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9/12/2011 The Chemical Basis of Life Energy levels of electrons *chemical behavior due to electrons Electron configuration and chemical properties: Covalent Bonds – A covalent bond forms when two atoms share one or more pairs of outer-shell electrons. – Atoms held together by covalent bonds form a molecule. Name Electron configuration (molecular formula) Hydrogen gas (H2) Structural formula Space-filling model Ball-and-stick model Single bond (a pair of shared electrons) Oxygen gas (O2) Double bond (two pairs of shared electrons) Methane (CH4) Ionic Bonds – When an atom loses or gains electrons, it becomes electrically charged. • Charged atoms are called ions. • Ionic bonds are formed between oppositely charged ions. Outer shell has 1 electron Na Sodium atom The outer electron is stripped from sodium and completes the chlorine atom’s outer shell Outer shell has 7 electrons Cl Chlorine atom Complete outer shells Na+ Sodium ion Cl− Chlorine ion The attraction between the ions—an ionic bond—holds them together Sodium chloride (NaCl) 1 9/12/2011 Hydrogen Bonds – Water is a compound in which the electrons in its covalent bonds are shared unequally. • This causes water to be a polar molecule, one with opposite charges on opposite ends. (slightly +) (slightly +) H H O (slightly –) Hydrogen bonding • Weak bonds formed between hydrogen and another atom – Surface tension of water • Important p as intramolecular bonds, giving shape to proteins and other biomolecules WATER AND LIFE – Life on Earth began in water and evolved there for 3 billion years. • Modern life remains tied to water. • Your cells are composed of 70%–95% water. 2 9/12/2011 Properties of Water • Numerous properties due to the hydrogen bonding Properties of Water • Cohesion • Adhesion Evaporation from the leaves Microscopic tubes SEM Cohesion due to hydrogen bonds between water molecules Properties of Water – Heat and temperature – Water can absorb and store large amounts of heat while only changing a few degrees in temperature. – Water can moderate temperatures. 3 9/12/2011 Properties of Water – Surface tension is the measure of how difficult it is to stretch or break the surface of a liquid. Water is the solvent of life *solution *solvent *solute *aqueous solution The Chemical Basis of Life Hydrophilic and hydrophobic substances *hydrophilic *hydrophobic The dissociation of water molecules H2O H+ + OH- Acids and bases *acid *base 4 9/12/2011 Acids, Bases and pH 14 Increasingly basic (lower H+ concentration) Oven cleaner Basic solution Increasingly acidic (greater H+ concentration) Neutral [H+] = [OH–] Neutral solution Acidic solution 13 Household 12 bleach Household ammonia 11 Milk of magnesia 10 9 Seawater 8 Human blood 7 Pure water 6 Urine 5 4 Tomato juice 3 Grapefruit juice, soft drink 2 Lemon juice, gastric juice 1 0 pH scale Carbon and Organic Chemistry – Carbon is a versatile atom. – Carbon forms large, complex, and diverse molecules necessary for life’s functions. – Organic compounds are carbon-based molecules. Structural formula Ball-and-stick model Space-filling model Carbon and Organic Chemistry • Variations in Carbon skeletons Carbon skeletons vary in length Carbon skeletons may be unbranched or branched Carbon skeletons may have double bonds, which can vary in location Carbon skeletons may be arranged in rings 5 9/12/2011 Chemical Components of Cells *structural isomers *geometric isomers *enantiomers Example of enantiomers: Hydrocarbons – Larger hydrocarbons form fuels for engines. – Hydrocarbons of fat molecules fuel our bodies. Carbon and Organic Chemistry • The unique properties of an organic compound depend not only on its carbon skeleton but also on the atoms attached to the skeleton – These atoms are called functional groups – Some common functional groups include: Hydroxyl group Carbonyl group Found in alcohols and sugars Found in sugars Amino group Carboxyl group Found in amino acids and urea in urine (from protein breakdown) Found in amino acids, fatty acids, and some vitamins 6