Sinh học - Chapter 3: Water and life

Water and LifeChapter 3Overview: The Molecule That Supports All of LifeWater is the biological medium on EarthAll living organisms require water more than any other substanceMost cells are surrounded by water, and cells themselves are about 70–95% waterThe abundance of water is the main reason the Earth is habitable© 2011 Pearson Education, Inc.Concept 3.1: Polar covalent bonds in water molecules result in hydrogen bondingThe water molecule is a polar molecule: the opposite ends have opposite chargesPolarity allows water molecules to form hydrogen bonds with each other© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.Animation: Water StructureRight-click slide/select “Play”Figure 3.2HydrogenbondPolar covalentbonds++++Concept 3.2: Four emergent properties of water contribute to Earth’s suitability for lifeFour of water’s properties that facilitate an environment for life areCohesive behaviorAbility to moderate temperatureExpansion upon freezingVersatility as a solvent© 2011 Pearson Education, Inc.Cohesion of Water MoleculesCollectively, hydrogen bonds hold water molecules together, a phenomenon called cohesionCohesion helps the transport of water against gravity in plantsAdhesion is an attraction between different substances, for example, between water and plant cell walls© 2011 Pearson Education, Inc.Figure 3.3AdhesionTwo types ofwater-conductingcellsCohesion300 mDirectionof watermovementSurface tension is a measure of how hard it is to break the surface of a liquidSurface tension is related to cohesion© 2011 Pearson Education, Inc.Figure 3.4Moderation of Temperature by WaterWater absorbs heat from warmer air and releases stored heat to cooler airWater can absorb or release a large amount of heat with only a slight change in its own temperature© 2011 Pearson Education, Inc.Heat and TemperatureKinetic energy is the energy of motionHeat is a measure of the total amount of kinetic energy due to molecular motionTemperature measures the intensity of heat due to the average kinetic energy of molecules© 2011 Pearson Education, Inc.The Celsius scale is a measure of temperature using Celsius degrees (°C)A calorie (cal) is the amount of heat required to raise the temperature of 1 g of water by 1°CThe “calories” on food packages are actually kilocalories (kcal), where 1 kcal = 1,000 calThe joule (J) is another unit of energy where 1 J = 0.239 cal, or 1 cal = 4.184 J© 2011 Pearson Education, Inc.Water’s High Specific HeatThe specific heat of a substance is the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1ºCThe specific heat of water is 1 cal/g/ºCWater resists changing its temperature because of its high specific heat© 2011 Pearson Education, Inc.Water’s high specific heat can be traced to hydrogen bondingHeat is absorbed when hydrogen bonds breakHeat is released when hydrogen bonds formThe high specific heat of water minimizes temperature fluctuations to within limits that permit life© 2011 Pearson Education, Inc.Figure 3.5Santa Barbara 73°Los Angeles(Airport) 75°Pacific Ocean 68°Santa Ana 84°Burbank90°San Bernardino100°Palm Springs106°Riverside 96°San Diego 72°40 miles70s (°F)80s90s100sEvaporative CoolingEvaporation is transformation of a substance from liquid to gasHeat of vaporization is the heat a liquid must absorb for 1 g to be converted to gasAs a liquid evaporates, its remaining surface cools, a process called evaporative cooling Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water© 2011 Pearson Education, Inc.Floating of Ice on Liquid WaterIce floats in liquid water because hydrogen bonds in ice are more “ordered,” making ice less denseWater reaches its greatest density at 4°CIf ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth© 2011 Pearson Education, Inc.Figure 3.6Hydrogen bondIce:Hydrogen bondsare stable Liquid water:Hydrogen bonds break and re-formWater: The Solvent of LifeA solution is a liquid that is a homogeneous mixture of substancesA solvent is the dissolving agent of a solutionThe solute is the substance that is dissolvedAn aqueous solution is one in which water is the solvent © 2011 Pearson Education, Inc.Water is a versatile solvent due to its polarity, which allows it to form hydrogen bonds easilyWhen an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell© 2011 Pearson Education, Inc.Figure 3.7ClClNa     Na   Water can also dissolve compounds made of nonionic polar moleculesEven large polar molecules such as proteins can dissolve in water if they have ionic and polar regions© 2011 Pearson Education, Inc.Figure 3.8++Hydrophilic and Hydrophobic SubstancesA hydrophilic substance is one that has an affinity for waterA hydrophobic substance is one that does not have an affinity for waterOil molecules are hydrophobic because they have relatively nonpolar bondsA colloid is a stable suspension of fine particles in a liquid© 2011 Pearson Education, Inc.Solute Concentration in Aqueous SolutionsMost biochemical reactions occur in waterChemical reactions depend on collisions of molecules and therefore on the concentration of solutes in an aqueous solution© 2011 Pearson Education, Inc.Molecular mass is the sum of all masses of all atoms in a moleculeNumbers of molecules are usually measured in moles, where 1 mole (mol) = 6.02 x 1023 molecules Avogadro’s number and the unit dalton were defined such that 6.02 x 1023 daltons = 1 gMolarity (M) is the number of moles of solute per liter of solution© 2011 Pearson Education, Inc.Concept 3.3: Acidic and basic conditions affect living organismsA hydrogen atom in a hydrogen bond between two water molecules can shift from one to the otherThe hydrogen atom leaves its electron behind and is transferred as a proton, or hydrogen ion (H+)The molecule with the extra proton is now a hydronium ion (H3O+), though it is often represented as H+The molecule that lost the proton is now a hydroxide ion (OH–)© 2011 Pearson Education, Inc.Water is in a state of dynamic equilibrium in which water molecules dissociate at the same rate at which they are being reformed© 2011 Pearson Education, Inc.Figure 3.UN022 H2OHydroxideion (OH)Hydroniumion (H3O+)+Though statistically rare, the dissociation of water molecules has a great effect on organismsChanges in concentrations of H+ and OH– can drastically affect the chemistry of a cell© 2011 Pearson Education, Inc.Concentrations of H+ and OH– are equal in pure waterAdding certain solutes, called acids and bases, modifies the concentrations of H+ and OH– Biologists use something called the pH scale to describe whether a solution is acidic or basic (the opposite of acidic)© 2011 Pearson Education, Inc.Acids and BasesAn acid is any substance that increases the H+ concentration of a solutionA base is any substance that reduces the H+ concentration of a solution© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.The pH ScaleIn any aqueous solution at 25°C the product of H+ and OH– is constant and can be written asThe pH of a solution is defined by the negative logarithm of H+ concentration, written asFor a neutral aqueous solution, [H+] is 10–7, so[H+][OH–] = 10–14pH = –log [H+]pH = –(–7) = 7 Acidic solutions have pH values less than 7Basic solutions have pH values greater than 7Most biological fluids have pH values in the range of 6 to 8© 2011 Pearson Education, Inc.Figure 3.UN05Acidic[H+] > [OH]Neutral[H+] = [OH]Basic[H+] [OH]H+H+H+H+H+H+H+H+OHOHH+OHH+OHOHOHOHH+H+H+H+OHOHOHOHOHOHOHH+11121314BuffersThe internal pH of most living cells must remain close to pH 7Buffers are substances that minimize changes in concentrations of H+ and OH– in a solutionMost buffers consist of an acid-base pair that reversibly combines with H+© 2011 Pearson Education, Inc.Figure 3.UN08