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Likewise, the other atoms found inliving tissues have incomplete outer electron shells and can donate, accept, orshare electrons with each other to form both molecules and ions (Figure 2-4).Becausean unfilled electron shell is less stable than a filled one, atoms withincomplete outer shells tend to interact with other atoms in a way that causesthem to either gain or lose enough electrons to achieve a completed outermostshell. This electron exchange occurs either by transferring electrons from oneatom to another or by sharing electrons between two atoms. These two strategies generate two types of chemical bonds between atoms: an ionic bond isformed when electrons are donated by one atom to another, whereas a coualentbondis formed when two atoms share a pair of electrons (Figure 2-5).
Often, thepair of electrons is shared unequally, with a partial transfer between two atomsthat attract electrons differently-one more electronegatiuethanthe other: thisintermediate strategy results in a polar coualentbond, as we shall discusslater.An H atom, which needs only one electron to fill its shell, generally acquiresit by electron sharing, forming one covalent bond with another atom; often thisbond is polar-meaning that the electrons are shared unequally. The other common elements in living cells-C, N, and O, with an incomplete second shell, andP and S, with an incomplete third shell (see Figure 2-4)-generally share electrons and achieve a filled outer shell of eight electrons by forming several covalent bonds. The number of electrons that an atom must acquire or lose (eitherby sharing or by transfer) to fill its outer shell is knorm as irs ualence.The crucial role of the outer electron shell in determining the chemical properties of an element means that, when the elements are listed in order of theiratomic number, there is a periodic recurrence of elements with similar properties: an element with, say, an incomplete second shell containing one electronwill behave in much the same way as an element that has filled its second shellA mole is X grams of a substance,where X is its relative molecularmass(molecularweight).A mole will contain5 x 102m3 o l e c u l e so f t h e s u b s t a n c e .1 m o l e o f c a r b o nw e i g h s1 2 g1 m o l e o f g l u c o s ew e i g h s 1 8 0g1 m o l e o f s o d i u mc h l o r i d ew e i g h s5 8 gMolar solutions have a concentrationof 1 mole of the substancein 1 liter ofs o l u t i o n .A m o l a r s o l u t i o n( d e n o t e da s1 M ) o f g l u c o s e{,o r e x a m p l e ,h a s, h i l e a m i l l i m o l asr o l u t i o n1 8 0g / 1 w(1 mM) has 180mg/|.The standardabbreviationfor gram is g;the abbreviationfor liter is l.Figure2-2 Molesand molar solutions.:,4.4:lpri',1:ll:rllr:ai:':ir:il!numanoodvEarth'scrustoc6Ef s o!a@aoE20ooI.:anoMgNaPanoKFigure2-3 The abundancesof somechemicalelementsin the nonlivingworld (the Earth'scrust)comparedwiththeir abundancesin the tissuesof ananimal.The abundanceof eachelementis expressedas a percentageof the totalnumberof atomspresentincludingwater.Thus,becauseof the abundanceofwater,more than 600loof the atoms in alivingorganismarehydrogenatoms.Therelativeabundanceof elementsis similari n a l l l i v i n gt h i n g s .48Chapter2: CellChemistryand Biosynthesistomic numberIe l e c t r o ns h e l |-Figure2-4 Filledand unfilledelectronshellsin somecommonelements.Allthe elementscommonlyfound in livingorganismshaveunfilledoutermostshells(red)andcan thus participatein chemicalreactionswith otheratoms.Forcomparison,someelementsthat haveonly filled shells(yellow)areshown;thesearechemicallyunreactive.&ae&&&&&&&&se&8*a€&*****&&&&&&e e e * | & # * t * , s & & & eeand has an incomplete third shell containing one electron.
The metals, forexample, have incomplete outer shells with just one or a few electrons,whereas,as we have just seen,the inert gaseshave full outer shells.This pattern gives riseto the famous periodic table of the elements, presented in Figure 2-6 with theelements found in living organisms highlighted.CovalentBondsFormby the Sharingof ElectronsAll the characteristics of a cell depend on the molecules it contains. A moleculeis defined as a cluster of atoms held together by covalent bonds; here electronsare shared between atoms to complete the outer shells,rather than being transferred between them. In the simplest possible molecule-a molecule of hydrogen (H2)-two H atoms, each with a single electron, share two electrons, whichis the number required to fill the first shell.
These shared electrons form a cloudof negative charge that is densestbetween the two positively charged nuclei andhelps to hold them together, in opposition to the mutual repulsion between likecharges that would otherwise force them apart. The attractive and repulsiveforces are in balance when the nuclei are separatedby a characteristic distance,called the bond length.Another property of any bond-covalent or noncovalent-is its bond strength,which is measured by the amount of energy that must be supplied to break thatbond.
This is often expressedin units of kilocalories per mole (kcal/mole), wherea kilocalorie is the amount of energy needed to raise the temperature of one literaromsaUY-,;ELECTRoNSImoleculecovalent bondpositiveronnegativetoni o n i cb o n dFigure2-5 Comparisonof covalentandionicbonds.Atomscanattaina morestablearrangementof electronsin theiroutermostshellby interactingwith oneanother.An ionicbond isformedwhenelectronsare transferredfrom one atomto the other.A covalentbond isformedwhen electronsare sharedbetweenatoms.The two casesshown representextremes;often,covalentbonds formwith a partialtransfer(unequalsharingofelectrons),resultingin a polarcovalentbond (seeFigure2-43).49THECHEMICALCOMPONENTSOFA CELLa t o m i cn u m b e ra t o m i cw e i g h t6789CNOF12111214232824202324252621KCaVCrMnFeCoNi39514016siPSclNa Mg1916141552425556592A2911643534305eCuZn59321911196553MoI96127of water by one degreeCelsius(centigrade).Thus if 1 kilocalorie must be suppliedto break 6 x 1023bonds of a specific type (that is, I mole of these bonds), then thestrength of that bond is I kcal/mole.
An equivalent, widely used measure ofenergy is the kilojoule, which is equal to 0.239kilocalories.To understand bond strengths, it is helpful to compare them with the average energiesof the impacts that molecules are constantly experiencing from collisions with other molecules in their environment (their thermal, or heat,energy), as well as with other sources of biological energy such as light and glucose oxidation (Figure 2-7).Typical covalent bonds are stronger than the thermal energies by a factor of 100, so they resist being pulled apart by thermalmotions and are normally broken only during specific chemical reactions withother atoms and molecules.
The making and breaking of covalent bonds areviolent events, and in living cells they are carefully controlled by highly specificcatalysts, called enzymes.Noncovalent bonds as a rule are much weaker; weshall see later that they are important in the cell in the many situations wheremolecules have to associateand dissociate readily to carry out their functions.\Mhereasan H atom can form only a single covalent bond, the other common atoms that form covalent bonds in cells-O, N, S, and B as well as the allimportant C atom-can form more than one.
The outermost shell of theseatoms, as we have seen, can accommodate up to eight electrons, and they formcovalent bonds with as many other atoms as necessary to reach this number.Oxygen, with six electrons in its outer shell, is most stable when it acquires anextra two electrons by sharing with other atoms and therefore forms up to twocovalent bonds. Nitrogen, with five outer electrons, forms a maximum of threecovalent bonds, while carbon, with four outer electrons, forms up to four covalent bonds-thus sharing four pairs of electrons (seeFigure 2-4).\.Vhen one atom forms covalent bonds with several others, these multiplebonds have definite arrangements in spacerelative to one another, reflecting theorientations ofthe orbits ofthe shared electrons.The covalent bonds ofsuch anatom are therefore characterized by specific bond angles as well as by bondlengths and bond energies (Figure 2-B).
The four covalent bonds that can formaround a carbon atom, for example, are arranged as if pointing to the four corners of a regular tetrahedron. The precise orientation of covalent bonds formsthe basis for the three-dimensional geometry of organic molecules.averaget h e r m a lm o t i o n sE NE R G YCONTENT( k c a l / m o l e0) .11noncovalentbondbreakagein waterATPhydrolysisin cellFigure2-6 Elementsorderedby theiratomicnumberform the periodictable.Elementsfall into groupsthat showsimilarpropertiesbasedon the numberin itsof electronseachelementpossessesoutershell.Forexample,Mg and Catendto giveawaythe two electronsin theiroutershells;C, N,and O completetheirThesecondshellsby sharingelectrons.four elementshighlightedin redconstitute99oloof the total number ofatomspresentin the humanbody.Anadditionalsevenelements,highlightedinofb/ue,together representabout 0.9olothe total.Otherelements,shownin green,arerequiredin traceamountsby humans.It remainsunclearwhetherthoseinelementsshownin yellowareessentialhumansor not.Thechemistryof life,ittheseems,is thereforepredominantlychemistryof lighterelements.Atomicweights,givenby the sum ofthe orotonsand neutronsin the atomicwill varywith the particularnucleus,isotopeof the element.Theatomicweightsshownherearethoseof themostcommonisotopeof eachelement.C-C bondbreakage[ ::.,tr,,+;:.1]*rrg|.f+€*#*#Ei#fflff10010greenlight1000completeglucoseoxidationFigure2-7 Someenergiesimportant forcells.Notethat theseenergiesarecomparedon a logarithmicscale50Chapter2: CellChemistryand Biosynthesis-ooxygen(A)-N-InitrogenI-c IcaroonFigure2-8 The geometry of covalentbonds.(A)Thespatialarrangementof thecovalentbondsthat can be formed byoxygen,nitrogen,and carbon.(B)Moleculesformed from theseatomshavea precisethree-dimensionalstructure,as shown here by ball-and-stickmodelsfor waterand propane.A structurecan be specifiedby the bondanglesand bond lengthsfor eachcovalentlinkage.Theatomsarecoloredaccordingto the following,generallyusedconvention:H, white;C, block;O, red; N, blue.water (H2O)(B)p r o p a n e( C H 3 - C H 2 - C H 3 )ThereAre DifferentTypesof CovalentBondsMost covalent bonds involve the sharing of two electrons, one donated by eachparticipating atom; these are called single bonds.
Some covalent bonds, however,involve the sharing of more than one pair of electrons. Four electrons can beshared, for example, two coming from each participating atom; such a bond iscalled a double bond. Double bonds are shorter and stronger than single bondsand have a characteristic effect on the three-dimensional geometry of moleculescontaining them. A single covalent bond between tvvo atoms generally allowsthe rotation of one part of a molecule relative to the other around the bond axis.A double bond prevents such rotation, producing a more rigid and less flexiblearrangement of atoms (Figure 2-9 and Panel 2-1, pp. 106-107).In some molecules, electrons are shared among three or more atoms, producing bonds that have a hybrid character intermediate between single anddouble bonds.
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