Chapter 8 - An Introduction To Metabolism

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Metabolic PathwaysChapter 8 - An Introduction toMetabolism A metabolic pathway begins with a specificmolecule and ends with a productEnzyme 1Enzyme 2BAReaction 1StartingmoleculeEnzyme 3CReaction 2DReaction 3Product Each step is catalyzed by a specificenzyme Catabolic pathways release energy bybreaking down complex molecules into simplercompounds Cellular respiration, the breakdown of glucosein the presence of oxygen, is an example of apathway of catabolism Anabolic pathways consume energy to buildcomplex molecules from simpler ones The synthesis of protein from amino acids is anexample of anabolism Bioenergetics is the study of how organismsmanage their energy resourcesForms of Energy Energy is the capacity to cause change– Energy exists in various forms, some of which canperform work Potential energy is energy thatmatter possesses because of itslocation or structure Kinetic energy is energy associated withmotion Chemical energy is potential energyavailable for release in a chemicalreaction Heat (thermal energy) is kinetic energyassociated with random movement of atomsor molecules Energy can be converted from oneform to another1

The Laws of Energy Transformation Thermodynamics is the study of energytransformations A closed system, such as that approximated byliquid in a thermos, is isolated from itssurroundings In an open system, energy and matter can betransferred between the system and itssurroundingsThe First Law of Thermodynamics According to the first law of thermodynamics,the energy of the universe is constant:– Energy can be transferred and transformed,but it cannot be created or destroyed The first law is also called the principle ofconservation of energy Organisms are open systemsThe Second Law of Thermodynamics During every energy transfer or transformation,some energy is unusable, and is often lost as heat Living cells unavoidably convert organized formsof energy to heat According to the second law of thermodynamics: Spontaneous processes occur without energyinput; they can happen quickly or slowly– Every energy transfer or transformationincreases the entropy (disorder) of theuniverseChemicalEnergy For a process to occur without energy input, itmust increase the entropy of the universeHeat EnergyBiological Order and Disorder Which reactions occur spontaneously andwhich require input of energy? So, we need to determine energy changes thatoccur in chemical reactions2

Free-Energy Change, G A living system’s free energy is energy that can do workwhen temperature and pressure are uniform, as in a livingcell The change in free energy ( G) during a process is relatedto the change in enthalpy, or change in total energy ( H),change in entropy ( S), and temperature in Kelvin (T): G H – T S Only processes with a negative G are spontaneous Spontaneous processes can be harnessed to perform workFree Energy, Stability, and Equilibrium Free energy is a measure of a system’s instability,its tendency to change to a more stable state During a spontaneous change, free energydecreases and the stability of a system increases Equilibrium is a state of maximum stability A process is spontaneous and can perform workonly when it is moving toward equilibriumFig. 8-5b More free energy (higher G) Less stable Greater work capacityIn a spontaneous change The free energy of the systemdecreases ( G 0) The system becomes morestable The released free energy canbe harnessed to do work Less free energy (lower G) More stable Less work capacitySpontaneouschangeSpontaneouschange(a) Gravitational motionSpontaneouschange(b) Diffusion(c) Chemical reactionFig. 8-6aFree Energy and MetabolismReactants Exergonic and Endergonic Reactions inMetabolism:– An exergonic reaction proceeds with a netrelease of free energy and is spontaneous– An endergonic reaction absorbs free energyfrom its surroundings and is nonspontaneousFree energy The concept of free energy can be applied tothe chemistry of life’s processesAmount ofenergyreleased( G 0)EnergyProductsProgress of the reaction(a) Exergonic reaction: energy released3

Fig. 8-6bEquilibrium and MetabolismFree energyProducts Reactions in a closed system eventually reachequilibrium and then do no workAmount ofenergyrequired( G 0)Energy Cells are not in equilibrium; they are open systemsexperiencing a constant flow of materials A defining feature of life is that metabolism isnever at equilibriumReactants A catabolic pathway in a cell releases free energyin a series of reactionsProgress of the reaction(b) Endergonic reaction: energy required G 0 G 0 G 0(a) An isolated hydroelectric system(b) An open hydroelectric systemATP and energy coupling G 0 Within a cell - three main kinds of work: G 0 G 0– Chemical– Transport– Mechanical To do work, cells manage energy resources byenergy coupling, the use energy released by anexergonic process to power an endergonic one(c) A multistep open hydroelectric system Most energy coupling in cells is mediated by ATP4

ATP ATP (adenosine triphosphate) is the cell’senergy shuttle Bonds between the phosphate groups are veryunstable ATP is composed of ribose (a sugar), adenine (anitrogenous base), and three phosphate groups Energy is released from ATP when the terminalphosphate bond is broken This release of energy comes from thechemical change to a state of lower freeenergy, not from the phosphate bondsthemselvesHow ATP Performs WorkPPP The three types of cellular work (mechanical,transport, and chemical) are powered by ATPAdenosine triphosphate (ATP) In the cell, the energy from the exergonicreaction of ATP ADP and Pi is used to drivean endergonic reactionH2OPi PPInorganic phosphateEnergy Overall, the coupled reactions are exergonicAdenosine diphosphate (ADP)NH2GluGlutamicacidNH3 G 3.4 kcal/molGluAmmoniaGlutamine(a) Endergonic reaction1 ATP phosphorylatesglutamic acid,making the aminoacid less stable.PGlu ATP NH3Glu ADP The recipient molecule is now phosphorylatedNH22 Ammonia displacesthe phosphate group,forming glutamine.PGluGlu ATP drives endergonic reactions byphosphorylation, transferring a phosphategroup to some other molecule, such as areactant Pi(b) Coupled with ATP hydrolysis, an exergonic reaction(c) Overall free-energy change5

Membrane proteinATP ADP PiSolute ATP is a renewable resource that isregenerated by addition of a phosphate groupto adenosine diphosphate (ADP)PiPSolute transported(a) Transport work: ATP phosphorylatestransport proteinsADP ATPPiVesicleCytoskeletal track The energy to phosphorylate ADP comes fromcatabolic reactions in the cell called The chemical potential energy temporarilystored in ATP drives most cellular workATPProtein movedMotor protein(b) Mechanical work: ATP binds noncovalentlyto motor proteins, then is hydrolyzedATP H2OEnergy fromcatabolism (exergonic,energy-releasingprocesses)ADP P iEnergy for cellularwork (endergonic,energy-consumingprocesses)6

Fig. 8-5b Spontaneous change Spontaneous change Spontaneous change (a) Gravitational motion (b) Diffusion (c) Chemical reaction Free Energy and Metabolism The concept of free energy can be applied to the chemistry of life’s processes Exergonic and Endergonic Reactions in Metabol

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