Answer the following questions in about 150 words each : (a) How do plants absorb nitrogen from the environment ? Describe the mechanism of nitrate assimilation and synthesis of amino acids in plants.

Nitrogen is an indispensable macronutrient for plant growth, vital for synthesizing proteins, nucleic acids (DNA and RNA), chlorophyll, and enzymes. Despite the atmosphere being approximately 78% nitrogen gas (N₂), most plants cannot directly utilize this form due to its stable triple bond. Instead, plants primarily absorb nitrogen from the soil in inorganic forms like nitrate (NO₃⁻) and ammonium (NH₄⁺), with nitrate generally being the predominant form in aerobic soils. This absorption and subsequent metabolic conversion into organic compounds are critical processes underpinning plant productivity and the global nitrogen cycle. 1. Nitrogen Absorption from the Environment Plants absorb nitrogen from the soil through their root systems in two main inorganic forms: Nitrate (NO₃⁻): This is the most readily available form of nitrogen in well-aerated soils due to the nitrification process carried out by soil bacteria. Nitrate ions move towards plant roots as they absorb water and are taken up by specific nitrate transporters (e.g., NRT1 and NRT2 families) using a proton gradient. Ammonium (NH₄⁺): Ammonium ions are also absorbed by plant roots via ammonium transporters. While less mobile than nitrate, they are efficiently utilized, particularly in anaerobic or acidic soils where nitrification is limited. Some plants, especially legumes, form a symbiotic relationship with nitrogen-fixing bacteria (like Rhizobia) in their root nodules. These bacteria convert atmospheric N₂ directly into ammonia (NH₃), which is then available to the plant, a process known as biological nitrogen fixation. 2. Mechanism of Nitrate Assimilation Nitrate assimilation is the process where absorbed inorganic nitrate is reduced to ammonium, a form that can be incorporated into organic molecules. This energy-intensive process typically occurs in two main steps: Nitrate Reduction: Nitrate (NO₃⁻) is first reduced to nitrite (NO₂⁻) in the cytosol. This reaction is catalyzed by the enzyme Nitrate Reductase (NR) , using NADH or NADPH as a reductant. Nitrite Reduction: The highly toxic nitrite (NO₂⁻) is then transported into the chloroplasts (in shoots) or plastids (in roots) and reduced to ammonium (NH₄⁺). This step is catalyzed by the enzyme Nitrite Reductase (NiR) , utilizing ferredoxin (in chloroplasts) or NADH (in plastids) as an electron donor. 3. Synthesis of Amino Acids Once ammonium (NH₄⁺) is generated from nitrate assimilation or directly absorbed, it is rapidly incorporated into amino acids to avoid cellular toxicity. The primary pathway for this is the **Glutamine Synthetase-Glutamate Synthase (GS-GOGAT) Pathway**: Glutamine Synthetase (GS): This enzyme catalyzes the first step, combining ammonium (NH₄⁺) with glutamate to form glutamine. This reaction requires ATP. Glutamate Synthase (GOGAT): Glutamine then reacts with α-ketoglutarate, catalyzed by GOGAT, to produce two molecules of glutamate. One glutamate molecule is reused by GS, and the other serves as a precursor for the synthesis of other amino acids. This glutamate, along with glutamine, acts as a central hub for the synthesis of all other 20 amino acids required for protein synthesis and other metabolic processes in the plant. In essence, plants employ sophisticated mechanisms to acquire and metabolize nitrogen, a critical element for all life forms. Starting with the absorption of nitrate and ammonium from the soil, coupled with biological nitrogen fixation in some species, plants undertake a two-step nitrate assimilation process involving nitrate and nitrite reductases. The resultant ammonium is then swiftly converted into essential amino acids, primarily through the highly efficient GS-GOGAT pathway, ensuring the continuous synthesis of proteins and other nitrogenous compounds vital for growth, development, and overall plant function. Understanding these pathways is crucial for optimizing agricultural practices and improving nitrogen use efficiency in crops.

Why can't plants directly use atmospheric nitrogen (N₂)?

Plants lack the enzyme nitrogenase, which is required to break the strong triple bond in atmospheric nitrogen (N₂) and convert it into a usable form like ammonia. Only certain microorganisms possess this enzyme.

What are the environmental implications of inefficient nitrogen assimilation?

Inefficient nitrogen assimilation and overuse of nitrogen fertilizers can lead to environmental problems such as nitrate leaching into groundwater (causing water pollution and eutrophication), and the emission of nitrous oxide (N₂O), a potent greenhouse gas, into the atmosphere.

Nitrogen Absorption and Assimilation in Plants | UPSC Mains AGRICULTURE-PAPER-II 2025

Model Answer

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Introduction

Nitrogen is an indispensable macronutrient for plant growth, vital for synthesizing proteins, nucleic acids (DNA and RNA), chlorophyll, and enzymes. Despite the atmosphere being approximately 78% nitrogen gas (N₂), most plants cannot directly utilize this form due to its stable triple bond. Instead, plants primarily absorb nitrogen from the soil in inorganic forms like nitrate (NO₃⁻) and ammonium (NH₄⁺), with nitrate generally being the predominant form in aerobic soils. This absorption and subsequent metabolic conversion into organic compounds are critical processes underpinning plant productivity and the global nitrogen cycle.

1. Nitrogen Absorption from the Environment

Plants absorb nitrogen from the soil through their root systems in two main inorganic forms:

Nitrate (NO₃⁻): This is the most readily available form of nitrogen in well-aerated soils due to the nitrification process carried out by soil bacteria. Nitrate ions move towards plant roots as they absorb water and are taken up by specific nitrate transporters (e.g., NRT1 and NRT2 families) using a proton gradient.
Ammonium (NH₄⁺): Ammonium ions are also absorbed by plant roots via ammonium transporters. While less mobile than nitrate, they are efficiently utilized, particularly in anaerobic or acidic soils where nitrification is limited.

Some plants, especially legumes, form a symbiotic relationship with nitrogen-fixing bacteria (like Rhizobia) in their root nodules. These bacteria convert atmospheric N₂ directly into ammonia (NH₃), which is then available to the plant, a process known as biological nitrogen fixation.

2. Mechanism of Nitrate Assimilation

Nitrate assimilation is the process where absorbed inorganic nitrate is reduced to ammonium, a form that can be incorporated into organic molecules. This energy-intensive process typically occurs in two main steps:

Nitrate Reduction: Nitrate (NO₃⁻) is first reduced to nitrite (NO₂⁻) in the cytosol. This reaction is catalyzed by the enzyme Nitrate Reductase (NR), using NADH or NADPH as a reductant.
Nitrite Reduction: The highly toxic nitrite (NO₂⁻) is then transported into the chloroplasts (in shoots) or plastids (in roots) and reduced to ammonium (NH₄⁺). This step is catalyzed by the enzyme Nitrite Reductase (NiR), utilizing ferredoxin (in chloroplasts) or NADH (in plastids) as an electron donor.

3. Synthesis of Amino Acids

Once ammonium (NH₄⁺) is generated from nitrate assimilation or directly absorbed, it is rapidly incorporated into amino acids to avoid cellular toxicity. The primary pathway for this is the **Glutamine Synthetase-Glutamate Synthase (GS-GOGAT) Pathway**:

Glutamine Synthetase (GS): This enzyme catalyzes the first step, combining ammonium (NH₄⁺) with glutamate to form glutamine. This reaction requires ATP.
Glutamate Synthase (GOGAT): Glutamine then reacts with α-ketoglutarate, catalyzed by GOGAT, to produce two molecules of glutamate. One glutamate molecule is reused by GS, and the other serves as a precursor for the synthesis of other amino acids.

This glutamate, along with glutamine, acts as a central hub for the synthesis of all other 20 amino acids required for protein synthesis and other metabolic processes in the plant.

Conclusion

In essence, plants employ sophisticated mechanisms to acquire and metabolize nitrogen, a critical element for all life forms. Starting with the absorption of nitrate and ammonium from the soil, coupled with biological nitrogen fixation in some species, plants undertake a two-step nitrate assimilation process involving nitrate and nitrite reductases. The resultant ammonium is then swiftly converted into essential amino acids, primarily through the highly efficient GS-GOGAT pathway, ensuring the continuous synthesis of proteins and other nitrogenous compounds vital for growth, development, and overall plant function. Understanding these pathways is crucial for optimizing agricultural practices and improving nitrogen use efficiency in crops.

Answer Length

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Answer the following questions in about 150 words each : (a) How do plants absorb nitrogen from the environment ? Describe the mechanism of nitrate assimilation and synthesis of amino acids in plants.

Model Answer

Introduction

1. Nitrogen Absorption from the Environment

2. Mechanism of Nitrate Assimilation

3. Synthesis of Amino Acids

Conclusion

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Legumes and Rhizobium Symbiosis

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