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Chapter 4: Outline

Chapter 4: Outline. Thermodynamics First Law Second Law Free Energy Standard free energy changes Coupled reactions Hydrophobic effect (revisited) Role of ATP. Bioenergetics.

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Chapter 4: Outline

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  1. Chapter 4: Outline • Thermodynamics • First Law • Second Law • Free Energy • Standard free energy changes • Coupled reactions • Hydrophobic effect (revisited) • Role of ATP

  2. Bioenergetics • Each formation or breakdown of a biomolelcule involves an associated energy change. Thermodynamics is the field of chemistry that studies these energy changes. • The goal of thermodynamics is to predict whether a reaction will occur spontaneously which, in a chemical sense, means it will continue without energy input once started.

  3. 4.1 Thermodynamics • The heat and energy transformations studied by thermodynamics take place in a system (defined by the investigator) connected to the surroundings (the rest of the universe). • Closed system: energy exchanged between system and surroundings. • Open system-matter and energy exchanged between system and surroundings.

  4. First Law-1 • Energy is neither created nor destroyed. • or DE = q+w • DE is the change in the internal energy and is a state function, i. e. independent of path. • q is heat and is not a state function. • w is work and is not a state function.

  5. First Law-2 • Biochemical systems function at constant pressure, volume, and temperature. • H(enthalpy) = E + PV or DH = DE • and DH = q (heat flow) • The change in enthalpy for a reaction is calculated using the equation • DHreactants = DHproducts – DHreactants • -DH is exothermic +DH is endothermic

  6. First Law-3 • Given the equation and the DHf values, calculate the DH for the reaction. • 6 CO2 + 6 H2O  C6H12O6 + 6 O2 • kJ/mol kJ/mol • C6H12O6 -304.7 CO2 -94.0 • O2 0 H2O -68.4 [1*-304.7+6*0]-[6*-94.0+6*-68.4)]=+670 kJ Prod - Reactants

  7. Second Law • With a spontaneous reaction, the entropy of the universe increases. • DSuniv = DSsystem + Dssurroundings • In irreversible processes, entropy is a driving force.

  8. 4.2 Gibb’s Free Energy • Gibb’s free energy change (DG) is the most useful thermodynamic function for predicting reaction spontaneity. • The two other thermodynamic quantities that contribute to the value for DG: • DH=enthalpy change (energy change measured at constant pressure) • DS=entropy change (related to the state of disorder in a system)

  9. Gibb’s Free Energy: 2 • The three thermodynamic quantities are related by the following equation: • DG = DH -TDSsys • For biochemists, DG is usually measured at 25 oC, one atm for a gas, and at a concentration of 1 M for solutes except hydronium ion which is at pH 7. • These conditions specify a standard DG represented as DGo’ .

  10. Gibb’s Free Energy: 3 • In a spontaneous reaction: • free energy decreases, DG is negative • energy is released by the reaction • reaction is said to be exergonic • In a nonspontaneous reaction: • free energy increases, DG is positive • energy is absorbed by the reaction • reaction is said to be endergonic

  11. Examples, DG values • (From standard tables) DGo’,kJ/mol • (kcal/mol) • Exergonic reaction: • ATP + H2O  ADP + Pi -30.5 (-7.3) • Endergonic reaction: • glucose-6-phosphate to • fructose-6-phosphate + 1.7 (+0.4)

  12. DGo and Keq DG= DGo + RT ln [C]c[D]d [A]a[B]b • For the reaction • aA + bB = cC + dD At equilibrium, DG= 0 DGo = - RT ln Keq

  13. Coupled Reactions • Frequently in biochemistry two reactions are “coupled” or run as a pair. One reaction is endergonic but the second reaction is exergonic. The sum of the reactions (and the DG changes) is overall exergonic and consequently the reaction pair is overall spontaneous. • This is shown on the next slide.

  14. Coupled Reactions: 2 • DGo’(kcal/mol) • glucose-6-P  fructose-6-P + 0.4 • fructose-6-P + ATP  • fructose-1,6-bisP + ADP - 3.4 • glucose-6-P + ATP  • fructose-1,6-bisP + ADP - 3.0 • The overall reaction 3 (sum of 1+2) is exergonic. Sum of DGo’ 1 + DGo’ 2 is DGo’ 3 or –3.0 kcal/mol. Overall reaction is spontaneous

  15. 4.3 Bioenergetics and ATP ADP + Pi = -7.3 kcal/mol AMP + PPi = -7.7 kcal/mol PPi 2 Pi = -8 kcal/mol • Hydrolysis of adenosine triphosphate (ATP) provides the free energy to drive most endergonic reactions.

  16. ATP • Drives several processes: • Biosynthesis of biomolecules • Active transport across membranes • Mechanical work (e. g. muscle contraction) • Can carry phosphoryl groups from higer-energy compounds to lower-energy compounds.

  17. DGo’for ROPO32- + H2O • kJ/mol kcal/mol • Glucose-6-P -13.8 -3.3 • Fructose-6-P -15.9 -3.8 • ATPAMP + PPi -32.3 -7.7 • ATPADP + Pi -30.5 -7.3 • P-creatine -43.1 -10.3 • Glycerate-1,3-bP -40.4 -11.8 • Phosphoenolpyruvate -61.9 -14.8

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