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Question

A novel organism is discovered that utilizes a unique metabolic pathway where the standard free energy change ($\Delta$G°) for the overall reaction is +25 kJ/mol.  Which of the following strategies could this organism employ to make this pathway thermodynamically feasible?

Mr. NARESH KUMAR K · Accepted Answer

## CORE INFORMATION
✓ **Answer**: C
## CONCEPT FRAMEWORK
 **Primary Concept**: Thermodynamics of biological systems, specifically coupling of endergonic and exergonic reactions.
 **Related Topics**: Gibbs Free Energy, Reaction spontaneity, Metabolic pathways.
 **Prerequisites**: Understanding of Gibbs Free Energy ($\Delta G$), its relationship to spontaneity ($\Delta G < 0$ for spontaneous reactions), and the concept of coupling reactions.
## SOLUTION PATHWAY
**Given Information**:
  $\Delta G\u00b0 = +25 kJ/mol$ (positive, indicating a non-spontaneous reaction)
  The reaction is endergonic (requires energy input).
**Method & Steps**:
 1. Analyze the given options in the context of making the reaction thermodynamically feasible (spontaneous, $\Delta G < 0$).
  Reasoning: A positive $\Delta G$ means the reaction is not spontaneous under standard conditions. To make it feasible, we need to lower the overall $\Delta G$ to a negative value.
 2. Evaluate each option:
  A. Increasing product concentration shifts the equilibrium towards reactants, increasing $\Delta G$. Incorrect.
  B. Decreasing reactant concentration also shifts the equilibrium towards reactants, increasing $\Delta G$. Incorrect.
  C. Coupling the reaction to a highly exergonic reaction means combining it with a reaction that releases a large amount of free energy ($\Delta G << 0$). The combined $\Delta G$ will be the sum of the individual $\Delta G$ values. If the exergonic reaction's $\Delta G$ is sufficiently negative, the overall $\Delta G$ can become negative, making the combined process spontaneous. Correct.
  D. Decreasing temperature may affect the equilibrium constant, but doesn't inherently make a positive $\Delta G$ reaction spontaneous.  The effect is dependent on the enthalpy ($\Delta H$) and entropy ($\Delta S$) of the reaction. Incorrect.  In many biological systems, lowering the temperature would actually reduce the rate of reaction and make it even less feasible.
 3. Conclusion:
  Result: Coupling the endergonic reaction to a highly exergonic reaction is the only strategy that guarantees a negative overall $\Delta G$, making the pathway thermodynamically feasible.
  Verification: The principle of coupling reactions is a fundamental aspect of metabolism in living organisms. Many metabolic pathways rely on this strategy to drive otherwise non-spontaneous reactions.
 ## OPTION ANALYSIS
A :x: Increasing product concentration will shift the equilibrium to the left, making $\Delta G$ more positive. 
B :x: Decreasing reactant concentration will also shift the equilibrium to the left, making $\Delta G$ more positive.
C ✓: Coupling with an exergonic reaction (one with a negative $\Delta G$) can result in a net negative $\Delta G$ for the overall process, making it spontaneous.
D :x: Temperature change affects the equilibrium constant, but doesn't guarantee a negative $\Delta G$ for a reaction with a positive standard free energy change.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing $\Delta G$ with $\Delta G\u00b0$:  $\Delta G\u00b0$ is the standard free energy change, while $\Delta G$ is the actual free energy change under specific conditions.  The actual $\Delta G$ can be manipulated, but $\Delta G\u00b0$ is a constant at standard conditions.
  2.Neglecting the importance of coupling reactions: Biological systems frequently use coupling to drive non-spontaneous reactions.
 **Quick Tips**:
  1.Remember that for a reaction to be spontaneous, $\Delta G$ must be negative. 
  2.Coupling is a key strategy in biological systems to overcome thermodynamic barriers.

Mr. NARESH KUMAR K · Answer

Increase the concentration of products.

Mr. NARESH KUMAR K · Answer

Decrease the concentration of reactants.

Mr. NARESH KUMAR K · Answer

The correct Option is "Couple the endergonic reaction to a highly exergonic reaction."

Mr. NARESH KUMAR K · Answer

Decrease the temperature of the reaction.

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