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Lect12_catalysts - Exam 1 The miracle of catalysis The idea...

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1 The miracle of catalysis The idea of “reaction specific” matches Day 11 Exam 1 Available this afternoon It’s apparent that the class could use more practice explaining / justifying answers You’ll have another chance at the last page, plus further practice on two more of the exam questions 15 points, due in Disc Th, details soon on Smartsite Life relies on chemical reactions All chemical reactions in a cell = metabolism Energy can be released from chemicals (organic and inorganic) In cells we call this catabolism: tearing things apart, increasing entropy Energy can be stored in organic molecules (components of the cell) In cells, this is anabolism: it builds molecules, reduces entropy Must understand something about reactions to talk about how life gets and uses energy! A + B C + D ---> (forward reaction) k F What determines rate (molecules reacting per unit time) for the forward reaction? A. Concentration of A B. Concentration of B C. The probability that A reacts with B D. All of the above The probability that two molecules will react is termed a “ Rate constant ”, or K A + B C + D ---> (forward reaction) k F Rate = molecules reacting/unit time = k F x[A]x[B] [concentrations] A + B C + D <--- (Back reaction) k B Rate constant of back reaction (k B ) is a probability that C and D will react. Back or reverse reaction: Reverse rate = molecules reacting/unit time = k B x[C]x[D] concentrations
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2 A + B ---> (forward reaction) k F <--- (Back reaction) k R At equilibrium Equal rates Products are favored (exist at higher concentration at equilibrium) in a “spontaneous” reaction [C]x[D] x k B [A]x[B] x k F = Forward rate Backwards rate C + D If you start with pure A and B A ‘spontaneous’ reaction Proceeds without continuous external inputs (is ‘downhill’) Releases energy (has a negative G, exergonic) Increases entropy (disorder) (usually) Favors products over reactants This type of reaction can fuel a cell (it releases energy overall) Energy is released . . . UNTIL equilibrium is achieved – then there is no net energy change! Since life requires energy input, it must PREVENT reactions from reaching equilibrium! Surprising thing #1: 1. The amount of energy released dictates only the equilibrium state. K E = k F / k B = [A][B] [C][D] Whether products or reactants predominate at equilibrium Energy released doesn’t dictate the RATE of the reaction or even which direction the reaction goes. This observation is fundamental to how metabolism works. Surprising thing #2: 2. The direction of reaction (and thus amount of energy you can get out) depends on not just the structure of the products and reactants, but also on their relative concentrations . This is important in real cells, because NONE of the molecules are at 1 Molar concentration!
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Lect12_catalysts - Exam 1 The miracle of catalysis The idea...

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