Concept 8 1 An organism s metabolism transforms matter and energy subject to the laws of thermodynamics Metabolism is the totality of an organism s chemical reactions Metabolism is an emergent property of life that arises from interactions between molecules within the cell A metabolic pathway begins with a specific molecule and ends with a product where each step is catalyzed by a specific enzyme Catabolic pathways release energy by breaking down complex molecules into simpler compounds e g breakdown of glucose in the presence of oxygen Cellular respiration Anabolic pathways consume energy to build complex molecules from simpler ones e g synthesis of protein from AA Fig 8 UN1 Enzyme 1 A Reaction 1 Starting molecule Enzyme 2 B Reaction 2 Enzyme 3 C Reaction 3 D Product Forms of Energy Energy is the capacity to cause change It exists in various forms Kinetic energy is energy associated with motion Heat thermal energy is kinetic energy associated with random movement of atoms or molecules Potential energy is energy that matter possesses because of its location or structure Chemical energy is potential energy available for release in a chemical reaction Energy can be converted from one form to another Thermodynamics is the study of energy transformations The First Law of Thermodynamics According to the first law of thermodynamics the energy of the universe is constant Energy can neither be created nor destroyed It can only be transformed or transferred The first law is also called the principle of conservation of energy The Second Law of Thermodynamics During every energy transfer or transformation some energy is unusable and is often lost as heat According to the second law of thermodynamics Every energy transfer or transformation increases the entropy disorder of the universe Fig 8 3 Heat Chemical energy a First law of thermodynamics CO2 H2O b Second law of thermodynamics Living cells unavoidably convert organized forms of energy to heat Spontaneous processes occur without energy input they can happen quickly or slowly For a process to occur without energy input it must increase the entropy of the universe Energy flows into an ecosystem in the form of light and exits in the form of heat Concept 8 2 The free energy change of a reaction tells us whether or not the reaction occurs spontaneously Free Energy Change G G2 G1 A living system s free energy is energy that can do work when temperature and pressure are uniform as in a living cell The change in free energy G during a process is related to 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 work Fig 8 5 More free energy higher G Less stable Greater work capacity In a spontaneous change The free energy of the system decreases G 0 The system becomes more stable The released free energy can be harnessed to do work Less free energy lower G More stable Less work capacity a Gravitational motion b Diffusion c Chemical reaction Free Energy Stability Equilibrium Metabolism Free energy is a measure of a system s instability its tendency to change to a more stable state During a spontaneous change free energy decreases and the stability of a system increases Equilibrium is a state of maximum stability The concept of free energy can be applied to the chemistry of lif e s processes An exergonic reaction proceeds with a net release of free energy and is spontaneous An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous Fig 8 6 Reactants Free energy Amount of energy released G 0 Energy Products Progress of the reaction a Exergonic reaction energy released Free energy Products Energy Reactants Progress of the reaction b Endergonic reaction energy required Amount of energy required G 0 Equilibrium and Metabolism Reactions in a closed system eventually reach equilibrium and then do no work Cells are not in equilibrium they are open systems experiencing a constant flow of materials A defining feature of life is that metabolism is never at equilibrium A catabolic pathway in a cell releases free energy in a series of reactions Closed and open hydroelectric systems can serve as analogies Fig 8 7 G 0 G 0 a An isolated hydroelectric system b An open hydroelectric system G 0 G 0 G 0 G 0 c A multistep open hydroelectric system Concept 8 3 ATP powers cellular work by coupling exergonic reactions to endergonic reactions To do work cells manage energy resources by energy coupling the use of an exergonic process to drive an endergonic one Most energy coupling in cells is mediated by ATP ATP adenosine triphosphate is the cell s energy shuttle The bonds between the phosphate groups of ATP s tail can be broken by hydrolysis Energy is released from ATP when the terminal phosphate bond is broken This release of energy comes from the chemical change to a state of lower free energy not from the phosphate bonds themselves Fig 8 8 ATP is composed of ribose a sugar adenine a nitrogenous base and three phosphate groups Adenine Phosphate groups Ribose Fig 8 10 NH2 Glu Glutamic acid NH3 G 3 4 kcal mol Glu Ammonia Glutamine a Endergonic reaction 1 ATP phosphorylates glutamic acid making the amino acid less stable P Glu ATP Glu ADP NH2 2 Ammonia displaces the phosphate group forming glutamine P Glu NH3 Glu Pi b Coupled with ATP hydrolysis an exergonic reaction c Overall free energy change In the cell the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction Overall the coupled reactions are exergonic ATP drives endergonic reactions by phosphorylation transferring a phosphate group to some other molecule such as a reactant The recipient molecule is now phosphorylated The chemical potential energy temporarily stored in ATP drives most cellular work Fig 8 11 Membrane protein P Solute Pi Solute transported a Transport work ATP phosphorylates transport proteins ADP ATP Vesicle Cytoskeletal track ATP Motor protein Protein moved b Mechanical work ATP binds noncovalently to motor proteins then is hydrolyzed Pi Fig 8 12 The Regeneration of ATP ATP is a renewable resource that is regenerated by addition of a phosphate group to adenosine diphosphate ADP ATP H2O Energy from catabolism exergonic energy releasing processes ADP P i Energy for cellular work endergonic energy consuming processes The energy to phosphorylate ADP comes from catabolic reactions in