The Question is from neet-botany mineral-nutrition n2-metab0lism, and it was created by the author B VENKAT

Question

A researcher isolates a mutant strain of *Rhizobium* that is unable to synthesize leghemoglobin. Which of the following is the MOST LIKELY consequence of this mutation on nitrogen fixation?

B VENKAT · Accepted Answer

## CORE INFORMATION
✓ **Answer**: B
## CONCEPT FRAMEWORK
 **Primary Concept**: Nitrogen fixation in root nodules and the role of leghemoglobin.
 **Related Topics**: Symbiotic nitrogen fixation, oxygen sensitivity of nitrogenase,  *Rhizobium*-legume interactions.
 **Prerequisites**: Understanding of the nitrogenase enzyme, its oxygen sensitivity, and the function of leghemoglobin in protecting nitrogenase.
## SOLUTION PATHWAY
**Given Information**:
  A mutant *Rhizobium* strain cannot produce leghemoglobin.
  Nitrogen fixation is the process of converting atmospheric nitrogen ($N_2$) into ammonia ($NH_3$).
**Method & Steps**:
 1. Understand the role of leghemoglobin.
  Reasoning: Leghemoglobin is an oxygen-binding protein found in the root nodules of leguminous plants. It is crucial for nitrogen fixation because the nitrogenase enzyme, responsible for nitrogen fixation, is extremely sensitive to oxygen. Leghemoglobin maintains a low oxygen concentration in the nodule, protecting nitrogenase from oxygen damage.
  Formula/Rule: No specific formula is needed here; it's a qualitative understanding.
 2. Consider the consequence of leghemoglobin absence.
  Working: If *Rhizobium* cannot synthesize leghemoglobin, the oxygen concentration in the root nodule will increase. This increased oxygen will inhibit nitrogenase activity, leading to reduced nitrogen fixation.
  Key Point: The nitrogenase enzyme is irreversibly inactivated by oxygen.
 3. Determine the most likely consequence.
  Result: Reduced nitrogenase activity due to oxygen toxicity.
  Verification: The absence of leghemoglobin directly impacts the oxygen environment within the nodule, affecting nitrogenase, the key enzyme for nitrogen fixation.
 ## OPTION ANALYSIS
A :x: Enhanced nitrogenase activity due to increased oxygen availability is incorrect because nitrogenase is inhibited by oxygen.
B ✓: Reduced nitrogenase activity due to oxygen toxicity is correct because increased oxygen levels will inhibit nitrogenase.
C :x: No change in nitrogenase activity as leghemoglobin is not directly involved is incorrect because leghemoglobin plays a crucial role in protecting nitrogenase from oxygen.
D :x: Increased nitrogen fixation due to higher free oxygen levels is incorrect because higher oxygen levels inhibit nitrogenase activity.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing the role of leghemoglobin:  Students might incorrectly associate leghemoglobin with directly enhancing nitrogenase activity rather than protecting it from oxygen.
  How to avoid: Understand the oxygen sensitivity of nitrogenase and the protective role of leghemoglobin.
  2.Ignoring the oxygen sensitivity of nitrogenase: Students may overlook the critical role of oxygen in inhibiting nitrogenase activity.
  How to avoid: Emphasize the oxygen sensitivity of the nitrogenase enzyme.
 **Quick Tips**:
  1.Remember that leghemoglobin acts as an oxygen buffer in root nodules.
  2.Nitrogenase + Oxygen = Inactivation

B VENKAT · Answer

Enhanced nitrogenase activity due to increased oxygen availability

B VENKAT · Answer

The correct Option is "Reduced nitrogenase activity due to oxygen toxicity"

B VENKAT · Answer

No change in nitrogenase activity as leghemoglobin is not directly involved

B VENKAT · Answer

Increased nitrogen fixation due to higher free oxygen levels

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