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.

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

Gibb's free energy and work Free energy (G) = Enthalpy (H) - Temperature (T) x Entropy (S)  G =  H - T  S G Reaction progress G Exergonic  G < 0 Endergonic  G > 0

Free energy and chemical equilibrium For the reaction: A + B  C + D  G = ΔG o + RTln([C][D]/[A][B])  G o = 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] = K eq ; therefore  G o = -RTlnK eq

Reactions proceed toward equilibrium All chemical reactions are theoretically reversible. ΔG = RTln([C][D]/[A][B]) – RTlnK eq [C][D]/[A][B] < K eq G [C][D]/[A][B] > K eq Progression regression

Q: Are living cells in chemical equilibrium?

Cells maintain disequilibrium The products of a chemical reaction are siphoned off as reactants of other reactions Campbell & Reece, Biology, 8 th ed.

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 + NH 4 +  glutamine  G = +3.4 kcal/mol ATP  ADP + P i  G = -7.3 kcal/mol net  G = -3.9 kcal/mol

9 Adenosine triphosphate (ATP) Hydrolysis of ATP H2OH2O Adenosine diphosphate (ADP) Inorganic phosphate (P i ) HH Maureen Knabb West Chester U.

What drives biological reactions? – part B What are enzymes and how do cells use them? All images are from Wikimedia unless otherwise indicated.

Q: The oxidation of glucose to CO 2 and H 2 O is highly exergonic; ΔG o = -636 kcal/mole. Why doesn’t glucose spontaneously combust?

Activation energy determines reaction rate Free energy determines the equilibrium point, but not the reaction rate.

Enzymes are biological catalysts The enzyme-substrate complex creates a transition state with lower activation energy than the uncatalyzed reaction.

Enzyme-catalyzed reactions show saturation kinetics V max - 1/2 V max KMKM [substrate]

Enzyme inhibitors - competitive

allosteric regulation of enzymes

Feedback regulation Inhibition by the end-product (negative feedback) Activation by end-product or metabolite (positive feedback) A2BConvertase AB C Product Metabol.