Chapter 2Overview: A Chemical Connection to BiologyCompoundsImportant Elements of LifeSlide 5Subatomic ParticlesRadioactive IsotopesSlide 8Slide 9Fig. 2.10Fig. 2-10-2Fig. 2-10-3Slide 13Electron OrbitalsCovalent BondsCovalent Bonds in Four MoleculesSlide 17Slide 18Ionic BondsSlide 20Hydrogen BondsVan der Waals InteractionsSlide 23Molecular Shape and FunctionChemical ReactionsPhotosynthesisYou should now be able to:PowerPoint Lectures for Biology, Eighth EditionNeil Campbell and Jane ReeceCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsChapter 2Chapter 2The Chemical Context of LifeOverview: A Chemical Connection to BiologyCopyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings•The bombardier beetle uses chemistry to defend itselfCompounds•A compound–Is a substance consisting of two or more elements combined in a fixed ratio–Has characteristics different from those of its elementsSodium ChlorideSodium Chloride+Figure 2.2Important Elements of LifeTable 2.1•Trace elements–Are required by an organism in only minute quantities (less than 0.01%)(a) Nitrogen deficiency (b) Iodine deficiency•The effects of essential element deficienciesFigure 2.3Nucleus(a)(b)In this even more simplifiedmodel, the electrons areshown as two small bluespheres on a circle around thenucleus.Cloud of negativecharge (2 electrons)ElectronsThis model represents theelectrons as a cloud ofnegative charge, as if we hadtaken many snapshots of the 2electrons over time, with eachdot representing an electron‘sposition at one point in time.•Simplified models of an atomFigure 2.5Subatomic ParticlesRadioactive IsotopesCancerous throat tissueFigure 2.7•Some applications of radioactive isotopes in biological research are:–Dating fossils–Tracing atoms through metabolic processes–Diagnosing medical disordersFig. 2-8(a) A ball bouncing down a flight of stairs provides an analogy for energy levels of electronsThird shell (highest energylevel)Second shell (higherenergy level)EnergyabsorbedFirst shell (lowest energylevel)Atomicnucleus(b)EnergylostFig. 2-9Hydrogen1HLithium3LiBeryllium4BeBoron5BCarbon6CNitrogen7NOxygen8OFluorine9FNeon10NeHelium2HeAtomic numberElement symbolElectron-distributiondiagramAtomic mass2He4.00FirstshellSecondshellThirdshellSodium11NaMagnesium12MgAluminum13AlSilicon14SiPhosphorus15PSulfur16SChlorine17ClArgon18ArFig. 2.10Electron-distributiondiagram(a)Neon, with two filled shells (10 electrons)First shell Second shellFig. 2-10-2Electron-distributiondiagram(a)(b)Separate electronorbitalsNeon, with two filled shells (10 electrons)First shell Second shell1s orbitalFig. 2.10Fig. 2-10-3Electron-distributiondiagram(a)(b)Separate electronorbitalsNeon, with two filled shells (10 electrons)First shell Second shell1s orbital 2s orbital Three 2p orbitalsxyzFig. 2.10Fig. 2.10Electron-distributiondiagram(a)(b)Separate electronorbitalsNeon, with two filled shells (10 electrons)First shell Second shell1s orbital 2s orbital Three 2p orbitals(c)Superimposed electronorbitals1s, 2s, and 2p orbitalsxyzElectron orbitals.Each orbital holdsup to two electrons.1s orbital 2s orbital Three 2p orbitals1s, 2s, and 2p orbitals(a) First shell (maximum 2 electrons)(b) Second shell (maximum 8 electrons)(c) Neon, with two filled shells (10 electrons)Electron-shell diagrams.Each shell is shown withits maximum number of electrons, grouped in pairs.xZYElectron OrbitalsIn each hydrogenatom, the single electronis held in its orbital byits attraction to theproton in the nucleus.1When two hydrogenatoms approach eachother, the electron ofeach atom is alsoattracted to the protonin the other nucleus.2The two electronsbecome shared in a covalent bond,forming an H2molecule.3Hydrogenatoms (2 H)Hydrogenmolecule (H2)Covalent Bonds•A covalent bond is the sharing of a pair of valence electrons by two atoms•In a covalent bond, the shared electrons count as part of each atom’s valence shellCopyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings•Single and double covalent bondsCovalent Bonds in Four MoleculesName andMolecularFormulaElectron-distributionDiagramLewis DotStructure andStructural FormulaSpace-fillingModel(a) Hydrogen (H2)(b) Oxygen (O2)(c) Water (H2O)(d) Methane (CH4)Figure 2.13This results in a partial negative charge on theoxygen and apartial positivecharge onthe hydrogens.H2O–OHH+ +Because oxygen (O) is more electronegative than hydrogen (H), shared electrons are pulled more toward oxygen.•In a polar covalent bond–The atoms have differing electronegativities–Share the electrons unequallyCovalent BondsCopyright © 2008 Pearson Education, Inc., publishing as Benjamin CummingsThe lone valence electron of a sodiumatom is transferred to join the 7 valenceelectrons of a chlorine atom.1 Each resulting ion has a completedvalence shell. An ionic bond can formbetween the oppositely charged ions.2Ionic BondsFigure 2.14Cl ClNaSodium atom Chlorine atomNa+Sodium ion(a cation)Cl–Chloride ion(an anion)Sodium chloride (NaCl)NaNa+Cl–Figure 2.15•Ionic compounds–Are often called salts, which may form crystalsHydrogen Bonds – + +Water(H2O)Ammonia(NH3)OHH + –NHHHA hydrogenbond results from the attraction between thepartial positive charge on the hydrogen atom of water and the partial negative charge on the nitrogen atom of ammonia.++Figure 2.16•A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom•In living cells, the electronegative partners are usually oxygen or nitrogen atomsVan der Waals Interactions•If electrons are distributed asymmetrically in molecules or atoms, they can result in “hot spots” of positive or negative charge•Van der Waals interactions are attractions between molecules that are close together as a result of these charges•Collectively, such interactions can be strong, as between molecules of a gecko’s toe hairs and a wall surfaceFig. 2-17s orbitalThree porbitals(a) Hybridization of orbitalsTetrahedronFour hybrid orbitalsSpace-fillingModelBall-and-stickModelHybrid-orbital Model(with ball-and-stickmodel superimposed)Unbondedelectronpair104.5ºWater (H2O)Methane (CH4)(b) Molecular-shape modelszxy•Molecular shape–Determines how biological molecules recognize and respond to one another with specificityMolecular Shape and FunctionFig. 2.18(a) Structures of endorphin and morphine(b)
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