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lecture 23 - BIO 311C Be sure to see Jingjie before 4:00...

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BIO 311C March 19, 2008 Be sure to see Jingjie before 4:00 p.m. Friday (March 21) if you believe we have made an error in grading your Exam 2. We will not consider changes after that date.
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The study of changes in energy which occur during chemical reactions is a field of chemistry called chemical thermodynamics . These energy changes are called chemical energetics . For a chemical reaction in general: K eq = [Products] [ Reactants] Review Δ G° = (- 5.7 kJ/mole) (log 10 K eq ) Chemical reactions with K eq greater than 1 have Δ G° values less than zero. This can be expressed qualitatively by stating that “they are exergonic”, or “they are spontaneous”, or “they proceed in the forward direction”, under standard conditions . Chemical reactions with K eq less than 1 have Δ G° values greater than zero. This can be expressed qualitatively by stating that “they are endergonic”, or “they proceed in the reverse direction”, under standard conditions . *
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[X] actual Consider the reaction: X Y , with K eq = 0.01 Now suppose that within a living cell the steady-state concentration of X is 1,000 times higher than the steady-state concentration of Y. Under this non-standard set of conditions, the reaction is "pushed in the forward direction by the large excess of X over Y. The energy requirements under this non-standard set of conditions can be determined if the value of Δ G° (energy requirement under standard conditions) is known. Δ G = + 11.4 kJ/mole + (5.7 kJ/mole) ( log 10 [1/1000] ) Δ G = Δ G° + (5.7 kJ/mole) ( log 10 [Y] actual ) For this reaction: = - 5.7 kJ/mole * Then: Δ G° = (- 5.7 kJ/mole) (log 10 0.01) = + 11.4 kJ/mole) The reaction can be written: X Y Thus, a reaction can be changed from endergonic to exergonic by changing the relative steady-state concentrations of the substrates.
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A reaction that tends to proceed in the reverse direction can be made to go in the forward direction in a cell by the cell’s appropriately adjusting the steady-state concentrations of substrates. X Y endergonic reaction X Y Substrate X continually supplied by other reactions to keep its concentration high, thereby "pushing" this reaction Substrate Y continually removed by other reactions to keep its concentration low, thereby "pulling" this reaction Ö A cell can sometimes push and pull a reaction in order to make it more exergonic. These pushing reactions that produce X as a product and pulling reactions that use y as a reactant are said to be coupled to the reaction X Y.
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