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What drives biological reactions? – part A
• Energy – how do the laws of thermodynamics affect living organisms?
• Are living cells in chemical equilibrium?
All images are from Wikimedia unless otherwise indicated.
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Which law of thermodynamics do energy pyramids illustrate?
• First law: energy cannot be created or destroyed• Second law: in any energy conversion, some energy
is wasted; moreover, the entropy of any closed system always increases.
1 million Joules of sunlight
10,000 J primary producers
1,000 J
100 J
Primary consumers
Secondary consumers
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Gibb's free energy and work
• Free energy (G) = Enthalpy (H) - Temperature (T) x Entropy (S)
• G = H - TS
G
Reaction progress
G
Reaction progress
ExergonicG < 0
EndergonicG > 0
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Free energy and chemical equilibrium
• For the reaction: A + B C + DG = ΔGo + RTln([C][D]/[A][B])
Go = standard free energy change, at pH7 and 1 Molar concentrations of reactants and products;
R = the gas constant; T = absolute temperature in degrees Kelvin At equilibrium, G = 0, and [C][D]/[A][B] = Keq;
therefore
Go = -RTlnKeq
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Reactions proceed toward equilibrium
All chemical reactions are theoretically reversible.
ΔG = RTln([C][D]/[A][B]) – RTlnKeq
[C][D]/[A][B] < Keq
G [C][D]/[A][B] > Keq
Progression regression
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Q: Are living cells in chemical equilibrium?
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Cells maintain disequilibrium
The products of a chemical reaction are siphoned off as reactants of other reactions
Campbell & Reece, Biology, 8th ed.
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How do cells build their macromolecules (accomplish work), when anabolic reactions are endergonic?
Free energy changes are additive.
Cells couple endergonic reactions with exergonic reactions.
Glutamate + NH4+ glutamine G = +3.4 kcal/mol
ATP ADP + Pi G = -7.3 kcal/mol
net G = -3.9 kcal/mol
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9
Adenosine triphosphate (ATP)
Hydrolysisof ATP
H2O
Adenosine diphosphate (ADP)Inorganicphosphate (Pi)
H H
Maureen KnabbWest Chester U.
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What drives biological reactions? – part B
• What are enzymes and how do cells use them?
All images are from Wikimedia unless otherwise indicated.
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Q: The oxidation of glucose to CO2 and H2O is highly exergonic; ΔGo = -636 kcal/mole. Why doesn’t glucose spontaneously combust?
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Activation energy determines reaction rate
Free energy determines the equilibrium point, but not the reaction rate.
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Enzymes are biological catalysts
The enzyme-substrate complex creates a transition state with lower activation energy than the uncatalyzed reaction.
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Enzyme-catalyzed reactions show saturation kinetics
Vmax-
1/2 Vmax
KM [substrate]
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Enzyme inhibitors - competitive
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allosteric regulation of enzymes
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Feedback regulation
Inhibition by the end-product (negative feedback)
Activation by end-product or metabolite (positive feedback)
A2BConvertase
A B C Product
Metabol.