Water and the Fitness of the Environment Chapter 3 Water and the Fitness of the Environment

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 http://www.sumanasinc.com/webcontent/animations/content/propertiesofwater/water.html Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Concept 3.1: The polarity of water molecules results in hydrogen bonding The water molecule is a polar molecule: The opposite ends have opposite charges Polarity allows water molecules to form hydrogen bonds with each other Water is polar because the oxygen atom has a stronger electronegative pull on shared electrons in the molecule than do the hydrogen atoms Figure 3.2 Hydrogen bonds between water molecules Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Cohesion & Adhesion Collectively, hydrogen bonds hold water molecules together, a phenomenon called cohesion the attraction of water molecules to other water molecules as a result of hydrogen bonding Cohesion due to hydrogen bonding contributes to the transport of water and dissolved nutrients against gravity in plants Adhesion is the clinging of one substance to another Adhesion of water to cell walls by hydrogen bonds helps to counter the downward pull of gravity on the liquids passing through plants Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 3-3 Adhesion Water-conducting cells Direction of water movement Cohesion Figure 3.3 Water transport in plants - transport of water against gravity in plants 150 µm Cohesion and adhesion work together to give capillarity – the ability of water to spread through fine pores or to move upward through narrow tubes against the force of gravity.

Surface Tension The high surface tension of water, resulting from the collective strength of its hydrogen bonds, allows the water strider to walk on the surface of the pond. Surface tension is directly related to the cohesive property of water – it is a measurement of how difficult it is to stretch or break the surface of a liquid. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Moderation of Temperature Water moderates air temperature by absorbing heat from air that is warmer and releasing the stored heat to air that is cooler Water can absorb or release a large amount of heat with only a slight change in its own temperature The ability of water to stabilize temperature stems from its relatively high specific heat This is the amount of heat that must be absorbed or lost for 1g of a substance to change its temperature by 1°C Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Water’s 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 High specific heat of water is due to hydrogen bonding – H-bonds tend to restrict molecular movement, so when we add heat energy to water, it must break bonds first rather than increase molecular motion. A greater input of energy is required to raise the temperature of water than the temperature of air! Minimizes temperature fluctuations to within limits that permit life A large body of water can absorb and store a huge amount of heat from the sun in the daytime and during summer while warming up only a few degrees. And at night and during winter, gradually cooling water can warm the air. THE HIGH SPECIFIC HEAT OF WATER TENDS TO STABILIZE OCEAN TEMPERATURES – CREATING A FAVORABLE ENVIRONMENT FOR LIFE. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Water’s High Specific Heat The world is covered mostly by water. Large bodies of water can absorb and store a huge amount of heat from the sun in the daytime and during summer while warming up only a few degrees. Gradually cooling water warms air at night. Organisms are made primarily of water, so these properties help to maintain body temperatures. This is vital because many biologically important chemical reactions take place in a very narrow temperature range. A large body of water can absorb and store a huge amount of heat from the sun in the daytime and during summer while warming up only a few degrees. And at night and during winter, gradually cooling water can warm the air. THE HIGH SPECIFIC HEAT OF WATER TENDS TO STABILIZE OCEAN TEMPERATURES – CREATING A FAVORABLE ENVIRONMENT FOR LIFE. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 9

Evaporation is transformation of a substance from liquid to gas 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 The high amount of energy required to vaporize water has a wide range of effects: Helps stabilize temperatures in organisms and bodies of water Evaporation of sweat from human skin dissipates body heat and helps prevent overheating on a hot day or when excess heat is generated by strenuous activity. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Due to geometry of water molecule, they must move slightly apart to maintain the max number of H bonds in a stable structure. So at Zero degrees Celsius, an open latticework is formed, allowing air in – thus ice becomes less dense than liquid water floats on top of the water. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Hydrogen bonds are stable Hydrogen bonds break and re-form Fig. 3-6 Hydrogen bond Ice Hydrogen bonds are stable Figure 3.6 Ice: crystalline structure and floating barrier Liquid water Hydrogen bonds break and re-form

A solution is a liquid that is a homogeneous mixture of substances The Solvent of Life A solution is a liquid that is a homogeneous mixture of substances Solvent (dissolving agent) Solute (substance that is dissolved) An aqueous solution is one in which water is the solvent Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Hydration Shell http://www. sumanasinc A hydration shell refers to the sphere of water molecules around each dissolved ion in an aqueous solution Water will work inward from the surface of the solute until it dissolves all of it (provided that the solute is soluble in water) If a spoonful of salt (or other ionic substance) is placed in water, the ions in the salt and the water molecules have a mutual affinity owing to the attraction between opposite charges. O is negative and attracts to positive sodium. H is positive and attracts to negative chlorine. As a result, water will surround the individual sodium and chloride ions, separating and shielding them from one another (called a hydration shell). WATER IS THE SOLVENT OF LIFE – molecules within a living system must be broken down in order to be used by the system! Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 14

Acids and Bases 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

Figure 3.9 The pH scale and pH values of some aqueous solutions 1 Battery acid Gastric juice, lemon juice 2 H+ H+ H+ H+ OH– 3 Vinegar, beer, wine, cola OH– H+ H+ Increasingly Acidic [H+] > [OH–] H+ H+ 4 Acidic solution Tomato juice Black coffee 5 Rainwater 6 Urine OH– Saliva OH– Neutral [H+] = [OH–] H+ H+ OH– 7 Pure water OH– OH– H+ Human blood, tears H+ H+ 8 Seawater Neutral solution 9 Figure 3.9 The pH scale and pH values of some aqueous solutions 10 Increasingly Basic [H+] < [OH–] Milk of magnesia OH– OH– 11 OH– H+ OH– OH– Household ammonia OH– H+ OH– 12 Basic solution Household bleach 13 Oven cleaner 14

For a neutral aqueous solution [H+] is 10–7 = –(–7) = 7 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

CO2 + H2O <= H2CO3 => HCO3- + H+ Buffers The internal pH of most living cells must remain close to pH 7 Buffers are substances that minimize changes in concentrations of H+ and OH– in a solution They do so by accepting hydrogen ions from the solution when they are in excess and donating hydrogen ions when they are depleted Most buffers consist of an acid-base pair that reversibly combines with H+ CO2 + H2O <= H2CO3 => HCO3- + H+ Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

You should now be able to: List and explain the properties of water and how they result from polarity and hydrogen bonding. Distinguish between the following sets of terms: hydrophobic and hydrophilic; a solute, a solvent, and a solution. Define acid, base, and pH; describe the importance of buffers in physiological systems. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings