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lecture0219audio - ‑‑‑ ‑ 2X> X X ΔG = 3 kcal...

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Bioenergetics For many reactions inside cells (e.g.  the  synthesis of polymers from monomers),  standard free energy change is positive,  so how come these reactions still occur?     use of coupled reactions a) Removal of product through a reaction  with a negative  G o > B + C   ‑‑‑ G o ’ = +2 kcal/mole  C  > D         ‑‑‑ G o ’ =  7 kcal/mole Overall: A  > B+D    ‑‑‑ G o ’ =  5 kcal/mole  
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Coupled reactions  b) Use of an activated intermediate. We wish to make the polymer X X from X  but the reaction 2X < > X X has a  Δ G of +3  ‑‑ kcal/mole. So we first phosphorylate X using ATP 2ATP + 2X  > 2X P + 2ADP ‑‑‑‑ Then combine two of the phosphorylated X’s        2X P  > X X + 2Pi ‑‑ These reactions are equivalent to: 2ATP  > 2ADP + 2Pi        Δ G =  11.5 kcal
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Unformatted text preview: ‑‑‑ ‑ 2X > X X ΔG = +3 kcal ‑‑‑ ‑ So overall ΔG = 8.5 kcal ‑ “Steady states” Many reactions inside cells are coupled to other reactions or are driven by the constant removal of their products. As a result most reactions in cells are NOT at equilibrium but are in a “ steady state ” , in which the concentrations of reactants are constant but there is a steady net flow of material through the coupled reactions. Figure 3.7 ATP<-> ADP + Pi ∆ G o ’ = - RT ln K eq -7.3 = - RT ln( [ADP] eq. [Pi] eq /[ATP] eq ) So at equilibrium [ADP] >>[ATP] (assuming Pi levels are normal) Dead! Equilibrium Live! Steady state – continuous ATP production...
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