BIOLOGY CONCEPTS & CONNECTIONS Fourth Edition Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Neil A. Campbell Jane B. Reece Lawrence G. Mitchell Martha R. Taylor From PowerPoint ® Lectures for Biology: Concepts & Connections CHAPTER 2 The Chemical Basis of Life Modules 2.9 – 2.17

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Atoms in a covalently bonded molecule may share electrons equally, creating a nonpolar molecule If electrons are shared unequally, a polar molecule is created 2.9 Water is a polar molecule THE PROPERTIES OF WATER

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings –This makes the oxygen end of the molecule slightly negatively charged –The hydrogen end of the molecule is slightly positively charged –Water is therefore a polar molecule In a water molecule, oxygen exerts a stronger pull on the shared electrons than hydrogen Figure 2.9 (–) O (+) HH

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The charged regions on water molecules are attracted to the oppositely charged regions on nearby molecules –This attraction forms weak bonds called hydrogen bonds 2.10 Overview: Water’s polarity leads to hydrogen bonding and other unusual properties Figure 2.10A Hydrogen bond

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Like no other common substance, water exists in nature in all three physical states: Figure 2.10B –as a solid –as a liquid –as a gas

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Due to hydrogen bonding, water molecules can move from a plant’s roots to its leaves Insects can walk on water due to surface tension created by cohesive water molecules 2.11 Hydrogen bonds make liquid water cohesive Figure 2.11

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings It takes a lot of energy to disrupt hydrogen bonds –Therefore water is able to absorb a great deal of heat energy without a large increase in temperature –As water cools, a slight drop in temperature releases a large amount of heat 2.12 Water’s hydrogen bonds moderate temperature

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings –A water molecule takes a large amount of energy with it when it evaporates –This leads to evaporative cooling Figure 2.12

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Molecules in ice are farther apart than those in liquid water 2.13 Ice is less dense than liquid water Figure 2.13 Hydrogen bond ICE Hydrogen bonds are stable LIQUID WATER Hydrogen bonds constantly break and re-form

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings –Ice is therefore less dense than liquid water, which causes it to float –If ice sank, it would seldom have a chance to thaw –Ponds, lakes, and oceans would eventually freeze solid

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Solutes whose charges or polarity allow them to stick to water molecules dissolve in water –They form aqueous solutions 2.14 Water is a versatile solvent Figure 2.14 Ions in solution Salt crystal Cl – Na + Cl – – –– – – Na

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings A compound that releases H + ions in solution is an acid, and one that accepts H + ions in solution is a base Acidity is measured on the pH scale: –0-7 is acidic –8-14 is basic –Pure water and solutions that are neither basic nor acidic are neutral, with a pH of The chemistry of life is sensitive to acidic and basic conditions

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The pH scale Figure 2.15 pH scale Acidic solution Neutral solution Basic solution Increasingly ACIDIC (Higher concentration of H + ) Increasingly BASIC (Lower concentration of H + ) NEUTRAL [H + ] = [OH – ] Lemon juice; gastric juice Grapefruit juice Tomato juice Urine PURE WATER Seawater Milk of magnesia Household ammonia Household bleach Oven cleaner Human blood H+H+ OH –

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Cells are kept close to pH 7 by buffers Buffers are substances that resist pH change –They accept H + ions when they are in excess and donate H + ions when they are depleted –Buffers are not foolproof

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Some ecosystems are threatened by acid precipitation Acid precipitation is formed when air pollutants from burning fossil fuels combine with water vapor in the air to form sulfuric and nitric acids 2.16 Connection: Acid precipitation threatens the environment Figure 2.16A

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings –These acids can kill fish, damage buildings, and injure trees –Regulations, new technology, and energy conservation may help us reduce acid precipitation Figure 2.16B

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings In a chemical reaction: –reactants interact –atoms rearrange –products result 2.17 Chemical reactions rearrange matter REARRANGEMENTS OF ATOMS 2 H 2 +O2O2  2 H 2 O Figure 2.17A

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings –Living cells carry out thousands of chemical reactions that rearrange matter in significant ways Figure 2.17B Beta-caroteneVitamin A (2 molecules)