Bioenergetics Bryant Miles Basis of Thermodynamics Every living cell and organism must perform work to stay alive, to grow and to reproduce. The ability to harvest energy from nutrients or photons of light and to channel it into biological work is the miracle of life. Living organisms carry out a remarkable variety of energy transductions. The biological energy transductions obey the physical laws that govern all natural processes, including the laws of thermodynamics. 1stLaw of Thermodynamics The energy of the universe remains constant. 2ndLaw of Thermodynamics All spontaneous processes increase the entropy of the universe. State functionsdepend only on the initial and final conditions not on path taken between the initial and final conditions. They are independent of path. The important state functions for the study of biological systems are: G,the Gibbs free energywhich is equal to the total amount of energy capable of doing work during a process at constant temperature and pressure. •If ΔG is negative, then the process is spontaneous and termed exergonic. •If ΔG is positive, then the process is nonspontaneous and termed endergonic. •If ΔG is equal to zero, then the process has reached equibrium. H, the Enthalpy which is the heat content of the system. •When ΔH is negative the process produces heat and is termed exothermic. •When ΔH is positive the process absorbs heat and is termed endothermic. S, the Entropyis a quantitative expression of the degree of randomness or disorder of the system. •When ΔS is positive then the disorder of the system has increased. •When ΔS is negative then the disorder of the system has decreased. The conditions of biological systems are constant temperature and pressure. Under such conditions the relationships between the change in free energy, enthalpy and entropy can be described by the expression where T is the temperature of the system in Kelvin. ΔG = ΔH −TΔS Equilibrium Constants All spontaneous processes proceed until equilibrium is reached. Consider the following chemical reaction. A + B C + Dk1k2The forward rate of product formation is = k1[A][B] The reverse rate of reactant formation is = k2[C][D] At equilibrium the concentrations of products and reactants are such that forward and reverse rates are equal k1[Aeq][Beq] = k2[Ceq][Deq]. A little algebra and presto
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