In the nitrogen cycle, nitrogen fixation refers specifically to which of the following processes?

Introduction / Context:
This question belongs to environmental and biological chemistry, focusing on the nitrogen cycle. Nitrogen fixation is a key step that makes atmospheric nitrogen available to living organisms. Although nitrogen gas is abundant in the atmosphere, most organisms cannot use it directly. Understanding the precise meaning of nitrogen fixation helps you distinguish it from other processes in the nitrogen cycle, such as nitrification, assimilation and ammonification.

Given Data / Assumptions:

• The process in question is nitrogen fixation.
• Options include assimilation of nitrate, utilisation of nitrogen gas, conversion of organic nitrogen to proteins, conversion of N2 to NH3 and conversion of ammonia to nitrate.
• We assume standard textbook definitions for steps in the nitrogen cycle.
• Biological nitrogen fixation is typically carried out by certain bacteria and cyanobacteria.

Concept / Approach:
Nitrogen fixation is defined as the conversion of atmospheric molecular nitrogen, N2, into ammonia (NH3) or related forms that can be assimilated by plants. This process can occur biologically via nitrogen fixing bacteria or industrially in processes like the Haber–Bosch synthesis of ammonia. Once ammonia is formed, nitrifying bacteria can convert it into nitrite and nitrate, and plants can assimilate these ions to build amino acids and proteins. Assimilation of nitrate, conversion of ammonia to nitrate and conversion of organic nitrogen to proteins are different steps, not nitrogen fixation itself.

Step-by-Step Solution:
Step 1: Recall that atmospheric nitrogen exists mostly as N2, a very stable molecule with a triple bond between nitrogen atoms. Step 2: Most plants and animals cannot directly use N2 because breaking the triple bond requires a large amount of energy. Step 3: Nitrogen fixation refers to the process of converting N2 into ammonia (NH3) or ammonium ions (NH4+), which can then enter biological pathways. Step 4: Biological nitrogen fixation is performed by specialised microorganisms, often in association with leguminous plants, using the enzyme nitrogenase. Step 5: Industrial nitrogen fixation in the Haber process converts N2 and H2 into NH3 under high temperature and pressure with a catalyst. Step 6: Other steps such as assimilation (uptake of nitrate by plants), nitrification (conversion of ammonia to nitrate) and protein synthesis (conversion of organic nitrogen to proteins) are distinct from nitrogen fixation. Step 7: Therefore, nitrogen fixation is best described as the conversion of molecular nitrogen (N2) into ammonia.

Verification / Alternative check:
Diagrams of the nitrogen cycle in textbooks show a specific arrow labelled nitrogen fixation from atmospheric N2 to ammonia or related compounds in soil. Further arrows represent nitrification (NH3 to NO2− and NO3−), assimilation (uptake of nitrate by plants and conversion into organic nitrogen), and denitrification (return of nitrogen to the atmosphere). Only the first step, capturing N2 from the atmosphere and reducing it to ammonia, is called nitrogen fixation. The fact that nitrogenase enzymes and the Haber process are described as nitrogen fixing also supports this definition.

Why Other Options Are Wrong:
Assimilation of nitrate by plants is a step in which plants absorb nitrate from soil and reduce it to ammonium to incorporate into amino acids; it is not nitrogen fixation. Direct utilisation of nitrogen gas by animals does not occur, as animals cannot use N2 directly, so this option is incorrect by definition. Conversion of organic nitrogen to proteins occurs within organisms and involves biosynthesis, not fixation of atmospheric nitrogen. Conversion of ammonia into nitrate is nitrification, a separate process carried out by nitrifying bacteria, not nitrogen fixation. Therefore these options do not match the definition of nitrogen fixation.

Common Pitfalls:
Students sometimes loosely use the term nitrogen fixation to refer to any step that moves nitrogen through the cycle, leading to confusion between fixation, nitrification, assimilation and ammonification. Another mistake is to think that nitrogen fixation is plant uptake of nitrate, ignoring the crucial role of bacteria in converting N2 to ammonia. To avoid these errors, remember that fixation always involves capturing atmospheric N2 and reducing it to ammonia, while other terms describe transformations of nitrogen once it is already in biologically available forms.

Final Answer:
Nitrogen fixation is the conversion of molecular nitrogen (N2) from the atmosphere into ammonia (NH3).