SBI4U Biochemistry Properties of Water

Transcription

1 SBI4U Biochemistry PropertiesofWater WATERANDTHEFITNESSOFTHE ENVIRONMENT THEEFFECTSOFWATER SPOLARITY The polarity of water molecules results in hydrogenbonding Organisms depend on the cohesion of water molecules WatermoderatestemperaturesonEarth Oceans and lakes don t freeze solid because icefloats Wateristhesolventoflife As astronomers study newly discovered planets orbitingdistantstars,theyhopetofindevidence ofwateronthesefar offworlds,forwateristhe substancethatmakespossiblelifeasweknowit here on Earth. All organisms familiar to us are made mostly of water and live in an environment dominated by water. Water is the biological medium here on Earth, and possibly onotherplanetsaswell. Life on Earth began in water and evolved there for 3 billion years before spreading onto land. Modernlife,eventerrestrial(land dwelling)life, remains tied to water. Most cells are surrounded by water, and cells themselves are about 70 95% water. Three quarters of Earth s surfaceissubmergedinwater.althoughmostof thiswaterisinliquidform,waterisalsopresent on Earth as ice and vapor. Water is the only common substance to exist in the natural environment in all three physical states of matter:solid,liquid,andgas.thesethreestates of water are visible in the photograph on this page,aviewofearthfromspace. TheabundanceofwaterisamajorreasonEarth ishabitable.inaclassicbookcalledthefitness of the Environment, Lawrence Henderson highlightstheimportanceofwatertolife.while acknowledging that life adapts to its environment through natural selection, Henderson emphasizes that for life to exist at all, the environment must first be a suitable abode. Your objective in this reading is to develop a conceptual understanding of how watercontributestothefitnessofearthforlife. THEEFFECTSOFWATER SPOLARITY Water is so common that it is easy to overlook thefactthatitisanexceptionalsubstancewith many extraordinary qualities. Following the theme of emergent properties, we can trace water s unique behavior to the structure and interactionsofitsmolecules. The polarity of water molecules results in hydrogenbonding Studied in isolation, the water molecule is deceptively simple. Its two hydrogen atoms are joined to the oxygen atom by single covalent bonds. Because oxygen is more electronegative thanhydrogen,theelectronsofthepolarbonds spend more time closer to the oxygen atom. In other words, the bonds that hold together the atoms in a water molecule are polar covalent bonds, with the oxygen region of the molecule having a partial negative charge and the hydrogens having a partial positive charge. The water molecule, shaped something like a wide V, is a polar molecule, meaning that opposite endsofthemoleculehaveoppositecharges(see figure2.13). The anomalous properties of water arise from attractionsbetweenthesepolarmolecules.the attraction is electrical; a slightly positive hydrogen of one molecule is attracted to the slightly negative oxygen of a nearby molecule. The two molecules are thus held together by a hydrogen bond (FIGURE 3.1). Each water molecule can form hydrogen bonds to a maximum of four neighbors, and at any given AdaptedfromBiology,SixthEd.,Campbell&Reece.,PearsonEducationInc.,2002.

2 SBI4U Biochemistry PropertiesofWater moment,manyofthemoleculesinasampleof liquid water are linked in this way. The extraordinary qualities of water are emergent propertiesresultingfromthehydrogenbonding that orders molecules into a higher level of structuralorganization. Fig 3 1.Hydrogen bonds between water molecules.thechargedregionsofapolarwater molecule are attracted to oppositely charged partsofneighboringmolecules.(theoxygenhas a slight negative charge; the hydrogens have a slight positive charge.) Each molecule can hydrogen bondtoamaximumoffourpartners. At any instant in liquid water at 37 C (human bodytemperature),about15%ofthemolecules are bonded to four partners in short lived clusters. Wewillexaminefourofwater spropertiesthat contribute to the fitness of Earth as an environmentforlife:water scohesivebehavior, itsabilitytostabilizetemperature,itsexpansion uponfreezing,anditsversatilityasasolvent. Organisms depend on the cohesion of water molecules Water molecules stick to each other as a result ofhydrogenbonding.whenwaterisinitsliquid form,itshydrogenbondsareveryfragile,about one twentiethasstrongascovalentbonds.they form, break, and re form with great frequency. Each hydrogen bond lasts only a few trillionths of a second, but the molecules are constantly forming new bonds with a succession of partners. Thus, at any instant, a substantial percentage of all the water molecules are bonded to their neighbors, making water more structured than most other liquids. Collectively, the hydrogen bonds hold the substance together,aphenomenoncalledcohesion. Cohesion due to hydrogen bonding contributes to the transport of water against gravity in plants. Water reaches the leaves through microscopic vessels that extend upward from the roots (FIGURE 3.2). Water that evaporates fromaleafisreplacedbywaterfromthevessels in the veins of the leaf. Hydrogen bonds cause water molecules leaving the veins to tug on molecules farther down in the vessel, and the upward pull is transmitted along the vessel all the way down to the roots. Adhesion, the clingingofonesubstancetoanother,alsoplays a role. Adhesion of water to the walls of the vessels helps counter the downward pull of gravity. Fig 3 2.Water transport in plants. Evaporation from leaves pulls water upward from the roots throughmicroscopictubescalledxylemvessels, in this case located in the trunk of a tree. Cohesion due to hydrogen bonding helps hold together the column of water within a vessel. Adhesion of the water to the vessel wall also helpsinresistingthedownwardpullofgravity. Related to cohesion is surface tension, a measureofhowdifficultitistostretchorbreak the surface of a liquid. Water has a greater surface tension than most other liquids. At the interface between water and air is an ordered arrangement of water molecules, hydrogenbondedtooneanotherandtothewaterbelow. This makes the water behave as though coated with an invisible film. We can observe the surface tension of water by slightly overfilling a drinking glass; the water will stand above the rim. Water s surface tension also allows us to skip rocks on a pond. In a more biological example, some animals can stand, walk, or run AdaptedfromBiology,SixthEd.,Campbell&Reece.,PearsonEducationInc.,2002.

3 SBI4U Biochemistry PropertiesofWater on water without breaking the surface(figure 3.3). Fig 3 3.Walking on water. The high surface tension of water, resulting from the collective strengthofitshydrogenbonds,allowsthewater strider to walk on a pond without breaking the surface. WatermoderatestemperaturesonEarth Water stabilizes air temperatures by absorbing heat from air that is warmer and releasing the stored heat to air that is cooler. Water is effectiveasaheatbankbecauseitcanabsorbor release a relatively large amount of heat with only a slight change in its own temperature. To understand this quality of water, we must first lookbrieflyatheatandtemperature. HeatandTemperature Anything that moves has kinetic energy, the energy of motion. Atoms and molecules have kineticenergybecausetheyarealwaysmoving, although not necessarily in any particular direction. The faster a molecule moves, the greater its kinetic energy. Heat is a measure of the total quantity of kinetic energy due to molecular motion in a body of matter. Temperaturemeasurestheintensityofheatdue to the average kinetic energy of the molecules. When the average speed of the molecules increases, a thermometer records this as a rise in temperature. Heat and temperature are related, but they are not the same. A swimmer crossing the English Channel has a higher temperature than the water, but the ocean containsfarmoreheatbecauseofitsvolume. Whenevertwoobjectsofdifferenttemperature are brought together, heat passes from the warmertothecoolerbodyuntilthetwoarethe same temperature. Molecules in the cooler object speed up at the expense of the kinetic energyofthewarmerobject.anicecubecoolsa drink not by adding coldness to the liquid, but byabsorbingheatastheicemelts. Throughout this course, we will use the Celsius scale to indicate temperature (Celsius degrees are abbreviated as C). At sea level, water freezes at 0 C and boils at 100 C. The temperatureofthehumanbodyaverages37 C, andcomfortableroomtemperatureisabout20 25 C. Oneconvenientunitofheatisthecalorie(cal).A calorieistheamountofheatenergyittakesto raise the temperature of 1 g of water by 1 C. Conversely,acalorieisalsotheamountofheat that1gofwaterreleaseswhenitcoolsby1 C.A kilocalorie (kcal), 1,000 cal, is the quantity of heat required to raise the temperature of 1 kilogram(kg)ofwaterby1 C.(The"calories"on foodpackagesareactuallykilocalories.)another energy unit used in this book is the joule (J). One joule equals cal; a calorie equals 4.184J. Water shighspecificheat The ability of water to stabilize temperature stems from its relatively high specific heat. The specific heat of a substance is defined as the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1 C. We already know water s specificheatbecausewehavedefinedacalorie astheamountofheatthatcauses1gofwater tochangeitstemperatureby1 C.Therefore,the specific heat of water is 1 calorie per gram per degree Celsius, abbreviated as 1 cal/g/ C. Compared with most other substances, water hasanunusuallyhighspecificheat.forexample, ethyl alcohol, the type of alcohol in alcoholic beverages,hasaspecificheatof0.6cal/g/ C. Because of the high specific heat of water relativetoothermaterials,waterwillchangeits temperature less when it absorbs or loses a givenamountofheat.thereasonyoucanburn your fingers by touching the metal handle of a pot on the stove when the water in the pot is still lukewarm is that the specific heat of water is ten times greater than that of iron. In other AdaptedfromBiology,SixthEd.,Campbell&Reece.,PearsonEducationInc.,2002.

4 SBI4U Biochemistry PropertiesofWater words, it will take only 0.1 cal to raise the temperatureof1gofiron1 C.Specificheatcan be thought of as a measure of how well a substance resists changing its temperature when it absorbs or releases heat. Water resists changing its temperature; when it does change its temperature, it absorbs or loses a relatively large quantity of heat for each degree of change. We can trace water s high specific heat, like many of its other properties, to hydrogen bonding. Heat must be absorbed in order to break hydrogen bonds, and heat is released when hydrogen bonds form. A calorie of heat causes a relatively small change in the temperatureofwaterbecausemuchoftheheat energyisusedtodisrupthydrogenbondsbefore the water molecules can begin moving faster. And when the temperature of water drops slightly, many additional hydrogen bonds form, releasing a considerable amount of energy in theformofheat. What is the relevance of water s high specific heattolifeonearth?alargebodyofwatercan absorb and store a huge amount of heat from the sun in the daytime and during summer, while warming up only a few degrees. At night and during winter, the gradually cooling water can warm the air. This is the reason coastal areasgenerallyhavemilderclimatesthaninland regions. The high specific heat of water also tends to stabilize ocean temperatures, creating a favorable environment for marine life. Thus, becauseofitshighspecificheat,thewaterthat covers most of Earth keeps temperature fluctuations on land and in water within limits that permit life. Also, because organisms are madeprimarilyofwater,theyaremoreableto resistchangesintheirowntemperaturesthanif theyweremadeofaliquidwithalowerspecific heat. EvaporativeCooling Molecules of any liquid stay close together because they are attracted to one another. Molecules moving fast enough to overcome theseattractionscandeparttheliquidandenter theairasgas.thistransformationfromaliquid to a gas is called vaporization, or evaporation. Recall that the speed of molecular movement varies and that temperature is the average kinetic energy of molecules. Even at low temperatures, the speediest molecules can escapeintotheair.someevaporationoccursat anytemperature;aglassofwater,forexample, willeventuallyevaporateatroomtemperature. If a liquid is heated, the average kinetic energy ofmoleculesincreasesandtheliquidevaporates morerapidly. Heat of vaporization is the quantity of heat a liquidmustabsorbfor1gofittobeconverted fromtheliquidtothegaseousstate.compared withmostotherliquids,waterhasahighheatof vaporization. To evaporate each gram of water at room temperature, about 580 cal of heat is needed nearly double the amount needed to vaporizeagramofalcoholorammonia.water s high heat of vaporization is another emergent propertycausedbyhydrogenbonds,whichmust bebrokenbeforethemoleculescanmaketheir exodusfromtheliquid. Water s high heat of vaporization helps moderate Earth s climate. A considerable amount of solar heat absorbed by tropical seas is consumed during the evaporation of surface water. Then, as moist tropical air circulates poleward, it releases heat as it condenses to formrain. Asaliquidevaporates,thesurfaceoftheliquid that remains behind cools down. This evaporative cooling occurs because the "hottest" molecules, those with the greatest kinetic energy, are the most likely to leave as gas.itisasifthe100fastestrunnersatacollege transferred to another school; the average speedoftheremainingstudentswoulddecline. Evaporative cooling of water contributes to the stabilityoftemperatureinlakesandpondsand also provides a mechanism that prevents terrestrial organisms from overheating. For example, evaporation of water from the leaves of a plant helps keep the tissues in the leaves from becoming too warm in the sunlight. Evaporation of sweat from human skin dissipates body heat and helps prevent overheatingonahotdayorwhenexcessheatis generated by strenuous activity (FIGURE 3.4). Highhumidityonahotdayincreasesdiscomfort because the high concentration of water vapor intheairinhibitstheevaporationofsweatfrom thebody. AdaptedfromBiology,SixthEd.,Campbell&Reece.,PearsonEducationInc.,2002.

5 SBI4U Biochemistry PropertiesofWater Fig 3 4.Evaporative cooling. Because of water s highheatofvaporization,evaporationofsweat significantlycoolsthebodysurface. Oceans and lakes don t freeze solid because icefloats Waterisoneofthefewsubstancesthatareless denseasasolidthanasaliquid.inotherwords, ice floats. While other materials contract when they solidify, water expands. The cause of this exotic behavior is, once again, hydrogen bonding. At temperatures above 4 C, water behaves like other liquids, expanding as it warmsandcontractingasitcools.waterbegins to freeze when its molecules are no longer moving vigorously enough to break their hydrogen bonds. As the temperature reaches 0 C,thewaterbecomeslockedintoacrystalline lattice, each water molecule bonded to the maximum of four partners (FIGURE 3.5). The hydrogen bonds keep the molecules at "arm s length,"farenoughaparttomakeiceabout10% less dense (10% fewer molecules for the same volume) than liquid water at 4 C. When ice absorbsenoughheatforitstemperaturetorise above 0 C, hydrogen bonds between molecules are disrupted. As the crystal collapses, the ice melts, and molecules are free to slip closer together. Water reaches its greatest density at 4 Candthenbeginstoexpandasthemolecules move faster. Keep in mind, however, that even in liquid water, many of the molecules are connected by hydrogen bonds, though only transiently: The hydrogen bonds are constantly breakingandreforming. Fig 3 5.The structure of ice. In ice, each moleculeishydrogen bondedtofourneighbors in a three dimensional crystal. Because the crystalisspacious,icehasfewermoleculesthan anequalvolumeofliquidwater.inotherwords, iceislessdensethanliquidwater. The ability of ice to float because of the expansion of water as it solidifies is an important factor in the fitness of the environment. If ice sank, then eventually all ponds, lakes, and even oceans would freeze solid, making life as we know it impossible on Earth. During summer, only the upper few inchesoftheoceanwouldthaw.instead,when a deep body of water cools, the floating ice insulates the liquid water below, preventing it fromfreezingandallowinglifetoexistunderthe frozensurface(figure3.6). Fig 3 6.Floating ice and the fitness of the environment. Floating ice becomes a barrier that protects the liquid water below from the colder air. The invertebrates shown here are called krill; they were photographed beneath theantarcticice. AdaptedfromBiology,SixthEd.,Campbell&Reece.,PearsonEducationInc.,2002.

6 SBI4U Biochemistry PropertiesofWater Wateristhesolventoflife A sugar cube placed in a glass of water will dissolve. The glass will then contain a uniform mixture of sugar and water; the concentration ofdissolvedsugarwillbethesameeverywhere in the mixture. A liquid that is a completely homogeneous mixture of two or more substances is called a solution. The dissolving agent of a solution is the solvent, and the substance that is dissolved is the solute. In this case, water is the solvent and sugar is the solute. An aqueous solution is one in which wateristhesolvent. Themedievalalchemiststriedtofindauniversal solvent,onethatwoulddissolveanything.they learned that nothing works better than water. However, water is not a universal solvent; if it were, it could not be stored in any container, including our cells. But water is a very versatile solvent,aqualitywecantracetothepolarityof thewatermolecule. Suppose,forexample,thatacrystaloftheionic compound sodium chloride is placed in water (FIGURE 3.7). At the surface of the crystal, the sodium and chloride ions are exposed to the solvent. These ions and the water molecules have a mutual affinity through electrical attraction. The oxygen regions of the water molecules are negatively charged and cling to sodium cations. The hydrogen regions of the water molecules are positively charged and are attracted to chloride anions. As a result, water molecules surround the individual sodium and chloride ions, separating and shielding them from one another. The sphere of water molecules around each dissolved ion is called a hydration shell. Working inward from the surface of the salt crystal, water eventually dissolves all the ions. The result is a solution of two solutes, sodium and chloride, homogeneously mixed with water, the solvent. Other ionic compounds also dissolve in water. Seawater, for instance, contains a great variety ofdissolvedions,asdolivingcells. Fig3 7.Acrystaloftablesaltdissolvinginwater. Asphereofwatermolecules,calledahydration shell,surroundseachsoluteion. A compound does not need to be ionic to dissolveinwater;compoundsmadeupofpolar molecules, such as sugars, are also watersoluble. Such compounds dissolve when water molecules surround each of the solute molecules. Even molecules as large as proteins candissolveinwateriftheyhaveionicandpolar regions on their surface (FIGURE 3.8). Many different kinds of polar compounds are dissolved(alongwithions)inthewaterofsuch biologicalfluidsasblood,thesapofplants,and theliquidwithinallcells.wateristhesolventof life. Fig 3 8.A water soluble protein. Even a molecule as large as a protein can dissolve in waterifithasenoughionicandpolarregionson AdaptedfromBiology,SixthEd.,Campbell&Reece.,PearsonEducationInc.,2002.

7 SBI4U Biochemistry PropertiesofWater itssurface.themassofpurplehererepresentsa single such protein molecule, which is being surroundedbywatermolecules. HydrophilicandHydrophobicSubstances Whether ionic or polar, any substance that has an affinity for water is said to be hydrophilic (from the Greek hydro, water, and philios, loving). This term is used even if the substance doesnotdissolve becausethemoleculesaretoo large,forinstance.cotton,aplantproduct,isan exampleofahydrophilicsubstancethatabsorbs water without dissolving. Cotton consists of giant molecules of cellulose, a compound with numerousregionsofpartialpositiveandpartial negative charges associated with polar bonds. Water adheres to the cellulose fibers. Thus, a cotton towel does a great job of drying the body, yet does not dissolve in the washing machine.celluloseisalsopresentinthewallsof water conducting vessels in a plant; you read earlier how the adhesion of water to these hydrophilicwallsfunctionsinwatertransport. There are, of course, substances that do not have an affinity for water. Substances that are non ionic and nonpolar actually seem to repel water; these substances are termed hydrophobic (from the Greek phobos, fearing). An example from the kitchen is vegetable oil, which, as you know, does not mix stably with watery substances such as vinegar. The hydrophobic behavior of the oil molecules resultsfromaprevalenceofnonpolarbonds,in this case bonds between carbon and hydrogen, which share electrons almost equally. Hydrophobicmoleculesrelatedtooilsaremajor ingredients of cell membranes. (Imagine what would happen to a cell if its membrane dissolved.) SoluteConcentrationinAqueousSolutions Biologicalchemistryis"wet"chemistry.Mostof the chemical reactions that occur in organisms involve solutes dissolved in water. To carry out experiments on the chemistry of life, it is important to learn how to calculate the concentrations of solutes dissolved in aqueous solutions. To understand chemical reactions, we need to know how many atoms and molecules are involved. Suppose we wanted to prepare an aqueous solution of table sugar having a specified concentration of sugar molecules (a certainnumberofsolutemoleculesinacertain volume of solution). Because counting or weighing individual molecules is not practical, we usually measure substances in units called moles. A mole (mol) is equal in number to the molecular weight of a substance, but upscaled from daltons to units of grams. Imagine weighing out 1 mol of table sugar (sucrose), which has the molecular formula C 12 H 22 O 11. In round numbers, a carbon atom weighs 12 daltons, a hydrogen atom weighs 1 dalton, and an oxygen atom weighs 16 daltons. Molecular weightisthesumoftheweightsofalltheatoms in a molecule; thus, the molecular weight of sucrose is 342 daltons. To obtain 1 mol of sucrose, we weigh out 342 g, the molecular weightofsucroseexpressedingrams. Thepracticaladvantageofmeasuringaquantity of chemicals in moles is that a mole of one substance has exactly the same number of molecules as a mole of any other substance. If substance A has a molecular weight of 10 daltonsandsubstancebhasamolecularweight of100daltons,then10gofawillhavethesame numberofmoleculesas100gofb.thenumber of molecules in a mole, called Avogadro s number, is 6.02 X A mole of table sugar contains 6.02 X sucrose molecules and weighs 342 g. A mole of ethyl alcohol (C 2 H 6 O) also contains 6.02 X molecules, but it weighs only 46 g because the molecules are smaller than those of sucrose. Measuring in molesmakesitconvenientforscientistsworking inthelaboratorytocombinesubstancesinfixed ratiosofmolecules. How would we make a liter (L) of solution consisting of 1 mol of sucrose dissolved in water? We would weigh out 342 g of sucrose and then gradually add water, while stirring, until the sugar was completely dissolved. We wouldthenaddenoughwatertobringthetotal volumeofthesolutionupto1l.atthatpoint, we would have a 1 molar (1 M) solution of sucrose.molarity thenumberofmolesofsolute per liter of solution is the unit of concentration most often used by biologists for aqueous solutions. AdaptedfromBiology,SixthEd.,Campbell&Reece.,PearsonEducationInc.,2002.