Question:
In nitrogen fixation, to reduce nitrogen to ammonia, which one of the following shows the correct order of electron flow?
In nitrogen fixation, to reduce nitrogen to ammonia, which one of the following shows the correct order of electron flow?
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Nitrogenase is a two component system; electrons travel from ferredoxin to the reductase (Fe protein) and only then to nitrogenase (MoFe protein), never in reverse.
Updated On: Jul 16, 2026
- Ferredoxin -> reductase -> nitrogenase
- NADH -> reductase -> nitrogenase
- Ferredoxin -> nitrogenase -> reductase
- Reductase -> NADH -> nitrogenase
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The Correct Option is A
Solution and Explanation
Step 1: Understanding the Question:
Biological nitrogen fixation reduces atmospheric \(N_2\) gas to ammonia (\(NH_3\)) using the nitrogenase enzyme complex, and the question asks for the correct order in which electrons travel before they reach \(N_2\).
Step 2: Key Formula or Approach:
The nitrogenase system is made of two separate proteins that work together: dinitrogenase reductase (the reductase, or Fe protein) and dinitrogenase (the nitrogenase, or MoFe protein). Electrons do not jump straight from a donor to \(N_2\); they pass through a defined relay.
Step 3: Detailed Explanation:
The overall reduction is
\[ N_2 + 8H^+ + 8e^- + 16\,ATP \rightarrow 2NH_3 + H_2 + 16\,ADP + 16\,P_i \]
Electrons first come from ferredoxin, a small iron sulfur protein that is kept reduced by cellular reactions, such as pyruvate oxidation in bacteria.
Ferredoxin passes these electrons to the reductase component (Fe protein, dinitrogenase reductase), which uses ATP hydrolysis to change shape and deliver the electrons one at a time.
The reductase then transfers each electron to nitrogenase (the MoFe protein, dinitrogenase), which holds the FeMo-cofactor where \(N_2\) is bound and progressively reduced to \(NH_3\).
So the true relay is ferredoxin, then reductase, then nitrogenase, matching option (A).
Option (B) is wrong because NADH is not the direct electron donor to the reductase component in this classical pathway; ferredoxin (or in some organisms flavodoxin) plays that role.
Option (C) reverses the middle two steps, sending electrons to nitrogenase before the reductase, which is physically impossible since the reductase is the only component that can accept electrons directly from ferredoxin and hand them onward.
Option (D) puts reductase before NADH and garbles the order entirely, with no basis in the actual electron transport chain of nitrogenase.
Step 4: Final Answer:
Electrons flow from ferredoxin to the reductase (Fe protein) and then to nitrogenase (MoFe protein), which reduces \(N_2\) to \(NH_3\). \[ \boxed{\text{(A) Ferredoxin} \rightarrow \text{reductase} \rightarrow \text{nitrogenase}} \]
Biological nitrogen fixation reduces atmospheric \(N_2\) gas to ammonia (\(NH_3\)) using the nitrogenase enzyme complex, and the question asks for the correct order in which electrons travel before they reach \(N_2\).
Step 2: Key Formula or Approach:
The nitrogenase system is made of two separate proteins that work together: dinitrogenase reductase (the reductase, or Fe protein) and dinitrogenase (the nitrogenase, or MoFe protein). Electrons do not jump straight from a donor to \(N_2\); they pass through a defined relay.
Step 3: Detailed Explanation:
The overall reduction is
\[ N_2 + 8H^+ + 8e^- + 16\,ATP \rightarrow 2NH_3 + H_2 + 16\,ADP + 16\,P_i \]
Electrons first come from ferredoxin, a small iron sulfur protein that is kept reduced by cellular reactions, such as pyruvate oxidation in bacteria.
Ferredoxin passes these electrons to the reductase component (Fe protein, dinitrogenase reductase), which uses ATP hydrolysis to change shape and deliver the electrons one at a time.
The reductase then transfers each electron to nitrogenase (the MoFe protein, dinitrogenase), which holds the FeMo-cofactor where \(N_2\) is bound and progressively reduced to \(NH_3\).
So the true relay is ferredoxin, then reductase, then nitrogenase, matching option (A).
Option (B) is wrong because NADH is not the direct electron donor to the reductase component in this classical pathway; ferredoxin (or in some organisms flavodoxin) plays that role.
Option (C) reverses the middle two steps, sending electrons to nitrogenase before the reductase, which is physically impossible since the reductase is the only component that can accept electrons directly from ferredoxin and hand them onward.
Option (D) puts reductase before NADH and garbles the order entirely, with no basis in the actual electron transport chain of nitrogenase.
Step 4: Final Answer:
Electrons flow from ferredoxin to the reductase (Fe protein) and then to nitrogenase (MoFe protein), which reduces \(N_2\) to \(NH_3\). \[ \boxed{\text{(A) Ferredoxin} \rightarrow \text{reductase} \rightarrow \text{nitrogenase}} \]
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