Chapter Outline
INTRODUCTION Bioenergetics: How Energy Behaves in Living Systems Metabolism: The Sum of All Chemical Reactions Carried Out by an Organism Anabolism: expend energy to make or transform bonds Catabolism: harvest energy when bonds broken WHAT IS ENERGY? Energy: The Ability to Do Work Exists in two states Kinetic energy: energy of motion Potential energy: stored energy that has the capacity of moving Living organisms transform potential energy into kinetic energy fig 8.1 Thermodynamics: The Study of Energy Energy is readily measured by its conversion into heat Unit of heat: 1000 calories = 1 kilocalorie (kcal) OXIDATION-REDUCTION: THE FLOW OF ENERGY IN LIVING THINGS Life Exists on Earth Because It Is Able to Capture Energy From the Sun Energy from the sun transformed into chemical energy fig 8.2 Process called photosynthesis Done by plants, algae and certain bacteria Combine water and carbon dioxide to make sugars Energy stored in covalent bonds between sugar atoms Oxidation-Reduction Reactions Class of reactions that pass electrons from one molecule to another Oxidation: atom or molecule loses an electron, becomes oxidized Oxygen strongly attracts electrons Oxygen is most common electron acceptor in biological systems Reduction: atom or molecule gains an electron and is reduced Reactions occur together, electron transfers from one atom to other fig 8.3 Reactions play key role in flow of energy through biological systems Light adds energy and boosts electron to higher energy level Transfer of electrons often accompanied by transfer of protons Hydrogen atom: proton plus electron Oxidation involves removal of hydrogen atoms Reduction involves addition of hydrogen atoms Example: redox reaction of photosynthesis Hydrogen atoms transferred from water to carbon dioxide Carbon dioxide reduced to form glucose One mole of glucose stores 686 kcal of energy Example: redox reaction of cellular respiration Hydrogen atoms transferred from glucose to oxygen Glucose is oxidized Oxidation of glucose releases 686 kcal of energy THE LAWS OF THERMODYNAMICS DESCRIBE HOW ENERGY CHANGES First Law of Thermodynamics Energy can be transformed but not created or destroyed Total amount of energy in the universe remains constant Animals transfer food potential energy into their own chemical bonds fig 8.4 Energy is not lost but may be changed into other forms Converted to kinetic energy, light, electricity Also dissipated as heat Second Law of Thermodynamics All objects tend to become less ordered, disorder is increasing Entropy : measure of disorder of a system = S FREE ENERGY Bonds Between Atoms Hold Molecules Together Free energy: energy available to break and form chemical bonds = G Enthalpy: energy within a cell that is available to do work = H Temperature = T Free Energy = Ordering Influences - Disordering Influences G = H - TS Change in free energy: ΔG = ΔH - TΔS Negative ΔG: Exergonic reactions Products contain less free energy or more disorder than reactants Reactions occur spontaneously, release excess usable free energy Positive ΔG: Endergonic Reactions Products contain more free energy than the reactants Reactions do not occur spontaneously, requires input of energy fig 8.5 ACTIVATION ENERGY: PREPARING MOLECULES FOR REACTION Reactions Require an Input of Energy to Get Started Must break chemical bonds before new bonds can be created Activation energy: required to destabilize existing chemical bonds fig 8.6a Catalysis: Stressing Chemical Bonds Making Them Easier to Break fig 8.6b Catalyst: substance that carries out catalysis Cannot violate basic laws of thermodynamics Accelerates reaction in both forward and reverse directions Direction of reaction dependent on free energy ENZYMES: BIOLOGICAL CATALYSTS Enzymes: Agents That Carry Out Catalysis in Living Organisms Are generally proteins with specialized shapes Permit temporary associations with the molecules that are reacting Lower activation energy required for new bonds to form Bring two substrates together in the correct orientation Stress particular bonds of a substrate Example: formation of carbonic acid from carbon dioxide and water Reaction proceeds in either direction Reaction is slow because of a great activation energy Carbonic anhydrase: enzyme that speeds the reaction Enzymes given the name of their substrate with the ending -ase Thousands of Different Enzymes Exist Each enzyme catalyzes a different reaction Different cells contain different complements of enzymes How Enzymes Work Globular protein enzymes possess surface clefts called active sites fig 8.7 Enzymes are specific in their choice of substrate Amino acid side groups of enzyme react with substrate The substrate must fit precisely into the active site Induced fit: binding may induce shape adjustments in the protein fig 8.8 Substrate itself may act as activator FACTORS AFFECTING ENZYME ACTIVITY Temperature fig 8.9a Disrupts hydrogen bonds and hydrophobic interactions Alters protein shape and peptide arms Enzymes function best within narrow range, temperature optimum Hot spring bacteria proteins have strong bonds in peptide arms pH Hydrogen ion concentration fig 8.9b Disrupts bonds between oppositely charged amino acids With more H+ ions fewer negative, more positive charges occur Most enzymes have a pH optimum Enzymes that function in acids retain 3-D shape when many H+ present Inhibitors and Activators Activity dependent on presence of specific substances Substances bind to enzyme and change its shape When shape changes activity is altered Inhibitors change shape and shut off activity Competitive inhibitors bind at same site as substrate Non-competitive inhibitors bind at different site fig 8.10 Feedback inhibition: end product inhibits reaction early in pathway Allosteric site: region where non-competitive inhibitor binds Allosteric inhibitor binds to allosteric site to reduce enzyme activity fig 8.10b Activators bind to allosteric sites Keep enzymes in active configuration Increase enzyme activity Coenzymes and Other Cofactors Cofactors Additional components that aid enzyme action Many metallic trace elements are cofactors Coenzyme Nonprotein organic molecule functions as cofactor , include vitamins Serve as acceptors for electron pairs in redox reactions, shuttle energy Example: nicotinamide adenine dinucleotide (NAD+) fig 8.11 Important biological hydrogen acceptor NAD+ acquires an electron and hydrogen to become reduced NADH NADH carries energy of electron and hydrogen around in cells ATP: THE ENERGY CURRENCY OF LIFE Adenosine Triphosphate (ATP) Is the Chief Energy Currency of All Cells ATP Molecule Composed of Three Subunits fig 8.12 Five-carbon ribose sugar serves as the backbone Adenine composed of two C-N rings attaches to the ribose Nitrogen has unshared electrons Weakly attracts hydrogen atoms Called a nitrogenous base (one of four in DNA) Triphosphate group attaches to the ribose Covalent bonds linking phosphates are high-energy Bonds are readily broken and energy transferred ATP 9 ADP + Pi + 7.3 kcal/mole Adenosine diphosphate = ADP Pi is inorganic phosphate group Cells Use ATP to Drive Endergonic Reactions Products possess more energy than the reactants Can power cell activities Terminal high-energy bond is more exergonic than others Activation energy is usually less than 7 kcal/mole Cells contain a pool of ATP, ADP and phosphate Cells do not stockpile ATP but create it as needed ATP constantly recycled fig 8.13 BIOCHEMICAL PATHWAYS: THE ORGANIZATIONAL UNITS OF METABOLISM Reactions in Biological Systems Occur in Sequence Product of one reaction becomes substrate for another fig 8.14 Organized units of metabolism Location of enzymes helps map out model of pathway fig 8.15 How Biochemical Pathways Evolved First primitive biochemical processes Energy-rich molecules scavenged from the environment Molecules existed in the existing organic soup Catalyzed reactions were simple one-step processes As energy-rich molecules were depleted only those cells that could synthesize energy-rich molecules could survive Energy utilizing reaction became coupled to energy-producing reaction Evolution of pathways works backwards Occur one step at a time Final reactions generally evolve first, initial reaction evolves last How Biochemical Pathways Are Regulated Output of pathways must be controlled Primitive organisms evolved feedback mechanisms Enzymes have secondary binding sites, bind nonsubstrate molecules Binding alters shape of enzyme which changes its activity Activity can be increased as well as decreased Enzymes controlled in this manner are allosteric Feedback inhibition fig 8.16 Second binding site binds with end product Prevents enzyme activity Lack of enzyme activity shuts down pathway Stops production of product