B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 11
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Without a plasmamembrane, the cell could not maintain its integrity as a coordinated chemicalsystem.The molecules that form a membrane have the simple physicochemicalproperty of being amphiphilic—that is, consisting of one part that is hydrophobic (water-insoluble) and another part that is hydrophilic (water-soluble). Suchmolecules placed in water aggregate spontaneously, arranging their hydrophobic portions to be as much in contact with one another as possible to hide themfrom the water, while keeping their hydrophilic portions exposed.
Amphiphilicmolecules of appropriate shape, such as the phospholipid molecules that comprise most of the plasma membrane, spontaneously aggregate in water to createa bilayer that forms small closed vesicles (Figure 1–9). The phenomenon can bedemonstrated in a test tube by simply mixing phospholipids and water together;under appropriate conditions, small vesicles form whose aqueous contents areisolated from the external medium.Although the chemical details vary, the hydrophobic tails of the predominantmembrane molecules in all cells are hydrocarbon polymers (–CH2–CH2–CH2–),and their spontaneous assembly into a bilayered vesicle is but one of many examples of an important general principle: cells produce molecules whose chemicalproperties cause them to self-assemble into the structures that a cell needs.The cell boundary cannot be totally impermeable.
If a cell is to grow and reproduce, it must be able to import raw materials and export waste across its plasmamembrane. All cells therefore have specialized proteins embedded in their membrane that transport specific molecules from one side to the other. Some of thesemembrane transport proteins, like some of the proteins that catalyze the fundamental small-molecule reactions inside the cell, have been so well preserved overthe course of evolution that we can recognize the family resemblances betweenthem in comparisons of even the most distantly related groups of living organisms.The transport proteins in the membrane largely determine which moleculesenter the cell, and the catalytic proteins inside the cell determine the reactionsthat those molecules undergo.
Thus, by specifying the proteins that the cell is tomanufacture, the genetic information recorded in the DNA sequence dictates theentire chemistry of the cell; and not only its chemistry, but also its form and itsbehavior, for these too are chiefly constructed and controlled by the cell’s proteins.A Living Cell Can Exist with Fewer Than 500 GenesThe basic principles of biological information transfer are simple enough, but howcomplex are real living cells? In particular, what are the minimum requirements?We can get a rough indication by considering a species that has one of the smallest known genomes—the bacterium Mycoplasma genitalium (Figure 1–10).
Thisorganism lives as a parasite in mammals, and its environment provides it withmany of its small molecules ready-made. Nevertheless, it still has to make all thelarge molecules—DNA, RNAs, and proteins—required for the basic processes ofheredity. It has about 530 genes, about 400 of which are essential. Its genome of580,070 nucleotide pairs represents 145,018 bytes of information—about as muchas it takes to record the text of one chapter of this book. Cell biology may be complicated, but it is not impossibly so.The minimum number of genes for a viable cell in today’s environments isprobably not less than 300, although there are only about 60 genes in the core setthat is shared by all living species.9phospholipidmonolayerOILphospholipidbilayerWATERMBoC6 m1.12/1.0910Chapter 1: Cells and GenomesSummaryThe individual cell is the minimal self-reproducing unit of living matter, and it consists of a self-replicating collection of catalysts. Central to this reproduction is thetransmission of genetic information to progeny cells.
Every cell on our planet storesits genetic information in the same chemical form—as double-stranded DNA. Thecell replicates its information by separating the paired DNA strands and using eachas a template for polymerization to make a new DNA strand with a complementary sequence of nucleotides. The same strategy of templated polymerization is usedto transcribe portions of the information from DNA into molecules of the closelyrelated polymer, RNA. These RNA molecules in turn guide the synthesis of proteinmolecules by the more complex machinery of translation, involving a large multimolecular machine, the ribosome.
Proteins are the principal catalysts for almostall the chemical reactions in the cell; their other functions include the selectiveimport and export of small molecules across the plasma membrane that forms thecell’s boundary. The specific function of each protein depends on its amino acidsequence, which is specified by the nucleotide sequence of a corresponding segmentof the DNA—the gene that codes for that protein. In this way, the genome of thecell determines its chemistry; and the chemistry of every living cell is fundamentallysimilar, because it must provide for the synthesis of DNA, RNA, and protein.
Thesimplest known cells can survive with about 400 genes.(A)5 µmTHE DIVERSITY OF GENOMES AND THE TREE OF LIFEThe success of living organisms based on DNA, RNA, and protein has been spectacular. Life has populated the oceans, covered the land, infiltrated the Earth’scrust, and molded the surface of our planet. Our oxygen-rich atmosphere, thedeposits of coal and oil, the layers of iron ores, the cliffs of chalk and limestoneand marble—all these are products, directly or indirectly, of past biological activity on Earth.Living things are not confined to the familiar temperate realm of land, water,and sunlight inhabited by plants and plant-eating animals.
They can be found inthe darkest depths of the ocean, in hot volcanic mud, in pools beneath the frozen surface of the Antarctic, and buried kilometers deep in the Earth’s crust. Thecreatures that live in these extreme environments are generally unfamiliar, notonly because they are inaccessible, but also because they are mostly microscopic.In more homely habitats, too, most organisms are too small for us to see withoutspecial equipment: they tend to go unnoticed, unless they cause a disease or rotthe timbers of our houses.
Yet microorganisms make up most of the total massof living matter on our planet. Only recently, through new methods of molecularanalysis and specifically through the analysis of DNA sequences, have we begunto get a picture of life on Earth that is not grossly distorted by our biased perspective as large animals living on dry land.In this section, we consider the diversity of organisms and the relationshipsamong them.
Because the genetic information for every organism is written inthe universal language of DNA sequences, and the DNA sequence of any givenorganism can be readily obtained by standard biochemical techniques, it is nowpossible to characterize, catalog, and compare any set of living organisms withreference to these sequences. From such comparisons we can estimate the placeof each organism in the family tree of living species—the “tree of life.” But beforedescribing what this approach reveals, we need first to consider the routes bywhich cells in different environments obtain the matter and energy they require tosurvive and proliferate, and the ways in which some classes of organisms dependon others for their basic chemical needs.Cells Can Be Powered by a Variety of Free-Energy SourcesLiving organisms obtain their free energy in different ways.
Some, such as animals,fungi, and the many different bacteria that live in the human gut, get it by feeding on other living things or the organic chemicals they produce; such organisms(B)0.2 µmFigure 1–10 Mycoplasma genitalium.(A) Scanning electron micrograph showingthe irregular shape of this small bacterium,reflecting the lack of any rigid cell wall.(B) Cross section (transmission electronmicrograph) of a Mycoplasma cell.
Of the530 genes of Mycoplasma genitalium,43 code for transfer, ribosomal, and othernon-messenger RNAs. Functions areknown, or can be guessed, for 339 of thegenes coding for protein: of these, 154are involved in replication, transcription,translation,and m1.14/1.10related processesMBoC6involving DNA, RNA, and protein; 98 in themembrane and surface structures of thecell; 46 in the transport of nutrients andother molecules across the membrane;71 in energy conversion and the synthesisand degradation of small molecules; and12 in the regulation of cell divisionand other processes. Note that thesecategories are partly overlapping, so thatsome genes feature twice. (A, fromS.
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