Lecture 5 (Sept 7) - Biological Sciences 110A Introduction...

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Lecture 5: Enzymatic function and catalyzed reactions  The laws of thermodynamics  Free energy change ( G) in chemical reactions  ATP: The energy currency of cells  Coupled reactions driven by ATP hydrolysis  Reduction-oxidation (redox) reactions  Enzyme catalysts reduce activation energy  Enzyme kinetics (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) are bound by the  laws of thermodynamics 1. Energy is conserved  . Energy can be converted from  one form 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 are not 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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The first and second laws are represented in the “free energy” 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 be either negative or positive.  Examples: C 6 H 12 O 6 (glucose) + 6 O 2 > 6 H 2 O + 6 CO 2 H  is negative and  S  is positive.  G o   = -686 kcal/mole. 2 H 2 + O 2 > 2 H 2 O Negative  S  is overcome by very negative  H.   G o is VERY negative. G = G products – G reactants G is  dependent upon reaction conditions and the concentrations of reactants and products. G o  (“standard  free energy change”) refers to the free energy change under standard conditions (25 o C, 1 atm, 1 M [reactants,  products]), and is used for comparing the thermodynamic favorability of reactions.
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