Часть 1 (1120999), страница 23
Текст из файла (страница 23)
As a result, pure water contains an equal, very low concentration of H3O+and OH- ions, both being present at 10-7M. (The concentration ofH2O in pure water is 55.5M.)Substancesthat releaseprotons to form H3O+when they dissolve in waterare termed acids. The higher the concentration of HsO*, the more acidic thesolution. As H3O* rises, the concentration of OH- falls, according to the equilibrium equation for water: [HsO*][OH-] = 1.0 x 10-la, where square bracketsdenote molar concentrations to be multiplied.
By tradition, the H3O+ concentration is usually referred to as the H+ concentration, even though nearly all H+in an aqueous solution is present as H3O+.To avoid the use of unwieldy numbers, the concentration of H+ is expressedusing a logarithmic scale called thepH scale, as illustrated in Panel 2-2 (pp.108-109). Pure water has a pH of 7.0,and is neutral-that is, neither acidic (pH < 7.0) nor basic (pH > 7.0).Figure2-1 2 Threerepresentationsof awater molecule.(A)The usuallineformula,indrawingof the structuralwhicheachatom is indicatedby itsstandardsymbol,and eachlinerepresentsa covalentbondjoiningtwomodel,inatoms.(B)A ball-and-stickby sphereswhichatomsarerepresentedof arbitrarydiameter,connectedby sticksrepresentingcovalentbonds.Unlike(A),representedbond anglesareaccuratelyin thistype of model(seealsoFiguremodel,in which2-8).(C)A space-fillingboth bond geometryand van derWaalsrepresented.radiiareaccuratelyTHECHEMICALCOMPONENTSOFA CELLo53HOHcH3-Co-H66acetic acid|| !:!lHrOhydroniumtonacetateton(A)HrO_-proton movesfrom onem o l e c u l et othe other(B)HrOtOH-hydroniumtonhydroxyltonBecausethe proton of a hydronium ion can be passedreadily to many typesof molecules in cells, altering their character,the concentration of H3O+inside acell (the acidi$ must be closely regulated.
The interior of a cell is kept close toneutrality, and it is buffered by the presence of many chemical groups that cantake up and releaseprotons near pH 7.The opposite of an acid is a base. Just as the defining property of an acid isthat it donates protons to a water molecule so as to raise the concentration ofH3O+ions, the defining property of a base is that it acceptsprotons so as to lowerthe concentration of H3O+ions, and thereby raise the concentration of hydroxylions (OH-). A base can either combine with protons directly or form hydroxylions that immediately combine with protons to produce H2O.
Thus sodiumhydroxide (NaOH) is basic (or alkaline) because it dissociatesin aqueous solution to form Na+ ions and OH- ions. Other bases,especially important in livingcells, contain NH2 groups. These groups directly take up a proton from water:-NH2 + H2O -+ -NHs* + OH-.All molecules that accept protons from water will do so most readily whenthe concentration of H3O* is high (acidic solutions). Likewise, molecules thatcan give up protons do so more readily if the concentration of H3O+in solutionis low (basic solutions), and thev will tend to receive them back if this concentration is high.FourTypesof NoncovalentAttractionsHelpBringMoleculesTogetherin CellsIn aqueous solutions, covalent bonds are 10-100 times stronger than the otherattractive forces between atoms, allowing their connections to define theboundaries of one molecule from another.
But much of biology depends on thespecific binding of different molecules to each other. This binding is mediatedby a group of noncovalent attractions that are individually quite weak, butwhose energies can sum to create an effective force between two separatemolecules. We have previously introduced three of these attractive forces: electrostatic attractions (ionic bonds), hydrogen bonds, and van der Waals attractions.
Table 2-l compares the strengths of these three types of noncoualentbonds with that of a typical covalent bond, both in the presence and in theTable2-1 Covalentand NoncovalentChemicalBondsCovalentNoncovalent:ionicxhydrogenvan derWaalsattraction(peratom)0.150.2s0.300.35908040.1*An ionicbond isan electrostaticattractionbetweentwo fullvcharoedatoms90310.1Figure2-13 Acids in water. (A)Thereactionthat takesolacewhen amoleculeof aceticaciddissolvesin water.(B)Watermoleculesarecontinuouslyexchangingprotonswith eachothertoform hydroniumand hydroxylions.Theseionsin turn rapidlyrecombineto formwatermolecules.54Chapter2: CellChemistryand Biosynthesisabsenceof water. Becauseof their fundamental importance in all biological systems, we summarize their properties here:Electrostatic attractions.
These result from the attractive forces betweenoppositely charged atoms. Electrostatic attractions are quite strong in theabsence of water. They readily form between permanent dipoles, but aregreatestwhen the two atoms involved are fully charged (ionic bonds).However,the polar water molecules cluster around both fully charged ions andpolar molecules that contain permanent dipoles (Figure 2-14). This greatlyreduces the attractivenessof these charged species for each other in mostbiological settings.Hydrogen bonds.
The structure of a typical hydrogen bond is illustrated inFigure 2-15. This bond represents a special form of polar interaction inwhich an electropositive hydrogen atom is partially shared by two electronegative atoms. Its hydrogen can be viewed as a proton that has partially dissociated from a donor atom, allowing it to be shared by a secondacceptor atom. Unlike a typical electrostatic interaction, this bond ishighly directional-being strongest when a straight line can be drawnbetween all three of the involved atoms.
As already discussed,water weakens these bonds by forming competing hydrogen-bond interactions withthe involved molecules.van der Waals attractions. The electron cloud around any nonpolar atomwill fluctuate, producing a flickering dipole. Such dipoles will transientlyinduce an oppositely polarized flickering dipole in a nearby atom. Thisinteraction generates a very weak attraction between atoms. But sincemany atoms can be simultaneously in contact when two surfaces fitclosely,the net result is often significant. Water does not weaken these socalled van der Waals attractions.The fourth effect that often brings molecules together in water is not, strictlyspeaking, a bond at all.
However, a very important hydrophobic force is causedby a pushing of nonpolar surfaces out of the hydrogen-bonded water network,where they would otherwise physically interfere with the highly favorable interactions between water molecules. Bringing any two nonpolar surfaces togetherreduces their contact with water; in this sense,the force is nonspecific.
Nevertheless, we shall see in Chapter 3 that hydrophobic forces are central to theproper folding of protein molecules.Panel 2-3 provides an overview of the four types of attractions justdescribed. And Figure 2-16 illustrates schematically how many such interactions can sum to hold together the matching surfaces of two macromolecules,even though each interaction by itself would be much too weak to be effective inthe face of thermal motions.tr_Figure2-14 How the dipoleson watermoleculesorientto reducethe affinityof oppositelychargedionsor polargroups for each other.A Cellls Formedfrom CarbonCompoundsFigure2-15 Hydrogenbonds.(A)Ball-and-stickmodelof a typicalhydrogenbond.Thedistancebetweenthe hydrogenand the oxygenatomhereis lessthan the sum of theirvanderWaalsradii,indicatinga partial(B)The mostcommonhydrogenbondsin cells.sharingof electrons."ouH(A)Having looked at the ways atoms combine into small molecules and how thesemolecules behave in an aqueous environment, we now examine the mainclassesof small molecules found in cells and their biological roles.We shall seethat a few basic categoriesof molecules, formed from a handful of different elements, give rise to all the extraordinary richness of form and behavior shown byliving things.If we disregard water and inorganic ions such as potassium, nearly all themolecules in a cell are based on carbon.
Carbon is outstanding among all the elements in its ability to form large molecules; silicon is a poor second. Becauseit issmall and has four electrons and four vacancies in its outermost shell, a carbonatom can form four covalent bonds with other atoms. Most important, one carbon atom can join to other carbon atoms through highly stable covalent C-Cu6-u,h y d r o g e nb o n d - 0 .
3 n m l o n goonoratomacceproratom.ou-rf"naUonO- 0 . 1n m l o n g(B)o-Hililililililroo-Hililililililtooo - H ilililililil|- H ililililililroH ililililililrO- H ilililililil|T H EC H E M I C ACL O M P O N E N TOSF A C E L L55bonds to form chains and rings and hence generatelarge and complex moleculeswith no obvious upper limit to their size (seePanel 2-1, pp. 106-107).The smalland large carbon compounds made by cells are called organic molecules.Certain combinations of atoms, such as the methyl (-CHs), hydroxyl (-OH),carboxyl (-COOH), carbonyl (-C=O), phosphate (-POs2-),sulfhydryl (-SH), andamino (-NHz) groups, occur repeatedly in organic molecules.
Each such chemical group has distinct chemical and physical properties that influence the behavior of the molecule in which the group occurs. The most common chemicalgroups and some of their properties are summarized in Panel 2-1, pp. 106-107.CellsContainFourMajorFamiliesof SmallOrganicMoleculesThe small organic molecules of the cell are carbon-based compounds that havemolecular weights in the range 100-1000 and contain up to 30 or so carbonatoms. They are usually found free in solution and have many different fates.Some are used as monomer subunits to construct the giant polymeric macromolecules-the proteins, nucleic acids, and large polysaccharides-of the cell.Others act as energy sources and are broken down and transformed into othersmall molecules in a maze of intracellular metabolic pathways.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
