Biochemistry 244 – Thermodynamics Quiz

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Questions and Answers

What does a positive Gibbs energy change (∆G°') indicate about a reaction?

The reaction is spontaneous.

The reaction is exergonic.

The reaction is at equilibrium.

The reaction is endergonic. (correct)

Given that the equilibrium constant (Keq) is less than 1, what can be inferred about the concentrations of reactants and products at equilibrium?

Products are favored at equilibrium.

Reactants are favored at equilibrium. (correct)

Both reactants and products are present in equal concentrations.

The reaction does not proceed to equilibrium.

What happens to the reaction A ⇋ B when 1M of both A and B are mixed under standard conditions?

The reaction proceeds to the left. (correct)

The reaction remains static with no movement.

The reaction proceeds to the right.

The reaction converts completely to products.

How does the Gibbs energy change (∆G) relate to the mass-action ratio (G)?

∆G decreases as G approaches equilibrium. Signup and view all the answers

What is the relationship between standard Gibbs energy change (∆G°) and the equilibrium constant (Keq)?

They are directly proportional. Signup and view all the answers

What is the value of Keq at equilibrium for the reaction A ⇋ B?

0.0177 Signup and view all the answers

Which condition results in an exergonic reaction based on the equation ∆G' = ∆Gº' + RT ln [B]/[A]?

When RT ln [B]/[A] < -10000 Signup and view all the answers

What strategy is mentioned for organisms to enable reactions with positive Gibbs energy changes?

Mass action to maintain 𝚪 < Keq Signup and view all the answers

If the [A]/[B] ratio is greater than 56.6, what can be inferred about the system?

The reaction favors the formation of A Signup and view all the answers

What does the term ΔGº' represent in biochemical reactions?

The change in Gibbs Free Energy under standard conditions Signup and view all the answers

What is the value of ∆G' when [A]/[B] = 60 at 25 °C?

-139.08 J/mol Signup and view all the answers

For the reaction glucose-6-phosphate ⇋ fructose-6-phosphate, what is the value of ∆Gº'?

+1.7 kJ/mol Signup and view all the answers

At equilibrium, what is the value of ∆G' for the reaction involving glucose-6-phosphate and fructose-6-phosphate?

0 kJ/mol Signup and view all the answers

In the context of chemical coupling, if a reaction has ∆Gº' of +10 kJ/mol, what must be true for the coupled reaction?

It must have a greater negative ∆Gº'. Signup and view all the answers

What is the total change in Gibbs free energy (∆Gº') when reactions A⇋B (∆Gº' = +10 kJ/mol) and C⇋D (∆Gº' = -31 kJ/mol) are coupled?

-21 kJ/mol Signup and view all the answers

Which strategy do living organisms use to allow reactions with positive Gibbs energy values to proceed spontaneously?

Both mass action and chemical coupling Signup and view all the answers

What effect does ATP hydrolysis typically have on endergonic reactions?

It makes them spontaneous. Signup and view all the answers

What happens to the free energy fraction for glucose-6-phosphate (fG6P) when ∆G' is less than 0?

fG6P is greater than 0.665 Signup and view all the answers

What are the two strategies that living organisms use to allow reactions with positive ∆G values to occur spontaneously?

Mass action and chemical coupling Signup and view all the answers

Which of the following describes chemical coupling in the context of biochemical reactions?

Linking an energetically unfavorable reaction to a favorable one Signup and view all the answers

Which example illustrates the concept of mass action effectively?

ATP hydrolysis coupled to glucose phosphorylation Signup and view all the answers

In a metabolic pathway, which of the following best depicts the utilization of mass action?

The Krebs cycle reactions utilizing acetyl-CoA Signup and view all the answers

What can be calculated when given the ∆G or ∆G° values for individual reactions in a metabolic pathway?

The ∆G or ∆G° for the net coupled reaction Signup and view all the answers

Study Notes

Gibbs Free Energy Change (∆G)

The Gibbs free energy change (∆G) indicates the direction and extent of a reaction’s drive towards equilibrium.
∆G is influenced by the equilibrium constant (Keq) and the mass-action ratio (G, the ratio of products to reactants).
The standard Gibbs free energy change (∆Gº) directly relates to Keq.

Calculation of ∆G

For the reaction A ⇋ B, if ∆Gº' = +10 kJ/mol, the reaction is endergonic.
When concentrations of A and B are equal at standard conditions, the equilibrium constant (Keq) will be less than 1.
At equilibrium, the concentration ratio [B]/[A] is approximately 0.0177.

Mass Action Effect

To drive a reaction with a positive ∆G, maintain the mass action ratio (G) below Keq.
Increasing substrate concentration or reducing product concentration helps achieve exergonic conditions (∆G' < 0).

Chemical Coupling

Reactions with positive ∆G can be made spontaneous (exergonic) by coupling with a more negative ∆G reaction.
Example: A ⇋ B (∆Gº' = +10 kJ/mol) can be coupled with C ⇋ D (∆Gº' = -31 kJ/mol) resulting in a net reaction A + C ⇋ B + D with ∆Gº' = -21 kJ/mol.

Equilibrium and Reaction Dynamics

At equilibrium, ∆G’ equals 0, indicating no net change in reactant or product concentrations.
When fG6P (glucose-6-phosphate) = 0.665 and fF6P (fructose-6-phosphate) = 0.335, it reflects the state at equilibrium for the conversion of glucose-6-phosphate to fructose-6-phosphate.

Living Organisms and Reactions

Organisms utilize two main strategies to drive reactions with positive ∆G:
- Mass Action: Keeping the mass-action ratio (G) below Keq.
- Chemical Coupling: Linking reactions to those with a stronger negative ∆G.

ATP Hydrolysis as a Coupling Example

ATP hydrolysis is often employed as a coupling mechanism to power endergonic reactions, making them exergonic.

Key Takeaway

The integration of thermodynamic principles (mass action and chemical coupling) is critical for biological processes, allowing cells to overcome energetically unfavorable reactions and maintain metabolic pathways.

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Description

Test your understanding of thermodynamic principles in biochemistry. This quiz focuses on the calculation of Gibbs free energy changes and the mass action principle as applied to carbohydrate metabolism. Key concepts include the relationship between reactant concentrations and energy changes.