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 - TS

G

Reaction progress

G

Reaction progress

ExergonicG < 0

EndergonicG > 0

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

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

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, 8th 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 + NH4+ glutamine G = +3.4 kcal/mol

ATP ADP + Pi G = -7.3 kcal/mol

net G = -3.9 kcal/mol

9

Adenosine triphosphate (ATP)

Hydrolysisof ATP

H2O

Adenosine diphosphate (ADP)Inorganicphosphate (Pi)

H H

Maureen KnabbWest 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 CO2 and H2O is highly exergonic; ΔGo = -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

Vmax-

1/2 Vmax

KM [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

A B C Product

Metabol.