395GLec02_09 - Introduction to Bioenergetics Thermodynamics...

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Introduction to Bioenergetics Thermodynamics Free energy Equilibrium Coupled reactions Steady state High energy bonds Redox potential
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Thermodynamics Review First Law : For any physical or chemical change, the total amount of energy in the universe remains constant. Energy may change form or be transported, but it cannot be created or destroyed. Second Law : In all natural processes, the entropy of the universe (total S) increases. - Key terms we will deal with in this chapter: - System / Surroundings / Universe - Energy, Heat, Work - influence on system 1 J = (Kg-m / sec ) ; 1 cal = 4.184 J - Enthalpy (H) and Entropy (S) H = E + PV or H = E + P V S = k lnW or S = k ln( W f /W i )
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Entropy of dilution S = k ln W where W = “ways” of arranging solute. There are N I ways to place first solute, N I -1 for second etc. BUT since N I >>>NA, these are all same, W I N I x N I x N I …=±±N I NA and W F = N F NA . The entropy upon dilution is S F -S I S = k ln N F NA –k± lnN I NA = k ln [N F NA / N I NA ] = N A k ln N F /N I For N A = 1 mole, S = R ln N F /N I and given that V F ~ N F etc. S = R ln V F /V I A similar, but more complicated derivation shows for a given solute: S = R ln C F /C I
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Heat and entropy • The statistical models are almost useless in Biochem. It can be shown, though, that entropy can be measured via heat change. • A small addition of heat to a hot system adds little entropy (already stirred up), but adding the same heat to a cold system makes a larger entropic change. S ~ q/T the lower the absolute T, the greater entropy change • The system we are focused on may increase or decrease entropy during a living reaction, but the entropy of the universe always increases. • If the temperature is NOT constant, we must sum heat change over each T as: S > dq/T
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An example of spontaneous reaction
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Free energy combines enthalpy and entropy G = H-TS this is property is non-linear with concentration (recall the logarithmic nature of the entropy) For any molecule, G varies with concentration. •G = G 0 + RT lnC •= G 0 + RT 2.303 logC G 0 + 5.7 KJ/mol logC
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As systems go to equilibrium, G goes to zero
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G controls Equilibrium In the system A + B C + D G = G C + G D -(G A + G B ) = G P -G R However, for each species G i = G i 0 + RTln C i therefore, G = G C 0 + RTln C C + G D 0 + RTln C D -(G A 0 + RTln C A
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395GLec02_09 - Introduction to Bioenergetics Thermodynamics...

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