Cell Bio Ch 5

[Reversibility] Are chemical reactions reversible according to this lecture?

Yes. Reactions can proceed in both the forward and reverse directions.

[Reversibility] What happens after reactants are mixed?

The reaction proceeds until equilibrium is reached.

[Equilibrium] What is true of the forward and reverse rates at equilibrium?

Forward rate = reverse rate.

[Equilibrium] What does it mean that the reaction is occurring equally in each direction at equilibrium?

Equal numbers of molecules are moving in the forward and reverse directions.

[Reaction Progress] Just after reactants are mixed and before equilibrium, what usually happens?

The reaction typically dominates in one direction.

[Equilibrium] Does equilibrium mean the reaction has stopped?

No. The reaction is still occurring in both directions, but the forward and reverse rates are equal.

[Favored Direction] How can the favored direction of a reaction be determined?

By comparing the relative amounts of reactants and products after equilibrium is reached.

[Keq] What is the equilibrium constant (Keq) described as in this lecture?

The ratio of product to reactant at equilibrium.

[Keq] What does Keq predict?

The favored direction of a reaction.

[Keq Example] What does the 9:1 product-to-reactant ratio tell you about the reaction before equilibrium?

The reaction proceeded strongly to the right.

[Keq] If Keq > 1, which direction is favored?

The right, or forward direction.

[Keq] If Keq < 1, which direction is favored?

The left, or reverse direction.

[Keq Memory] Quick rule for Keq?

Keq > 1 → forward/right favored. Keq < 1 → reverse/left favored.

[Thermodynamics] What determines equilibrium according to the lecture?

The laws of thermodynamics.

[First Law] What does the first law of thermodynamics state?

Energy is neither created nor destroyed in chemical reactions; it is converted from one form to another.

[Second Law] What does the second law of thermodynamics state in this lecture?

With each reaction, less energy is available to do work as more energy is converted to less useful forms.

[Second Law] What law is the second law associated with?

The law of entropy.

[Energy] What is the energy available to do work called?

Free energy.

[Energy] What determines Keq in terms of energy?

The relative free-energy levels of reactants and products.

[Free Energy] What kind of energy does each chemical have according to the lecture?

An inherent free energy.

[Free Energy] What symbol is used for Gibbs free energy?

G.

[Gibbs Free Energy] What is the equation for G?

G = H − TS.

[Gibbs Free Energy] In G = H − TS, what does H represent?

Enthalpy, or total energy.

[Gibbs Free Energy] In G = H − TS, what does T represent?

Temperature in Kelvin.

[Gibbs Free Energy] In G = H − TS, what does S represent?

Entropy.

[Delta G] How is ΔG defined for a reaction?

ΔG = Gproducts − Greactants.

[Delta G] What does ΔG compare?

The free energy of the products with the free energy of the reactants.

[Exergonic] If Greactants > Gproducts, what happens to free energy?

Free energy is released by the reaction.

[Exergonic] If Greactants > Gproducts, what sign does ΔG have?

Negative.

[Exergonic] What is a reaction with negative ΔG called?

Exergonic.

[Exergonic] Is an exergonic reaction spontaneous or nonspontaneous?

Spontaneous.

[Exergonic Memory] What three ideas go together for an exergonic reaction?

Greactants > Gproducts; ΔG is negative; the reaction is spontaneous.

[Endergonic] If Gproducts > Greactants, what happens to free energy?

Free energy is consumed by the reaction.

[Endergonic] If Gproducts > Greactants, what sign does ΔG have?

Positive.

[Endergonic] What is a reaction with positive ΔG called?

Endergonic.

[Endergonic] Is an endergonic reaction spontaneous or nonspontaneous?

Nonspontaneous.

[Endergonic Memory] What three ideas go together for an endergonic reaction?

Gproducts > Greactants; ΔG is positive; the reaction is nonspontaneous.

[Compare] Exergonic vs endergonic?

Exergonic releases free energy and has ΔG < 0; endergonic consumes free energy and has ΔG > 0.

[Sign Check] If ΔG = −5 kcal/mol, how would the lecture classify the reaction?

Exergonic and spontaneous.

[Sign Check] If ΔG = +5 kcal/mol, how would the lecture classify the reaction?

Endergonic and nonspontaneous.

[Factors Affecting G] What four conditions are listed as affecting G?

Concentration, temperature, pH, and pressure.

[Standard Free Energy] Why must conditions be standardized when comparing ΔG values of different reactions?

Because G is affected by concentration, temperature, pH, and pressure.

[Standard Free Energy] What is the standard free-energy change called?

ΔG°′.

[Standard Free Energy] Under what conditions is ΔG°′ defined in the lecture?

1 M reactants, 1 M products, STP, and pH 7.0.

[Standard Free Energy] What does the prime symbol in ΔG°′ correspond to in the lecture's definition?

The standard biochemical conditions listed on the slide, including pH 7.0.

[Concentration Effects] Do reactant and product concentrations affect G?

Yes.

[Concentration Effects] Does the relationship between concentration and G exist only at equilibrium?

No. It exists at all concentrations.

[Concentration Effects] Can GR′ and GP′ be calculated away from equilibrium?

Yes. They can be calculated for any given concentration.

[Calculating G] What equation is given for the free energy of reactant R?

GR′ = GR°′ + RT ln[R].

[Calculating G] What equation is given for the free energy of product P?

GP′ = GP°′ + RT ln[P].

[Calculating G] In GR′ = GR°′ + RT ln[R], what is GR′?

The free energy of R under the stated conditions.

[Calculating G] What is GR°′?

The standard free energy of R.

[Calculating G] What value is given for the gas constant R?

1.987 cal/mol/K.

[Calculating G] What does T represent in the concentration-dependent free-energy equation?

Temperature in Kelvin.

[Calculating G] What temperature in Kelvin corresponds to 25°C on the slide?

298 K.

[Calculating G] What does ln[R] mean?

The natural logarithm of the reactant concentration [R].

[Calculating G] What happens to the concentration term when [R] changes?

The RT ln[R] term changes, so the free energy of R changes.

[Concept Check] Why can changing concentrations influence whether a cellular reaction proceeds?

Because concentrations of reactants and products affect their free energies, and therefore affect the reaction's ΔG.

[Cellular Reactions] What kind of standard free-energy change do many cellular reactions have?

Many are endergonic and have a positive ΔG°′.

[Cellular Reactions] What two ways do cells use to overcome endergonic reactions?

1) Regulate concentrations of reactants and products, and 2) input energy.

[Concentration Strategy] What concentration change can help drive an endergonic reaction forward?

Increase reactant concentration and decrease product concentration.

[Concentration Strategy] Why does increasing [R] and decreasing [P] help?

Because reactant and product concentrations affect free energy and can make the reaction more favorable.

[Glycolysis Example] What reaction is used to demonstrate concentration control?

Glyceraldehyde 3-phosphate → 1,3-bisphosphoglycerate.

[Glycolysis Example] What ΔG°′ is given for glyceraldehyde 3-phosphate → 1,3-bisphosphoglycerate?

+1.5.

[Glycolysis Example] What do upstream reactions do to glyceraldehyde 3-phosphate (G3P)?

They produce G3P.

[Glycolysis Example] What do downstream reactions do to 1,3-bisphosphoglycerate (1,3-BPG)?

They utilize 1,3-BPG.

[Glycolysis Example] How do upstream and downstream reactions help the G3P → 1,3-BPG reaction?

Upstream reactions raise the reactant concentration, while downstream reactions lower the product concentration.

[Energy Input] What second strategy can cells use to drive an endergonic reaction?

Provide energy.

[Glutamine Example] What endergonic reaction is shown on slide 25?

Glutamic acid + NH3 → glutamine.

[Glutamine Example] What ΔG°′ is given for glutamic acid + NH3 → glutamine?

+3.4.

[ATP Hydrolysis] What ΔG°′ is given for ATP hydrolysis?

−7.3 kcal/mol.

[ATP Coupling] Why can ATP hydrolysis help drive an endergonic reaction?

ATP hydrolysis has a strongly negative ΔG°′ and can provide energy when the reactions are coupled.

[ATP Coupling] What intermediate is formed when glutamic acid reacts with ATP?

Glutamyl phosphate.

[ATP Coupling] What are the products of the first coupled step?

Glutamyl phosphate + ADP.

[ATP Coupling] What happens in the second coupled step?

Glutamyl phosphate + NH3 → glutamine + Pi.

[ATP Coupling] What is the overall ΔG°′ shown for the ATP-coupled glutamine-forming process?

−3.9 kcal/mol.

[ATP Coupling] Is the ATP-coupled overall process favorable by the lecture's ΔG rule?

Yes. Its ΔG°′ is negative (−3.9 kcal/mol).

[ATP Coupling] What happened to the sign of ΔG when the +3.4 reaction was coupled to ATP hydrolysis?

It became negative overall: −3.9 kcal/mol.

[ATP Coupling] How does the lecture demonstrate the net ΔG°′ of the coupled process?

+3.4 kcal/mol + (−7.3 kcal/mol) = −3.9 kcal/mol.

[ATP Coupling] What is the major lesson of the glutamine example?

A positive-ΔG°′ cellular reaction can be driven by coupling it to ATP hydrolysis.

[High-Yield Review] Equilibrium vs favored direction?

At equilibrium, forward rate = reverse rate. The relative amounts of products and reactants at equilibrium reveal which direction was favored before equilibrium.

[High-Yield Review] Keq sign/direction rule?

Keq > 1 → forward/right favored; Keq < 1 → reverse/left favored.

[High-Yield Review] What determines Keq according to this lecture?

The relative free-energy levels of reactants and products.

[High-Yield Review] ΔG equation?

ΔG = Gproducts − Greactants.

[High-Yield Review] Negative ΔG means what?

Exergonic, free energy released, spontaneous.

[High-Yield Review] Positive ΔG means what?

Endergonic, free energy consumed, nonspontaneous.

[High-Yield Review] What affects G?

Concentration, temperature, pH, and pressure.

[High-Yield Review] What are the two main ways cells drive endergonic reactions?

Manipulate reactant/product concentrations or couple the reaction to an energy source such as ATP hydrolysis.

[High-Yield Review] What concentration pattern favors the forward cellular reaction example?

High reactant concentration and low product concentration.

[High-Yield Review] What is the simplest way to remember ATP coupling from the lecture?

Pair a positive-ΔG°′ reaction with ATP hydrolysis, whose negative ΔG°′ can make the combined process negative overall.