The Question is from neet-botany mineral-nutrition , and it was created by the author D VINOD

Question

The free energy change ($\Delta G$) for nitrogen fixation is highly positive.  Which of the following BEST explains how biological nitrogen fixation occurs despite this energetic barrier?

D VINOD · Accepted Answer

## CORE INFORMATION
✓ **Answer**: C
## CONCEPT FRAMEWORK
 **Primary Concept**: Thermodynamics and biological processes
 **Related Topics**: Gibbs free energy, ATP hydrolysis, coupled reactions, enzyme catalysis
 **Prerequisites**: Understanding of Gibbs free energy ($\Delta G$), its relationship to spontaneity, and the concept of coupled reactions.
## SOLUTION PATHWAY
**Given Information**:
  $\Delta G$ for nitrogen fixation is highly positive (non-spontaneous).
  Biological nitrogen fixation occurs.
**Method & Steps**:
 1. Analyze the options to determine which one correctly explains how a non-spontaneous reaction can proceed biologically.
  Reasoning: A highly positive $\Delta G$ indicates the reaction is endergonic (requires energy input) and will not proceed spontaneously under standard conditions.  Biological systems often overcome this by coupling the reaction to an exergonic reaction.
 2. Evaluate each option:
  Option A: High temperatures can affect reaction rates, but they do not change the $\Delta G$.
  Option B: Catalysts lower the activation energy ($E_a$), making the reaction faster, but they do not change the $\Delta G$.
  Option C: Coupling nitrogen fixation to the hydrolysis of ATP provides the necessary energy input to drive the reaction forward, even though the $\Delta G$ of nitrogen fixation remains positive. ATP hydrolysis releases a large amount of free energy ($\Delta G < 0$), which can be used to drive the coupled reaction. 
  Option D: Leghemoglobin is involved in oxygen regulation in nitrogen fixation, but it does not alter the thermodynamics of the reaction itself.
 3. Conclusion:
  Result: Option C correctly explains how biological nitrogen fixation occurs despite a highly positive $\Delta G$. The reaction is coupled with the exergonic hydrolysis of ATP, providing the energy needed to make the overall process proceed.
  Verification: The concept of coupled reactions is well-established in biochemistry.  ATP hydrolysis is a highly exergonic reaction frequently used to drive endergonic processes.
 ## OPTION ANALYSIS
A :x:  Temperature affects reaction kinetics (rate), not thermodynamics ($\Delta G$).
B :x: Catalysts lower the activation energy, not the Gibbs free energy change.
C ✓: Correct. Coupling the endergonic nitrogen fixation reaction with the exergonic hydrolysis of ATP provides the required energy.
D :x: Leghemoglobin regulates oxygen levels, but doesn't change the $\Delta G$ of nitrogen fixation.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing activation energy ($E_a$) with Gibbs free energy ($\Delta G$): $E_a$ relates to reaction rate, while $\Delta G$ determines spontaneity.
  How to avoid: Understand the definitions and roles of $E_a$ and $\Delta G$ in chemical reactions.
  2.Assuming that a positive $\Delta G$ always means a reaction cannot occur: Biological systems use coupled reactions to overcome positive $\Delta G$ values.
  How to avoid:  Remember that coupled reactions allow non-spontaneous reactions to occur by harnessing the energy from a spontaneous reaction.
 **Quick Tips**:
  1.If a reaction has a positive $\Delta G$, it requires energy input to proceed.
  2.ATP hydrolysis is a key source of energy for many endergonic reactions in living systems.

D VINOD · Answer

The reaction proceeds spontaneously at high temperatures found in root nodules

D VINOD · Answer

Nitrogenase acts as a catalyst, lowering the activation energy but not the $\Delta G$

D VINOD · Answer

The correct Option is "Hydrolysis of a large number of ATP molecules coupled to the reaction"

D VINOD · Answer

The presence of leghemoglobin alters the thermodynamics, making $\Delta G$ negative

NEET Botany Mineral Nutrition MCQs

Select Topic

Question Tags

Question Tags

NEET Subjects

Our Bots

Company