B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 23
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(1996) The Biology of Xenopus. Oxford:Clarendon Press.Tyson JJ, Chen KC & Novak B (2003) Sniffers, buzzers, toggles andblinkers: dynamics of regulatory and signaling pathways in the cell.Curr. Opin. Cell Biol. 15, 221–231.Venter JC, Adams MD, Myers EW et al (2001) The sequence of thehuman genome. Science 291, 1304–1351.43CHAPTER2Cell Chemistry andBioenergeticsIt is at first sight difficult to accept the idea that living creatures are merely chemical systems. Their incredible diversity of form, their seemingly purposeful behavior, and their ability to grow and reproduce all seem to set them apart from theworld of solids, liquids, and gases that chemistry normally describes.
Indeed,until the nineteenth century animals were believed to contain a Vital Force—an“animus”—that was responsible for their distinctive properties.We now know that there is nothing in living organisms that disobeys chemicalor physical laws. However, the chemistry of life is indeed special. First, it is basedoverwhelmingly on carbon compounds, the study of which is known as organicchemistry. Second, cells are 70% water, and life depends largely on chemical reactions that take place in aqueous solution.
Third, and most important, cell chemistry is enormously complex: even the simplest cell is vastly more complicatedin its chemistry than any other chemical system known. In particular, althoughcells contain a variety of small carbon-containing molecules, most of the carbonatoms present are incorporated into enormous polymeric molecules—chains ofchemical subunits linked end-to-end. It is the unique properties of these macromolecules that enable cells and organisms to grow and reproduce—as well as todo all the other things that are characteristic of life.IN THIS CHAPTERTHE CHEMICAL COMPONENTSOF A CELLCATALYSIS AND THE USE OFENERGY BY CELLSHOW CELLS OBTAIN ENERGYFROM FOODTHE CHEMICAL COMPONENTS OF A CELLLiving organisms are made of only a small selection of the 92 naturally occurringelements, four of which—carbon (C), hydrogen (H), nitrogen (N), and oxygen(O)—make up 96.5% of an organism’s weight (Figure 2–1).
The atoms of these elements are linked together by covalent bonds to form molecules (see Panel 2–1, pp.90–91). Because covalent bonds are typically 100 times stronger than the thermalenergies within a cell, they resist being pulled apart by thermal motions, and theyare normally broken only during specific chemical reactions with other atoms andmolecules. Two different molecules can be held together by noncovalent bonds,atomic number1H1Heatomic weight5Li Be1119K39Ca Sc40Rb SrYTi23V51N14158O16169F1917NeArCr Mn Fe Co Ni Cu Zn Ga Ge As Se BrKr2420C12147ClAlNa Mg23B1112624524225552656275928592964Si283065P31S323479Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te96Cs Ba LaHf Ta W Re OsFr Ra AcRf DbIrPt Au Hg Tl Pb3553I127XeBi Po At RnFigure 2–1 The main elements in cells,highlighted in the periodic table.
Whenordered by their atomic number andarranged in this manner, elements fallinto vertical columns that show similarproperties. Atoms in the same verticalcolumn must gain (or lose) the samenumber of electrons to attain a filled outershell, and they thus behave similarly inbond or ion formation. Thus, for example,Mg and Ca tend to give away the twoelectrons in their outer shells. C, N, and Ooccur in the same horizontal row, and tendto complete their second shells by sharingelectrons.The four elements highlighted in redconstitute 99% of the total number ofatoms present in the human body.
Anadditional seven elements, highlighted inblue, together represent about 0.9% ofthe total. The elements shown in green arerequired in trace amounts by humans. Itremains unclear whether those elementsshown in yellow are essential in humans.The chemistry of life, it seems, is thereforepredominantly the chemistry of lighterelements. The atomic weights shown hereare those of the most common isotope ofeach element.44Chapter 2: Cell Chemistry and BioenergeticsATPhydrolysisin cellaveragethermal motionsENERGYCONTENT(kJ/mole)110noncovalent bondbreakage in waterC–C bondbreakage100100010,000 kJgreencompletelight glucose oxidationwhich are much weaker (Figure 2–2). We shall see later that noncovalent bondsare important in the many situations where molecules have to associate and dissociate readily to carry out their biological functions.Water Is Held TogetherMBoC6by HydrogenBondsm2.07/2.02The reactions inside a cell occur in an aqueous environment.
Life on Earth beganin the ocean, and the conditions in that primeval environment put a permanentstamp on the chemistry of living things. Life therefore hinges on the chemicalproperties of water, which are reviewed in Panel 2–2, pp. 92–93.In each water molecule (H2O) the two H atoms are linked to the O atom bycovalent bonds. The two bonds are highly polar because the O is strongly attractive for electrons, whereas the H is only weakly attractive. Consequently, there isan unequal distribution of electrons in a water molecule, with a preponderanceof positive charge on the two H atoms and of negative charge on the O.
Whena positively charged region of one water molecule (that is, one of its H atoms)approaches a negatively charged region (that is, the O) of a second water molecule, the electrical attraction between them can result in a hydrogen bond. Thesebonds are much weaker than covalent bonds and are easily broken by the random thermal motions that reflect the heat energy of the molecules. Thus, eachbond lasts only a short time. But the combined effect of many weak bonds can beprofound. For example, each water molecule can form hydrogen bonds throughits two H atoms to two other water molecules, producing a network in whichhydrogen bonds are being continually broken and formed. It is only because ofthe hydrogen bonds that link water molecules together that water is a liquid atroom temperature—with a high boiling point and high surface tension—ratherthan a gas.Molecules, such as alcohols, that contain polar bonds and that can formhydrogen bonds with water dissolve readily in water.
Molecules carrying charges(ions) likewise interact favorably with water. Such molecules are termed hydrophilic, meaning that they are water-loving. Many of the molecules in the aqueousenvironment of a cell necessarily fall into this category, including sugars, DNA,RNA, and most proteins. Hydrophobic (water-hating) molecules, by contrast, areuncharged and form few or no hydrogen bonds, and so do not dissolve in water.Hydrocarbons are an important example. In these molecules all of the H atoms arecovalently linked to C atoms by a largely nonpolar bond; thus they cannot formeffective hydrogen bonds to other molecules (see Panel 2–1, p.
90). This makes thehydrocarbon as a whole hydrophobic—a property that is exploited in cells, whosemembranes are constructed from molecules that have long hydrocarbon tails, aswe see in Chapter 10.Four Types of Noncovalent Attractions Help Bring MoleculesTogether in CellsMuch of biology depends on the specific binding of different molecules caused bythree types of noncovalent bonds: electrostatic attractions (ionic bonds), hydrogen bonds, and van der Waals attractions; and on a fourth factor that can pushmolecules together: the hydrophobic force. The properties of the four types ofnoncovalent attractions are presented in Panel 2–3 (pp.
94–95). Although eachFigure 2–2 Some energies importantfor cells. A crucial property of any bond—covalent or noncovalent—is its strength.Bond strength is measured by the amountof energy that must be supplied to breakit, expressed in units of either kilojoulesper mole (kJ/mole) or kilocalories per mole(kcal/mole). Thus if 100 kJ of energy mustbe supplied to break 6 × 1023 bonds ofa specific type (that is, 1 mole of thesebonds), then the strength of that bond is100 kJ/mole.
Note that, in this diagram,energies are compared on a logarithmicscale. Typical strengths and lengths of themain classes of chemical bonds are givenin Table 2–1.One joule (J) is the amount of energyrequired to move an object a distance ofone meter against a force of one Newton.This measure of energy is derived from theSI units (Système Internationale d’Unités)universally employed by physical scientists.A second unit of energy, often used bycell biologists, is the kilocalorie (kcal); onecalorie is the amount of energy needed toraise the temperature of 1 gram of water by1°C. One kJ is equal to 0.239 kcal (1 kcal= 4.18 kJ).THE CHEMICAL COMPONENTS OF A CELL45Figure 2–3 Schematic indicating how two macromolecules withcomplementary surfaces can bind tightly to one another throughnoncovalent interactions. Noncovalent chemical bonds have less than1/20 the strength of a covalent bond.
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