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Lecture 5 (Sept 4) - Biological S nce 110A I ntroduction to...

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Lecture5: Enzymatic function and catalyzed reactions Thelaws of thermodynamics Freeenergy change ( G) in chemical reactions ATP: Theenergy currency of cells Coupled reactions driven by ATP hydrolysis Reduction-oxidation (redox) reactions Enzymecatalysts reduce activation energy Enzymekinetics (Michaelis-Menten equation) Biological Sciences 110A: Introduction to Biology Kendal Broadie Reading in Chapter 3 (85-104) Karp
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All chemical systems (and cells) arebound by the laws of thermodynamics 1. Energy is conserved . Energy can beconverted from oneform to another, but cannot be created or destroyed. 2. The entropy (disorder) of the universe (or any closed system) always increases. Reactions go “downhill” from a higher to a lower energy state. Living cells are highly ordered, and seem to defy the second law… How? Cells arenot closed systems . Cells create and maintain order by using energy (light or chemical energy) obtained from their environment, some of which is returned to their environment as heat.
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Thefirst and second laws arerepresented in the “freeenergy” change ( G) of a chemical reaction G = H – T S H = enthalpy (~energy) change, T = temperature, S = entropy (~disorder) change H and S for a reaction can beeither negativeor positive. Examples: C 6 H 12 O 6 (glucose) + 6 O 2 > 6 H 2 O + 6 CO 2 H is negativeand S is positive. G o = -686 kcal/mole. 2 H 2 + O 2 > 2 H 2 O Negative S is overcomeby very negative H. G o is VERY negative. G = G products – G reactants G is dependent upon reaction conditions and theconcentrations of reactants and products. G o (“standard freeenergy change”) refers to thefreeenergy changeunder standard conditions (25 o C, 1 atm, 1 M [reactants, products]), and is used for comparing thethermodynamic favorability of reactions.
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