrom the Earth’s interior (Figure 1–11). Downward-percolating seawater is heatedand driven back upward as a submarine geyser, carrying with it a current ofchemicals from the hot rocks below.
A typical cocktail might include H2S, H2, CO,Mn2+, Fe2+, Ni2+, CH2, NH4+, and phosphorus-containing compounds. A denseSEAdark cloud ofhot, mineral-richwateranaerobic lithotrophicbacteriainvertebrateanimal communityhydrothermalventchimney made fromprecipitated metalsulfides2–3°Csea floor350°Ccontourpercolationof seawaterhot mineral solutionhot basaltFigure 1–11 The geology of a hothydrothermal vent in the ocean floor. Asindicated, water percolates down towardthe hot molten rock upwelling from theEarth’s interior and is heated and drivenback upward, carrying minerals leachedfrom the hot rock. A temperature gradientis set up, from more than 350°C near thecore of the vent, down to 2–3°C in thesurrounding ocean.
Minerals precipitatefrom the water as it cools, forming achimney. Different classes of organisms,thriving at different temperatures, live indifferent neighborhoods of the chimney. Atypical chimney might be a few meters tall,spewing out hot, mineral-rich water at aflow rate of 1–2 m/sec.12Chapter 1: Cells and GenomesFigure 1–12 Organisms living at adepth of 2500 meters near a vent inthe ocean floor.
Close to the vent, attemperatures up to about 120°C, variouslithotrophic species of bacteria and archaea(archaebacteria) live, directly fueled bygeochemical energy. A little further away,where the temperature is lower, variousinvertebrate animals live by feeding onthese microorganisms. Most remarkableare these giant (2 meter) tube worms,Riftia pachyptila, which, rather than feedon the lithotrophic cells, live in symbiosiswith them: specialized organs in theworms harbor huge numbers of symbioticsulfur-oxidizing bacteria. These bacteriaharness geochemical energy and supplynourishment to their hosts, which haveno mouth, gut, or anus. The tube wormsare thought to have evolved from moreconventional animals, and to have becomesecondarily adapted to life at hydrothermalvents.
(Courtesy of Monika Bright,University of Vienna, Austria.)geochemical energy andinorganic raw materialsbacteriamulticellular animals, e.g., tube worms1mpopulation of microbes lives in the neighborhood of the vent, thriving on this austere diet and harvesting free energy from reactions between the available chemicals. Other organisms—clams, mussels, and giant marine worms—in turn live offthe microbes at the vent, forming an entire ecosystem analogous to the world ofplants and animals that we belong to, but powered by geochemical energy insteadof light (Figure 1–12).MBoC6 m1.16/1.12Some Cells Fix Nitrogen and Carbon Dioxide for OthersTo make a living cell requires matter, as well as free energy.
DNA, RNA, andprotein are composed of just six elements: hydrogen, carbon, nitrogen, oxygen,sulfur, and phosphorus. These are all plentiful in the nonliving environment, inthe Earth’s rocks, water, and atmosphere. But they are not present in chemicalforms that allow easy incorporation into biological molecules.
Atmospheric N2and CO2, in particular, are extremely unreactive. A large amount of free energyis required to drive the reactions that use these inorganic molecules to makethe organic compounds needed for further biosynthesis—that is, to fix nitrogenand carbon dioxide, so as to make N and C available to living organisms. Manytypes of living cells lack the biochemical machinery to achieve this fixation; theyinstead rely on other classes of cells to do the job for them. We animals depend onplants for our supplies of organic carbon and nitrogen compounds.
Plants in turn,although they can fix carbon dioxide from the atmosphere, lack the ability to fixatmospheric nitrogen; they depend in part on nitrogen-fixing bacteria to supplytheir need for nitrogen compounds. Plants of the pea family, for example, harborsymbiotic nitrogen-fixing bacteria in nodules in their roots.Living cells therefore differ widely in some of the most basic aspects of theirbiochemistry. Not surprisingly, cells with complementary needs and capabilitieshave developed close associations. Some of these associations, as we see below,have evolved to the point where the partners have lost their separate identitiesaltogether: they have joined forces to form a single composite cell.The Greatest Biochemical Diversity Exists Among Prokaryotic CellsFrom simple microscopy, it has long been clear that living organisms can beclassified on the basis of cell structure into two groups: the eukaryotes and theTHE DIVERSITY OF GENOMES AND THE TREE OF LIFE132 µmspherical cellse.g., Streptococcusrod-shaped cellse.g., Escherichia coli,Vibrio choleraethe smallest cellse.g., Mycoplasma,Spiroplasmaspiral cellse.g., Treponema pallidumprokaryotes.
Eukaryotes keep their DNA in a distinct membrane-enclosed intracellular compartment called the nucleus. (The name is from the Greek, meaning “truly nucleated,” from the words eu, “well” or “truly,” and karyon, “kernel”or “nucleus.”) Prokaryotes have no distinct nuclear compartment to house theirDNA. Plants, fungi, and animals are eukaryotes; bacteria are prokaryotes, as arearchaea—a separate class of prokaryotic cells, discussed below.Most prokaryotic cells are small and simple in outward appearance (Figure1–13), and they live mostly as independent individuals or in loosely organizedcommunities, rather than as multicellular organisms. They are typically sphericalor rod-shaped and measure a few micrometers in linear dimension. They oftenhave a tough protective coat, called a cell wall, beneath which a plasma memMBoC6 m1.17/1.13brane encloses a single cytoplasmic compartment containing DNA, RNA, proteins, and the many small molecules needed for life.
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