ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Two Outline INTRODUCTION Organisms Are Chemical Machines Composed of molecules Content reshuffled via chemical reactions Water is most important molecule of life Beginning of Universe 20 billion years ago Residual energy still detectable Organisms composed of molecules that are composed of atoms ATOMS: THE STUFF OF LIFE Universe Composed of Matter All matter made of atoms fig 2.1 Very small size, resembling solar system Composed of smaller subatomic particles fig 2.2 Protons (+) and neutrons (0) in central nucleus Electrons (-) in circular orbits around nucleus Same number as protons to balance charge Dictates chemical activity Atomic number = number of protons Neutrons and protons have same mass Only protons have electrical charge Mass versus weight Mass is the amount of a substance Weight is the force of gravity exerted on it Atomic mass = mass of protons + mass of neutrons Mass measured in daltons Proton or neutron is roughly 1 dalton Electron is 1/1840 dalton, practically mass-less Subatomic particles seen indirectly via collisions Isotopes All atoms of an element have the same atomic number (proton number) An element cannot be broken into other substances by chemical means Isotopes of an element have: Same number of protons, different number of neutrons Same number of electrons, thus same chemical properties Example: carbon-12 versus carbon-13 and carbon-14 fig 2.3 Unstable forms, like carbon-14, decay Emit radioactive energy Half-life = time for half of a sample's atoms to decay Potential harmful side effects, exposure must be limited Electrons Electrically neutral atom has same number of electrons and protons Electron orbit maintained by electrical attraction In ions the number of electrons and protons are different Element that possesses a net electrical charge Positive charge if electron lost, a cation Negative charge if electron gained, an anion Electrons Determine the Chemical Behavior of Atoms Arrangement determines chemical properties of element Orbital describes probable, not actual location Shapes differ fig 2.4 Inner s orbitals are spherical More distant p orbitals are dumbbell-shaped Maximum number of two electrons per orbital Orbitals extremely far away from nucleus, atom mostly empty space Nuclei of different atoms rarely contact one another Electrons interact, determine chemical behavior Energy Within the Atom (-) electrons are attracted to (+) protons Energy required to keep electrons in orbit Electron energy of position is potential energy fig 2.5 Moving electron away from nucleus Requires energy Electron then has more potential energy Moving electron toward nucleus Releases energy Electron then has less potential energy Exchange of electrons between molecules fig 2.6 Oxidation is a loss of electrons Reduction is a gain of electrons Chemical energy stored in electrons by oxidation-reduction reactions Energy level schematics Electrons represented as concentric rings called energy levels fig 2.7 Electrons in outer most rings hold more energy Don't confuse energy levels and electron orbitals The Periodic Table fig 2.8 Eight groups of repeating chemical properties Based on interactions of valence electrons in outer shell Maximum of eight electrons in outer shell of elements important to life Elements at maximum are inert, not reactive Elements with one less than maximum are highly reactive Octet rule (rule of eight) states that atoms want their outer shell full CHEMICAL BONDS HOLD MOLECULES TOGETHER Molecule Is a Stable Group of Atoms Compounds are molecules containing more than one kind of element A chemical bond is the holding force Ionic Bonds Form Crystals fig 2.9 Atoms attracted by opposite electrical charges Atoms donate or receive electrons from other atoms Example: sodium chloride, common table salt Sodium atom, loses electron = Na+ Chlorine atom, accepts electron = Cl- Resulting atoms become charged ions, an ionic compound Bond forms by attraction of ions of opposite charges Not between two individual atoms Between one ion and all oppositely charged ions in vicinity Covalent Bonds Build Stable Molecules Two atoms share one or more pairs of valence electrons Example: single bonded diatomic hydrogen (H2) fig 2.10 Hydrogen has unpaired electron and unfilled outer level Two atoms combine, each nucleus shares two electrons Bond requires close proximity of atoms to one another Covalent bonds are very strong Double bond shares two pairs of electrons, stronger than a single bond Structural formulas: H - H or O = O Molecular formulas: H2 or O2 Chemical reactions Make and Break Chemical Bonds Involve shifting atoms without change in number or identity Reactants: original, pre-reaction molecules Products: molecules resulting from a reaction Influenced by several factors Temperature: heat increases rate Concentration: reactant versus product have opposite effect Catalyst: special substance increases rate Molecules with Several Covalent Bonds Atoms can share electrons with more than one other atom Example: carbon, has six electrons, four in the outer level To satisfy octet rule must gain four electrons Thus can form four chemical bonds THE ATOMS OF LIFE Distribution of Elements in Living Organisms tbl 2.1 Only eleven elements found in greater than trace amounts Elements are generally light, atomic mass less than 21 Most Abundant Elements: N, O, C, H All form covalently bonded molecules Possess breakable chemical bonds to make a variety of molecules Reflect predominance of water (H2O) in organisms Many form gaseous molecules that are soluble in water WATER : THE CRADLE OF LIFE Unique Properties of Water Necessary for Living Organisms fig 2.11 Exists as liquid at temperature of earth's surface Provides a medium in which other molecules can interact Composes two-thirds of most organisms Forms weak chemical associations Simple atomic structure, H2O fig 2.12 Water Acts Like a Magnet Electronegativity attracts electrons of water molecules Has distinct ends, each with a partial charge Polar molecule results from magnet like poles Polarity is crux of chemistry of water and life Charge separation results in polar nature Most stable configuration is tetrahedron, bond angle 104.5% Partial (d+) charges at apexes opposite hydrogens Partial (d-) charge at oxygen Polar molecules interact with one another Opposite charges attract, form hydrogen bonds fig 2.13 Bonds are transient, cumulative effects important Hydrogen bonds affect physical properties of water tbl 2.2 Water Clings to Polar Molecules Cohesion is attraction of water to water Results in surface tension of water fig 2.14 Causes things to get wet in water Adhesion is attraction of water to another molecule Attraction is electrostatic Results in capillary action, water rises in thin tube fig 2.15 Height inversely proportional to tube diameter Water Stores Heat Exhibits high specific heat Amount of heat to change temperature of a substance Associated with and proportional to polarity Thermal energy must first disrupt hydrogen bonds Heats up slowly Retains heat longer than surroundings Forms ice with decrease in temperature fig 2.16 Crystal-like lattice of hydrogen bonds Less dense than liquid water High heat of vaporization Amount of heat required to change water to vapor Evaporation of water produces cooling effect Water Is a Powerful Solvent Water molecules gather around charged molecules Example: table sugar (sucrose) Water forms hydrogen bonds with OH- groups of sucrose Each sugar molecule surrounded by cloud of water molecules Cloud is called the hydration shell Hydration shells form around ions fig 2.17 Water Organizes Nonpolar Molecules Water excludes nonpolar molecules Preferentially forms hydrogen bonds with itself Minimizes disruption of hydrogen bonding Hydrophobic: not soluble in water, nonpolar Hydrophilic: soluble in water, polar Hydrophobic exclusion Forces nonpolar molecules to associate together Shapes molecules with nonpolar regions Water Ionizes Ionization is spontaneous formation of ions Results from breaking of covalent bonds of water Proton (H+) dissociates from molecule Remainder of molecule is OH- Mole of a substance is its molecular mass Corresponds to combined atomic mass of all molecules Molar concentration of H+ ions in water is 10-7 mole/liter pH scale quantifies H+ concentration fig 2.18 pH = negative log of H+ ion concentration = -log[H+] Acid = low pH value, <7, high concentration of H+ Base = high pH value, >7, low concentration of H+ Scale is logarithmic, change of one on scale is really tenfold Changes in environmental pH caused by acid precipitation fig 2.19 Serious impact on living organisms Erodes even limestone and marble fig 2.20 Buffers pH of body fluids is about 7 Minimize changes in H+ and OH- concentration Act as reservoirs for H+ Donate to solutions when concentration falls Take from solutions when concentration increases Example: carbonic acid/bicarbonate in blood fig 2.21