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Chapter 5B: Energy & Enzymes 1. What is Energy? 2. Energy & Chemical Reactions 3. ATP 4. Enzymes & Metabolic Pathways 1. What is Energy? 2 Basic Forms of Energy Kinetic Energy (KE) • energy in motion or “released” energy: heat (molecular motion) *electric current (flow of charged particles) *light energy (radiation of photons) *mechanical energy (structural movement) *chemical energy (breaking covalent bonds, flow from high to low concentration) *forms of KE cells use to “do things” Potential Energy (PE) • stored energy (i.e., not yet released): gravitational potential *chemical bonds *chemical, charge gradients *sources of PE cells rely on Illustration of Kinetic & Potential Energy KE highest at B, lowest at A & C PE highest at A & C, lowest at B 2 Laws of Thermodynamics 1st Law of Thermodynamics Conservation of energy: “energy is neither created nor destroyed, but may be converted to other forms” 2nd Law of Thermodynamics Energy conversions: “in any energy conversion, some usable energy is always lost” • “Lost” energy = energy released as heat to the surroundings, energy that can’t be used NO energy conversion is 100% efficient! Energy conversion Fuel Waste products Heat energy Carbon dioxide Gasoline Combustion Kinetic energy of movement Water Oxygen Energy conversion in a car Heat Glucose Cellular respiration Oxygen Carbon dioxide Water Energy for cellular work Energy conversion in a cell 2. Energy & Chemical Reactions Exergonic Chemical Reactions Potential energy of molecules loss of PE • net release of energy Reactants Amount of energy released Energy released Products • loss of PE, converted to KE Endergonic Chemical Reactions gain of PE • increases PE Potential energy of molecules • net consumption of energy Products Energy required Reactants Amount of energy required gain of PE Photosynthesis is Endergonic • energy input from sunlight • stored in glucose Respiration is Exergonic • releases PE stored in glucose loss of PE Activation Energy (Ea) Whether endergonic or exergonic, all chemical reactions require some energy input for the reaction to proceed: the activation energy (Ea) • all reactions require some sort of “spark” • this is why sources of chemical PE are “stable” Mechanical Model of Activation Energy The upright bottle falling over is analogous to an exergonic reaction, yet it still requires some energy input for the bottle to tip over. 3. ATP Adenosine Triphosphate (ATP) Adenosine Chemical PE in molecules of ATP is the principal source of directly useable energy in cells. Triphosphate (ATP) Phosphate group Adenine Ribose hydrolysis • “food energy” is transferred to ATP by respiration + Adenosine Diphosphate (ADP) gain of PE ATP synthesis is Endergonic • energy input is needed to add a 3rd phosphate ATP “hydrolysis” is Exergonic loss of PE • breaking the covalent bond with the 3rd PO4 releases energy Coupling of Biochemical Reactions Exergonic reactions fuel (provide energy for) endergonic reactions in cells (i.e, they are “coupled”) • breakdown of glucose fuels ATP production exergonic endergonic exergonic endergonic • ATP hydrolysis fuels most cellular activities Examples of ATP-powered “Work” ATP Chemical work Mechanical work Transport work membrane protein P + motor protein solute P P Reactants P P P Product Molecule formed Protein moved ADP + P Solute transported 4. Enzymes & Metabolic Pathways Enzymes are Biological Catalysts Enzymes are proteins that increase the rates of reactions by lowering the E a • the enzymes themselves are not changed Reaction without enzyme EA without enzyme Energy EA with enzyme Reactants Reaction with enzyme Net change in energy (the same) Products Progress of the reaction • allows reactions to occur that otherwise could not The catalytic cycle of an enzyme 1 Enzyme available with empty active site Active site • every enzyme has a unique substrate & thus catalyzes a specific reaction Substrate (sucrose) 2 Substrate binds to enzyme with induced fit Glucose Enzyme (sucrase) Fructose H 2O 4 Products are released 3 Substrate is converted to products • cells produce 1000s of different enzymes, all of which are proteins encoded by a particular gene Metabolic Pathways Most biological processes, whether anabolic (building) or catabolic (breaking down), require a series of chemical reactions (i.e., a pathway) • each step in a metabolic pathway is catalyzed by a specific enzyme A Simple Metabolic Pathway Regulation of Metabolic Pathways Most metabolic pathways are regulated so that “too much” is not produced: • e.g., Feedback Inhibition: excess product inhibits the activity of an enzyme early in the pathway Inhibitors prevent substrate binding Substrate Active site Enzyme • provides a means to regulate enzyme activity Normal binding of substrate Competitive inhibitor Noncompetitive inhibitor Enzyme inhibition • many poisons inhibit enzyme activity Key Terms for Chapter 5B (5.10-16) • kinetic, potential energy • endergonic, exergonic, coupling of reactions • activation energy • ATP, enzyme, catalyst • substrate, active site • competitive, noncompetitive inhibition Relevant Review Questions: 2, 4, 7, 12, 14, 17