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Many smallmolecules have more than one role in the cell-for example, acting both as apotential subunit for a macromolecule and as an energy source. Small organicmolecules are much less abundant than the organic macromolecules, accounting for only about one-tenth of the total mass of organic matter in a cell (Table2-Z). As a rough guess,there may be a thousand different kinds of these smallmolecules in a typical cell.All organic molecules are slmthesized from and are broken down into thesame set of simple compounds. Both their slmthesis and their breakdown occurthrough sequences of limited chemical changes that follow definite rules.
As aconsequence, the compounds in a cell are chemically related and most can beclassified into a few distinct families. Broadly speaking, cells contain four majorfamilies of small organic molecules: lhe sugars, the fatty acids, the amino acids,and the nucleotides (Figure 2-17). Although many compounds present in cellsdo not fit into these categories,these four families of small organic molecules,together with the macromolecules made by linking them into long chains,account for a large fraction of cell mass (seeTable 2-2).SugarsProvidean EnergySourcefor Cellsand Arethe SubunitsofPolysaccharidesThe simplest sugars-the monosaccharides-are compounds with the generalformula (CH2O)2,where n is usually 3, 4, 5, 6,7 , or 8.
Sugars,and the moleculesmade from them, are also called carbohydratesbecause of this simple formula.Glucose,for example, has the formula C6H1206@igure 2-18). The formula, however,does not fully define the molecule: the same set of carbons, hydrogens, andTable2-2 TheTypesof MoleculesThat Forma BacterialCellWaterI n o r g a n i co n sSugarsand precursorsAminoacidsand precursorsNucleotidesand precursorsFattyacidsand precursorsO t h e rs m a lml olecules(proteiMacromoleculesns,nucleicacids,and polysaccharides)70110.40.410.22612025010010050-300-3000ilFigure2-16 Schematicindicatinghowwithtwo macromoleculescomplementarysurfacescan bindtightly to one anotherthroughnoncovalentinteractions,56Chapter2:CellChemistryand Biosynthesisb u i l d i n gb l o c k sof the celll a r g e ru n i t sof the cell-laltaaclg:---J+_AUlIgASlps"_-"__l+PROTEINSNUCLEICACIDS___NUcIi-oJlPSl**.-I+!woxygens can be joined together by covalent bonds in a variety ofways, creatingstructures with different shapes.As shown in Panel 2-4 (pp.1l2-113), for example, glucose can be converted into a different sugar-mannose or galactosesimply by switching the orientations of specific OH groups relative to the rest ofthe molecule.
Each of these sugars,moreover, can exist in either of two forms,called the D-form and the l-form, which are mirror images of each other. Setsofmolecules with the same chemical formula but different structures are calledisomers,and the subset of such molecules that are mirror-image pairs are calledoptical isomers.Isomers are widespread among organic molecules in general,and they play a major part in generating the enormous variety of sugars.Panel 2-4 presents an outline of sugar structure and chemistry. Sugarscanexist as rings or as open chains. In their open-chain form, sugars contain anumber of hydroxyl groups and either one aldehyde ( > C : O) or one ketoneH(> C: O) group.
The aldehyde or ketone group plays a special role. First, it canreact with a hydroxyl group in the same molecule to convert the molecule intoa ring; in the ring form the carbon of the original aldehyde or ketone group canbe recognized as the only one that is bonded to two oxygens.Second, once thering is formed, this same carbon can become further linked, via oxygen, to oneof the carbons bearing a hydroxyl group on another sugar molecule.
This creates a disaccharide such as sucrose,which is composed of a glucose and a fructose unit. Larger sugar polymers range from the oligosaccharldes(trisaccharides, tetrasaccharides,and so on) up to giant polysaccharides,wlr'ich can contain thousands of monosaccharideunits.The way that sugars are linked together to form poly'rnersillustrates somecommon features of biochemical bond formation. A bond is formed between an-OH group on one sugar and an -OH group on another by a condensation reaction, in which a molecule of water is expelled as the bond is formed (Figure2-19).
Subunits in other biological polymers, such as nucleic acids and proteins,are also linked by condensation reactions in which water is expelled.The bondscreated by all of these condensation reactions can be broken by the reverseprocessof hydrolysis, in which a molecule of water is consumed (seeFigure 2-19).CH,OHta -a)"\()HHilH\rCCH ,/ll\ OHl/Ho\lnC-CL]r\H(c)Figure2-17 Thefour main familiesofsmallorganicmoleculesin cells.Theseform the monomericsmallmoleculesbuildingblocks,or subunits,for mostofthe macromoleculesand otherof the cell.Some,suchastheassembliessugarsand the fatty acids,arealsoenergy50urce5.Figure2-18 The structureof glucose,apreviouslysimplesugar.As illustratedforwater(seeFigure2-12),any moleculecanin severalways.In thebe representedstructuralformulasshownin (A),(B)and(C),the atomsareshownas chemicalsymbolslinkedtogetherby linesrepresentingthe covalentbonds.Thethickenedlineshereare usedto indicatethe planeof the sugarring,in an attemptto emphasizethat the -H and -OHgroupsarenot in the sameplaneasthering.(A)Theopen-chainform of thissugar,which is in equilibriumwith themorestablecyclicor ringform in (B).(C)Thechairform is an alternativeway todrawthe cyclicmoleculethat reflectsthegeometrymoreaccuratelythan thestructuralformulain (B).(D)A spacefillingmodel,which,aswell as depictingthe three-dimensionalarrangementofthe atoms,alsousesthe van derWaalsradiito representthe surfacecontoursofthe molecule.(E)A ball-and-stickmodelin whichthe three-dimensionalarrangementof the atomsin spaceisshown.(H,white;C,black;O, red;N, blue.)IHE CHEMICALCOMPONENTSOFA CELLmonosaccharide57monosaccharideCONDENSATIONFigure2-19 The reactionof twomonosaccharidesto form aThisreactionbelongsto adisaccharide.generalcategoryof reactionstermedreactions,in which twocondensationjoin togetheras a resultof themoleculesThe reverselossof a watermolecule.reaction(in whichwateris added)istermed hydrolysrs.Note that the reactivecarbonat whichthe new bond is formed(on the monosaccharideon the /efthere)is the carbonjoinedto two oxygensasaresultof sugarringformation(seeFigurethis commontype of2-18),As indicated,covalentbond betweentwo sugarbondmoleculesis known as a glycosidic(seealsoFigure2-20).HYDROLYSISH:OH,Owater expelledwater consumed'""1,]"0 j" ^oflY.'."n'.?,'jBecause each monosaccharide has several free hydroxyl groups that canform a link to another monosaccharide (or to some other compound), sugarpolymers can be branched, and the number of possible polysaccharide structures is extremely large.
Even a simple disaccharide consisting of two glucoseunits can exist in eleven different varieties (Figure 2-2O), while three differenthexoses (CoHrzOo)can join together to make several thousand trisaccharides.For this reason it is a much more complex task to determine the arrangement ofsugarsin a polysaccharide than to determine the nucleotide sequenceof a DNAmolecule, where each unit is joined to the next in exactly the same way.The monosaccharide glucoseis a key energy source for cells.
In a series ofreactions, it is broken down to smaller molecules, releasing energy that the cellcan harness to do useful work, as we shall explain later. Cells use simple polysaccharides composed only of glucose units-principally glycogenin animals andstarchin plants-as energy stores.p1*6CH,OHCH]OHt_t_q"qfo'",r-Q 'p 1 *4CH,OHt-io..\_,/ICH,OHI',/-o\\-,/CH,OH,r-o\o,.f\_/|\i(Il * o(lRl* 2Figure2-20 Elevend isaccharidesconsistingof two D-glucoseunits.Althoughthesedifferonly in the type oflinkagebetweenthe two glucoseunits,distinct.Sincethethey arechemicallyassociatedwith proteinsoligosaccharidesand lipidsmay havesixor moredifferentkindsof sugarjoined in both linearandthroughbranchedarrangementsglycosidicbondssuchasthoseillustratedhere,the numberof distincttypesofthat can be usedin cellsoligosaccharidesis extremelylarge.Foran explanationseePanel2-4of s and p linkages,(pp.112-113).Shortb/acklinesending(Redlines"blind"indicateOH positions.bondmerelyindicatedisaccharideand'torners"do not implyorientationsextraatoms.)58Chapter2: CellChemistryand BiosynthesisSugars do not function only in the production and storage of energy.Theycan also be used, for example, to make mechanical supports.
Thus, the mostabundant organic chemical on Earth-the cellulose of plant cell walls-is apolysaccharide of glucose.Becausethe glucose-glucoselinkages in cellulose differ from those in starch and glycogen, however, humans cannot digest celluloseand use its glucose. Another extraordinarily abundant organic substance, thechitin of insect exoskeletonsand fungal cell walls, is also an indigestible polysaccharide-in this case a linear polymer of a sugar derivative called ly'-acetylgl.,cosamine (see Panel 2-4). Other polysaccharides are the main components ofslime, mucus, and gristle.Smaller oligosaccharidescan be covalently linked to proteins to form glycoproteins and to lipids to form glycolipids,both of which are found in cell membranes.As described in Chapter 10,most cell surfacesare clothed and decoratedwith glycoproteins and glycolipids in the cell membrane.
The sugar side chainson these molecules are often recognized selectively by other cells. And differences between people in the details of their cell-surface sugars are the molecular basis for the different major human blood groups, termed A, B, AB, and O.FattyAcidsAreComponentsof CellMembranes,asWellasaSourceof EnergyA fatty acid molecule, such as palmitic acid,has two chemically distinct regions(Figure 2-21). One is a long hydrocarbon chain, which is hydrophobic and notvery reactive chemically. The other is a carboxyl (-COOH) group, which behavesas an acid (carboxylic acid): it is ionized in solution (-COO-), extremelyhydrophilic, and chemically reactive.Almost all the fatty acid molecules in a cellare covalently linked to other molecules by their carboxylic acid group.The hydrocarbon tail of palmitic acid is saturated: it has no double bondsbetween carbon atoms and contains the maximum possible number of hydrogens. Stearic acid, another one of the common fatty acids in animal fat, is alsosaturated.
Some other fatty acids, such as oleic acid, have unsaturatedtails,withone or more double bonds along their length. The double bonds create kinks inthe molecules, interfering with their ability to pack together in a solid mass. It isthis that accounts for the difference between hard margarine (saturated) andliquid vegetable oils (polyunsaturated). The many different fatty acids found incells differ only in the length of their hydrocarbon chains and the number andposition ofthe carbon-carbon double bonds (seePanel2-5, pp.1l4-ll5).Fatty acids are stored in the cytoplasm of many cells in the form of dropletsof triacylglycerol molecules, which consist of three fatty acid chains joined to aglycerol molecule (seePanel 2-5); these molecules are the animal fats found inmeat, butter, and cream, and the plant oils such as corn oil and olive oil.
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