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 2.1 – 2.8

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Everything an organism is and does depends on chemistry Chemistry is dependent on the arrangement of atoms in molecules 2.1 The emergence of biological function starts at the chemical level ATOMS AND MOLECULES

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings A chemical element is a substance that cannot be broken down to other substances About 25 different chemical elements are essential to life 2.2 Life requires about 25 chemical elements

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Carbon, hydrogen, oxygen, and nitrogen (CHON) make up 96% of living matter Trace elements make up the other 4% Table 2.2

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Goiters are caused by iodine deficiency Figure 2.2

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Chemical elements combine in fixed ratios to form compounds Example: sodium + chlorine  sodium chloride 2.3 Elements can combine to form compounds

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The smallest particle of an element is an atom Different elements have different types of atoms An atom is made up of protons and neutrons located in a central nucleus The nucleus is surrounded by electrons 2.4 Atoms consist of protons, neutrons, and electrons

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Atoms are held together by attractions between the positively charged protons and negatively charged electrons Figure 2.4B B. Carbon atom Protons Neutrons Electrons Nucleus Neutrons are electrically neutral

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Atoms of each element are distinguished by a specific number of protons Table 2.4 –The number of neutrons may vary –Variant forms of an element are called isotopes –Some isotopes are radioactive

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Radioactive isotopes can be useful tracers for studying biological processes PET scanners use radioactive isotopes to create anatomical images 2.5 Connection: Radioactive isotopes can help or harm us Figure 2.5BFigure 2.5A

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Electrons are arranged in shells –The outermost shell determines the chemical properties of an atom –In most atoms, a full outer shell holds eight electrons 2.6 Electron arrangement determines the chemical properties

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Atoms whose shells are not full tend to interact with other atoms and gain, lose, or share electrons Figure 2.6 HYDROGEN (H) Atomic number = 1 CARBON (C) Atomic number = 6 NITROGEN (N) Atomic number = 7 OXYGEN (O) Atomic number = 8 Electron Outermost electron shell (can hold 8 electrons) First electron shell (can hold 2 electrons)

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings When atoms gain or lose electrons, charged atoms called ions are created –An electrical attraction between ions with opposite charges results in an ionic bond 2.7 Ionic bonds Figure 2.7A Na Sodium atom Cl Chlorine atom Na + Sodium ion Cl – Chloride ion Sodium chloride (NaCl) NaClNaCl + –

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Sodium and chloride ions bond to form sodium chloride, common table salt Figure 2.7B Na + Cl –

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Some atoms share outer shell electrons with other atoms, forming covalent bonds –Atoms joined together by covalent bonds form molecules 2.8 Covalent bonds

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Molecules can be represented in many ways Table 2.8

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 –the oxygen end of the molecule becomes slightly negatively charged –The hydrogen end of becomes slightly positively charged –Water is therefore a polar molecule In a water molecule, oxygen shares electrons unevenly with hydrogen Figure 2.9 (–) O (+) HH

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The charged regions are attracted to the oppositely charged regions on nearby molecules –This attraction forms weak bonds called hydrogen bonds 2.10 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 –water is able to absorb a lot of heat energy without a large increase in temperature –As water cools, a drop in temperature releases a lot of heat 2.12 Water’s hydrogen bonds moderate temperature

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings –A water molecule takes a lot 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