Chapter 2 The Chemical Context of Life PowerPoint Lectures for Biology Eighth Edition Neil Campbell and Jane Reece Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Overview A Chemical Connection to Biology The bombardier beetle uses chemistry to defend itself Copyright 2008 Pearson Education Inc publishing as Benjamin Cummings Compounds A compound Is a substance consisting of two or more elements combined in a fixed ratio Has characteristics different from those of its elements Figure 2 2 Sodium Chloride Sodium Chloride Important Elements of Life Table 2 1 Trace elements Are required by an organism in only minute quantities less than 0 01 The effects of essential element deficiencies Figure 2 3 a Nitrogen deficiency b Iodine deficiency Subatomic Particles Simplified models of an atom Cloud of negative charge 2 electrons Electrons Nucleus a This model represents the Figure 2 5 electrons as a cloud of negative charge as if we had taken many snapshots of the 2 electrons over time with each dot representing an electron s position at one point in time b In this even more simplified model the electrons are shown as two small blue spheres on a circle around the nucleus Radioactive Isotopes Some applications of radioactive isotopes in biological research are Dating fossils Tracing atoms through metabolic processes Diagnosing medical disorders Figure 2 7 Cancerous throat tissue Fig 2 8 a A ball bouncing down a flight of stairs provides an analogy for energy levels of electrons Third shell highest energy level Second shell higher energy level First shell lowest energy level b Atomic nucleus Energy absorbed Energy lost Fig 2 9 Hydrogen 1H Atomic mass First shell 2 He 4 00 Atomic number Helium 2He Element symbol Electrondistribution diagram Lithium 3Li Beryllium 4Be Boron 5B Carbon 6C Nitrogen 7N Oxygen 8O Fluorine 9F Neon 10Ne Silicon 14Si Phosphorus 15P Sulfur 16S Chlorine 17Cl Argon 18Ar Second shell Sodium Magnesium Aluminum 12Mg 11Na 13Al Third shell Fig 2 10 Neon with two filled shells 10 electrons a Electron distribution diagram First shell Second shell Fig Fig 2 10 2 2 10 Neon with two filled shells 10 electrons a Electron distribution diagram b Separate electron orbitals First shell 1s orbital Second shell Fig Fig 2 10 3 2 10 Neon with two filled shells 10 electrons a Electron distribution diagram b Separate electron orbitals First shell Second shell y x z 1s orbital 2s orbital Three 2p orbitals Fig 2 10 Neon with two filled shells 10 electrons a Electron distribution diagram b Separate electron orbitals First shell Second shell y x z 1s orbital 2s orbital Three 2p orbitals c Superimposed electron orbitals 1s 2s and 2p orbitals Electron Orbitals Electron orbitals Each orbital holds up to two electrons x Y Z 1s orbital 2s orbital Three 2p orbitals 1s 2s and 2p orbitals Electron shell diagrams Each shell is shown with its maximum number of electrons grouped in pairs a First shell maximum 2 electrons b Second shell maximum 8 electrons c Neon with two filled shells 10 electrons Covalent Bonds A covalent bond is the sharing of a pair of valence electrons by two atoms Hydrogen atoms 2 H In a covalent bond the shared electrons count as part of each atom s valence shell 1 In each hydrogen atom the single electron is held in its orbital by its attraction to the proton in the nucleus 2 When two hydrogen atoms approach each other the electron of each atom is also attracted to the proton in the other nucleus 3 Hydrogen molecule H2 Copyright 2008 Pearson Education Inc publishing as Benjamin Cummings The two electrons become shared in a covalent bond forming an H2 molecule Covalent Bonds in Four Molecules Single and double covalent bonds Name and Molecular Formula a Hydrogen H2 b Oxygen O2 c Water H2O d Methane CH4 Electrondistribution Diagram Lewis Dot Structure and Structural Formula Spacefilling Model Covalent Bonds In a polar covalent bond The atoms have differing electronegativities Share the electrons unequally Because oxygen O is more electronegative than hydrogen H shared electrons are pulled more toward oxygen This results in a partial negative charge on the oxygen and a partial positive charge on the hydrogens O Figure 2 13 H H H2O Copyright 2008 Pearson Education Inc publishing as Benjamin Cummings Ionic Bonds The lone valence electron of a sodium atom is transferred to join the 7 valence electrons of a chlorine atom 1 Na Sodium atom Figure 2 14 Cl Chlorine atom 2 Each resulting ion has a completed valence shell An ionic bond can form between the oppositely charged ions Na Na Sodium ion a cation Cl Cl Chloride ion an anion Sodium chloride NaCl Ionic compounds Are often called salts which may form crystals Na Cl Figure 2 15 Hydrogen Bonds A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom Water H2O H O H Ammonia NH3 In living cells the electronegative partners are usually oxygen or nitrogen atoms N H H H A hydrogen bond results from the attraction between the partial positive charge on the hydrogen atom of water and the partial negative charge on the nitrogen atom of ammonia Figure 2 16 Van 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 surface Fig 2 17 s orbital Four hybrid orbitals z x Three p orbitals y Tetrahedron a Hybridization of orbitals Space filling Model Ball and stick Hybrid orbital Model Model with ball and stick model superimposed Unbonded electron pair 104 5 Water H2O Methane CH4 b Molecular shape models Molecular Shape and Function Molecular shape Determines how biological molecules recognize and respond to one another with specificity Natural endorphin Key Carbon Hydrogen Morphine a Structures of endorphin and morphine Natural endorphin Endorphin receptors Morphine Brain cell b Binding to endorphin receptors Fig 2 18 Nitrogen Sulfur Oxygen Chemical Reactions 2 H2 O2 Reactants 2 H2 O Reaction Products Photosynthesis Is an example of a chemical reaction 6 CO2 6 H2O Figure 2 19 C6H12O6 6 O2 You should now be able to 1 Identify the four major elements 2 Distinguish between the following pairs of terms neutron and proton atomic number and mass
View Full Document
Unlocking...