Concept 3 1 The polarity of water molecules results in hydrogen bonding Water is the biological medium on Earth All living organisms require water more than any other substance Most cells are surrounded by water and cells themselves are about 70 95 water The abundance of water is the main reason the Earth is habitable The water molecule is a polar molecule The opposite ends have opposite charges Polarity allows water molecules to form hydrogen bonds with each other Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 3 2 Hydrogen bond H O H Concept 3 2 Four emergent properties of water contribute to Earth s fitness for life Four of water s properties that facilitate an environment for life are Cohesive behavior Ability to moderate temperature Expansion upon freezing Versatility as a solvent Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Cohesion Collectively hydrogen bonds hold water molecules together a phenomenon called cohesion Cohesion helps the transport of water against gravity in plants Adhesion is an attraction between different substances for example between water and plant cell walls Surface tension is a measure of how hard it is to break the surface of a liquid Surface tension is related to cohesion Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 3 3 Adhesion Water conducting cells Direction of water movement Cohesion 150 m Moderation of Temperature Water absorbs heat from warmer air and releases stored heat to cooler air Water can absorb or release a large amount of heat with only a slight change in its own temperature Kinetic energy is the energy of motion Temperature measures the intensity of heat due to the average kinetic energy of molecules Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Water s High Specific Heat The 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 1 C Water resists changing its temperature because of its high specific heat Water s high specific heat can be traced to hydrogen bonding Heat is absorbed when hydrogen bonds break Heat is released when hydrogen bonds form The high specific heat of water minimizes temperature fluctuations to within limits that permit life Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Evaporative Cooling Evaporation is transformation of a substance from liquid to gas Heat of vaporization is the heat a liquid must absorb for 1 g to be converted to gas As 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 Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 3 6 Insulation of Bodies of Water by Floating Ice Ice floats in liquid water because hydrogen bonds in ice are more ordered making ice less dense Water reaches its greatest density at 4 C If ice sank all bodies of water would eventually freeze solid making life impossible on Earth Hydrogen bond Ice Hydrogen bonds are stable Liquid water Hydrogen bonds break and re form The Solvent of Life A solution is a liquid that is a homogeneous mixture of substances A solvent is the dissolving agent of a solution The solute is the substance that is dissolved An aqueous solution is one in which water is the solvent Water is a versatile solvent due to its polarity which allows it to form hydrogen bonds easily When an ionic compound is dissolved in water each ion is surrounded by a sphere of water molecules called a hydration shell Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 3 7 Cl Cl Na How salt dissolves in water Na Fig 3 8 a Lysozyme molecule in a nonaqueous environment b Lysozyme molecule purple in an aqueous environment c Ionic and polar regions on the protein s surface attract water molecules Hydrophilic and Hydrophobic Substances A hydrophilic substance is one that has an affinity for water A hydrophobic substance is one that does not have an affinity for water oil Oil molecules are hydrophobic because they have relatively nonpolar bonds A colloid is a stable suspension of fine particles in a liquid Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Concept 3 3 Acidic and basic conditions affect living organisms A hydrogen atom in a hydrogen bond between two water molecules can shift from one to the other The 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 Water is in a state of dynamic equilibrium in which water molecules dissociate at the same rate at which they are being reformed Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 3 UN2 A proton in the hydrogen is ripped away from a water molecule making one hydronium and one hydroxide H O H H O H 2H2O H O H H O H Hydronium ion H3O Hydroxide ion OH Extra protons One less proton Effects of Changes in pH Concentrations of H and OH are equal in pure water Adding 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 An acid is any substance that increases the H concentration of a solution A base is any substance that reduces the H concentration of a solution Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings The pH Scale In any aqueous solution at 25 C the product of H and OH is constant and can be written as H OH 10 14 The pH of a solution is defined by the negative logarithm of H concentration written as pH log H For a neutral aqueous solution H is 10 7 7 7 Acidic solutions have pH values less than 7 Basic solutions have pH values greater than 7 Most biological fluids have pH values in the range of 6 to 8 Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 3 9 pH Scale 0 H H H H OH OH H H H H Acidic solution Increasingly Acidic H OH 1 Battery acid Gastric juice 2 lemon juice 3 Vinegar beer wine cola 4 Tomato juice 5 Black coffee Rainwater 6 Urine OH OH H H OH OH OH H H H Neutral H OH 8 Seawater OH OH H OH OH OH H OH Basic solution Increasingly Basic H OH