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Chapter 2The Chemical Basis of LifePowerPoint Lectures forBiology: Concepts & Connections, Sixth EditionCampbell, Reece, Taylor, Simon, and DickeyLecture by Richard L. MyersCopyright 2009 Pearson Education, Inc.

ELEMENTS, ATOMS,AND MOLECULESCopyright 2009 Pearson Education, Inc.

2.1 Living organisms are composed of about 25chemical elements Chemicals are at the base level of biologicalhierarchy They are arranged into higher and higher levels ofstructural organization– Arrangement eventually leads to formation of livingorganismsCopyright 2009 Pearson Education, Inc.

2.1 Living organisms are composed of about 25chemical elements Living organisms are composed of matter, whichis anything that occupies space and has mass(weight)– Matter is composed of chemical elements– Element—a substance that cannot be broken down to othersubstances– There are 92 elements in nature—only a few exist in a purestate– Life requires 25 essential elements; some are calledtrace elementsCopyright 2009 Pearson Education, Inc.

2.2 CONNECTION: Trace elements arecommon additives to food and water Some trace elements are required to preventdisease– Without iron, your body cannot transport oxygen– An iodine deficiency prevents production of thyroidhormones, resulting in goiterCopyright 2009 Pearson Education, Inc.

2.2 CONNECTION: Trace elements arecommon additives to food and water Several chemicals are added to food for a varietyof reasons– Help preserve it– Make it more nutritious– Make it look better Check out the ―Nutrition Facts‖ label on foods anddrinks you purchaseCopyright 2009 Pearson Education, Inc.

2.3 Elements can combine to form compounds Compound—a substance consisting of two ormore different elements combined in a fixed ratio– There are many compounds that consist of only twoelements– Table salt (sodium chloride or NaCl) is an example– Sodium is a metal, and chloride is a poisonous gas– However, when chemically combined, an edible compoundemergesCopyright 2009 Pearson Education, Inc.

SodiumChlorineSodium Chloride

2.3 Elements can combine to form compounds Many of the compounds in living organisms containcarbon, hydrogen, oxygen, and nitrogen– DNA, for example, contains all four of these elements Interestingly, different arrangements of elementsprovide unique properties for each compoundCopyright 2009 Pearson Education, Inc.

2.4 Atoms consist of protons, neutrons, andelectrons An atom is the smallest unit of matter that stillretains the properties of a element– Atoms are made of over a hundred subatomic particles,but only three are important for biological compounds– Proton—has a single positive electrical charge– Electron—has a single negative electrical charge– Neutron—is electrically neutralCopyright 2009 Pearson Education, Inc.

2.4 Atoms consist of protons, neutrons, andelectrons Elements differ in their number of protons,neutrons, and electrons Helium has two protons, two neutrons, and twoelectrons Carbon has six protons, six neutrons, and sixelectronsCopyright 2009 Pearson Education, Inc.

Massnumber 4

2.4 Atoms consist of protons, neutrons, andelectrons Neutrons and protons are packed in the atom’snucleus– The negative charge of electrons and the positivecharge of protons keep electrons near the nucleus– The number of protons is the atom’s atomic number– Carbon with 6 protons has an atomic number of 6– The mass number is the sum of the protons and neutronsin the nucleus (carbon-12 is written 12C)Copyright 2009 Pearson Education, Inc.

nsMassnumber 12

2.4 Atoms consist of protons, neutrons, andelectrons Although all atoms of an element have the sameatomic number, some differ in mass number– The variations are isotopes, which have the samenumbers of protons and electrons but different numbersof neutrons– One isotope of carbon has 8 neutrons instead of 6 (written14C)– Unlike 12C, 14C is an unstable (radioactive) isotope that givesoff energyCopyright 2009 Pearson Education, Inc.

2.5 CONNECTION: Radioactive isotopes canhelp or harm us Living cells cannot distinguish between isotopes ofthe same element– Therefore, when radioactive compounds are used inmetabolic processes, they act as tracers– Radioactivity can be detected by instruments With instruments, the fate of radioactive tracerscan be monitored in living organismsCopyright 2009 Pearson Education, Inc.

2.5 CONNECTION: Radioactive isotopes canhelp or harm us Biologists use radioactive tracers in research– Radioactive 14C was used to show the route of 14CO2 information of sugar during plant photosynthesisCopyright 2009 Pearson Education, Inc.

2.5 CONNECTION: Radioactive isotopes canhelp or harm us Radioactive tracers are frequently used in medicaldiagnosis Sophisticated imaging instruments are used todetect them– An imaging instrument that uses positron-emissiontomography (PET) detects the location of injectedradioactive materials– PET is useful for diagnosing heart disorders and cancerand in brain researchCopyright 2009 Pearson Education, Inc.

Healthy brainAlzheimer’s patient

2.5 CONNECTION: Radioactive isotopes canhelp or harm us In addition to benefits, there are also dangersassociated with using radioactive substances– Uncontrolled exposure can cause damage to somemolecules in a living cell, especially DNA– Chemical bonds are broken by the emitted energy,which causes abnormal bonds to formCopyright 2009 Pearson Education, Inc.

2.6 Electron arrangement determines thechemical properties of an atom Only electrons are involved in chemical activity Electrons occur in energy levels called electronshells– Information about the distribution of electrons is foundin the periodic table of the elementsCopyright 2009 Pearson Education, Inc.

ChlorineArgonSecondshellSodiumThirdshellMagnesium Aluminum

2.6 Electron arrangement determines thechemical properties of an atom An atom may have one, two, or three electronshells– The number of electrons in the outermost shelldetermines the chemical properties of the atom– The first shell is full with two electrons, whereas thesecond and third will hold up to eight electronsCopyright 2009 Pearson Education, Inc.

2.6 Electron arrangement determines thechemical properties of an atom Atoms want to fill their outer electron shells– To accomplish this, the atom can share, donate, orreceive electrons– This results in attractions between atoms calledchemical bondsCopyright 2009 Pearson Education, Inc.

2.7 Ionic bonds are attractions between ions ofopposite charge An ion is an atom or molecule with an electricalcharge resulting from gain or loss of electrons– When an electron is lost, a positive charge results;when one is gained, a negative charge results Two ions with opposite charges attract each other– When the attraction holds the ions together, it is calledan ionic bondAnimation: Ionic BondsCopyright 2009 Pearson Education, Inc.

Transfer ofelectronNaSodium atomClChlorine atom

Transfer ofelectronNaSodium atomClChlorine atom –Na Sodium ionCl–Chloride ionSodium chloride (NaCl)

2.8 Covalent bonds join atoms into moleculesthrough electron sharing A covalent bond results when atoms share outershell electrons– A molecule is formed when atoms are held together bycovalent bondsAnimation: Covalent BondsCopyright 2009 Pearson Education, Inc.

2.9 Unequal electron sharing creates polarmolecules Atoms in a covalently bonded molecule continuallycompete for shared electrons– The attraction (pull) for shared electrons is calledelectronegativity– More electronegative atoms pull harderCopyright 2009 Pearson Education, Inc.

2.9 Unequal electron sharing creates polarmolecules In molecules of only one element, the pull towardeach atom is equal, because each atom has thesame electronegativity– The bonds formed are called nonpolar covalentbondsCopyright 2009 Pearson Education, Inc.

2.9 Unequal electron sharing creates polarmolecules Water has atoms with different electronegativities– Oxygen attracts the shared electrons more stronglythan hydrogen– So, the shared electrons spend more time near oxygen– The result is a polar covalent bondCopyright 2009 Pearson Education, Inc.

2.9 Unequal electron sharing creates polarmolecules In H2O the oxygen atom has a slight negativecharge and the hydrogens have a slight positivecharge– Molecules with this unequal distribution of charges arecalled polar moleculesCopyright 2009 Pearson Education, Inc.

(–)(–)OH( )H( )

2.10 Hydrogen bonds are weak bonds importantin the chemistry of life Some chemical bonds are weaker than covalentbonds Hydrogen, as part of a polar covalent bond, willshare attractions with other electronegative atoms– Examples are oxygen and nitrogen Water molecules are electrically attracted tooppositely charged regions on neighboring molecules– Because the positively charged region is always ahydrogen atom, the bond is called a hydrogen bondAnimation: Water StructureCopyright 2009 Pearson Education, Inc.

Hydrogen bond

WATER’S LIFE-SUPPORTINGPROPERTIESCopyright 2009 Pearson Education, Inc.

2.11 Hydrogen bonds make liquid water cohesive Hydrogen bonding causes molecules to sticktogether, a property called cohesion– Cohesion is much stronger for water than other liquids– This is useful in plants that depend upon cohesion tohelp transport water and nutrients up the plantCopyright 2009 Pearson Education, Inc.

2.11 Hydrogen bonds make liquid water cohesive Cohesion is related to surface tension—ameasure of how difficult it is to break the surfaceof a liquid– Hydrogen bonds are responsible for surface tensionAnimation: Water TransportCopyright 2009 Pearson Education, Inc.

AdhesionWater-conductingcellsDirectionof watermovementCohesion150 µm

2.12 Water’s hydrogen bonds moderatetemperature Because of hydrogen bonding, water has a greaterability to resist temperature change than otherliquids– Heat is the energy associated with movement of atomsand molecules in matter– Temperature measures the intensity of heat Heat must be absorbed to break hydrogen bonds;heat is released when hydrogen bonds formCopyright 2009 Pearson Education, Inc.

2.13 Ice is less dense than liquid water Water can exist as a gas, liquid, and solid– Water is less dense as a solid, a property due tohydrogen bondingCopyright 2009 Pearson Education, Inc.

2.13 Ice is less dense than liquid water When water freezes, each molecule forms a stablehydrogen bond with four neighbors– A three-dimensional crystal results– There is space between the water molecules Ice is less dense than water, so it floatsCopyright 2009 Pearson Education, Inc.

Hydrogen bondIceHydrogen bondsare stableLiquid waterHydrogen bondsconstantly break and re-form

2.14 Water is the solvent of life A solution is a liquid consisting of a uniformmixture of two or more substances– The dissolving agent is the solvent– The substance that is dissolved is the soluteCopyright 2009 Pearson Education, Inc.

2.14 Water is the solvent of life Water is a versatile solvent that is fundamental tolife processes– Its versatility results from its polarity– Table salt is an example of a solute that will go intosolution in water– Sodium and chloride ions and water are attracted to eachother because of their chargesCopyright 2009 Pearson Education, Inc.

Ion insolutionSaltcrystal

2.15 The chemistry of life is sensitive to acidicand basic conditions A few water molecules can break apart into ions– Some are hydrogen ions (H )– Some are hydroxide ions (OH–)– Both are extremely reactive– A balance between the two is critical for chemical processesto occur in a living organismCopyright 2009 Pearson Education, Inc.

2.15 The chemistry of life is sensitive to acidicand basic conditions Chemicals other than water can contribute H to asolution– They are called acids– An example is hydrochloric acid (HCl)– This is the acid in your stomach that aids in digestion An acidic solution has a higher concentration of H than OH–Copyright 2009 Pearson Education, Inc.

2.15 The chemistry of life is sensitive to acidicand basic conditions Some chemicals accept hydrogen ions and removethem from solution– These chemicals are called bases– For example, sodium hydroxide (NaOH) provides OH–that combines with H to produce H2O (water)– This reduces the H concentrationCopyright 2009 Pearson Education, Inc.

2.15 The chemistry of life is sensitive to acidicand basic conditions A pH scale (pH potential of hydrogen) is usedto describe whether a solution is acidic or basic– pH ranges from 0 (most acidic) to 14 (most basic)– A solution that is neither acidic or basic is neutral(pH 7)Copyright 2009 Pearson Education, Inc.

pH scale01Acidic solutionIncreasingly ACIDIC(Higher concentration of H )Battery acid2 Lemon juice, gastric juice3 Grapefruit juice, soft drink,vinegar, beer4 Tomato juice5Rain water6 Human urineSalivaNEUTRAL[H ] OH–]7 Pure waterHuman blood,tears8SeawaterIncreasingly BASIC(Lower concentration of H )Neutral solution910Milk of magnesia11Household ammonia12Household bleach13Oven cleanerBasic solution14

Acidic solutionNeutral solutionBasic solution

2.17 EVOLUTION CONNECTION: The searchfor extraterrestrial life centers on the searchfor water An important question is, has life evolvedelsewhere?– Water is necessary for life as we know it The National Aeronautics and Space Administration(NASA) has evidence that water was onceabundant on Mars– Scientists have proposed that reservoirs of waterbeneath the surface of Mars could harbor microbial lifeCopyright 2009 Pearson Education, Inc.

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