Nitrogen Fixation in Plants: Biological Importance
1. What is nitrogen fixation, and why is it biologically important for plants?
Answer:
Nitrogen fixation is the process of converting atmospheric nitrogen (N₂) into ammonia (NH₃), which plants can assimilate.
- Importance:
- Nitrogen is a vital nutrient for plants, required for the synthesis of amino acids, proteins, nucleic acids, and chlorophyll.
- Atmospheric nitrogen is unavailable directly to most plants. Biological nitrogen fixation provides an accessible form of nitrogen, supporting plant growth and productivity.
2. Describe the role of nitrogenase in nitrogen fixation.
Answer:
Nitrogenase is the enzyme complex responsible for catalyzing the reduction of atmospheric nitrogen to ammonia.
- Structure: It comprises two components: Fe-protein and MoFe-protein.
- Mechanism:
- Nitrogenase binds to atmospheric nitrogen.
- ATP hydrolysis provides the energy needed.
- Electrons reduce N₂ to NH₃ in a stepwise manner.
- Sensitivity: Nitrogenase is oxygen-sensitive and functions optimally in anaerobic conditions.
3. Explain the symbiotic nitrogen fixation process in leguminous plants.
Answer:
Symbiotic nitrogen fixation occurs in a mutualistic relationship between legumes and Rhizobium bacteria.
- Steps:
- Recognition: Rhizobium recognizes and attaches to root hairs.
- Infection Thread Formation: The bacteria invade root hair cells and form an infection thread.
- Nodule Formation: Bacteria stimulate cortical cell division to form root nodules.
- Nitrogen Fixation: Rhizobium inside nodules converts atmospheric nitrogen into ammonia, which plants use.
- Mutual Benefit:
- Plants supply carbohydrates to Rhizobium.
- Rhizobium provides fixed nitrogen to plants.
4. What are free-living nitrogen-fixing organisms, and how do they contribute to nitrogen fixation?
Answer:
Free-living nitrogen-fixing organisms are non-symbiotic microbes capable of fixing atmospheric nitrogen independently. Examples include Azotobacter and Clostridium.
- Contribution:
- They enrich soil nitrogen content, particularly in natural ecosystems.
- Their activity supports plant growth, especially in nitrogen-deficient soils.
5. Discuss the significance of leghemoglobin in nitrogen fixation.
Answer:
Leghemoglobin is a red pigment found in root nodules of legumes.
- Functions:
- Binds oxygen, maintaining low oxygen levels in nodules.
- Protects the nitrogenase enzyme from inactivation by oxygen.
- Ensures an optimal environment for effective nitrogen fixation.
6. How do environmental factors influence biological nitrogen fixation?
Answer:
Biological nitrogen fixation is influenced by:
- Oxygen Levels: High oxygen inhibits nitrogenase activity.
- Temperature: Optimal temperature is essential for enzyme function.
- Soil pH: Neutral to slightly acidic pH enhances microbial activity.
- Availability of Nutrients: Adequate phosphorus and iron support bacterial growth.
- Moisture: Sufficient soil moisture facilitates bacterial survival and nodule formation.
7. Differentiate between symbiotic and non-symbiotic nitrogen fixation.
Answer:
| Aspect | Symbiotic Nitrogen Fixation | Non-Symbiotic Nitrogen Fixation |
|---|---|---|
| Organisms Involved | Rhizobium, Frankia | Azotobacter, Clostridium |
| Relationship | Mutualistic | Independent |
| Host | Specific plants (e.g., legumes) | None |
| Efficiency | High | Relatively low |
8. Explain the role of cyanobacteria in nitrogen fixation.
Answer:
Cyanobacteria, also known as blue-green algae, are photosynthetic organisms capable of nitrogen fixation.
- Examples: Anabaena, Nostoc.
- Role:
- Fix nitrogen in rice paddies and other aquatic ecosystems.
- Form symbiotic relationships with plants like Azolla.
- Enhance soil fertility by increasing available nitrogen.
9. What are the steps involved in biological nitrogen fixation?
Answer:
- Nitrogenase Activation: Enzyme complex is activated in suitable conditions.
- ATP Hydrolysis: Provides energy for nitrogen fixation.
- Reduction of N₂: Nitrogen is reduced to ammonia (NH₃) through a multi-step reaction.
- Ammonia Utilization: NH₃ is incorporated into amino acids and other organic compounds.
10. Discuss the importance of nitrogen fixation in sustainable agriculture.
Answer:
- Reduces dependency on synthetic nitrogen fertilizers.
- Enhances soil fertility and supports crop rotation systems.
- Promotes eco-friendly farming practices.
- Reduces environmental pollution caused by excessive fertilizer use.
11. What is denitrification, and how does it differ from nitrogen fixation?
Answer:
Denitrification is the process by which nitrates (NO₃⁻) are converted back into nitrogen gas (N₂) by bacteria like Pseudomonas.
- Difference:
- Nitrogen fixation adds nitrogen to soil, while denitrification removes it.
- Fixation is essential for plant growth; denitrification reduces soil fertility.
12. Explain the role of Nif genes in nitrogen fixation.
Answer:
Nif genes encode the proteins necessary for nitrogen fixation.
- Functions:
- Code for nitrogenase enzyme components.
- Regulate nitrogen fixation processes.
- Coordinate ATP synthesis and electron transfer for nitrogen reduction.
13. How does the Haber-Bosch process compare to biological nitrogen fixation?
Answer:
- Haber-Bosch Process: Industrial method for synthesizing ammonia using high pressure and temperature.
- Biological Nitrogen Fixation: Natural process mediated by microbes under ambient conditions.
- Comparison:
- Biological fixation is eco-friendly and sustainable.
- Haber-Bosch requires significant energy input, contributing to carbon emissions.
14. What is nitrogenase activity, and how is it measured?
Answer:
Nitrogenase activity refers to the enzyme’s ability to fix nitrogen.
- Measured using:
- Acetylene Reduction Assay: Acetylene gas is reduced to ethylene, indicating nitrogenase function.
- Isotopic Tracing: Incorporating labeled nitrogen isotopes into compounds.
15. Discuss the significance of Frankia in nitrogen fixation.
Answer:
Frankia is a nitrogen-fixing actinobacterium forming symbiosis with non-leguminous plants like Alnus.
- Importance:
- Fixes nitrogen in temperate and arid regions.
- Enhances growth of non-leguminous crops.
16. What are nitrogen-fixing biofertilizers? Provide examples.
Answer:
Nitrogen-fixing biofertilizers are microbial products enhancing soil fertility.
- Examples:
- Rhizobium inoculants for legumes.
- Azotobacter for cereals.
- Cyanobacteria for rice paddies.
17. Explain the process of nodule formation in leguminous plants.
Answer:
- Rhizobia attach to root hairs.
- Root hairs curl and form an infection thread.
- Bacteria invade cortical cells, inducing nodule formation.
- Nodules develop vascular tissues for nutrient exchange.
18. How do plants assimilate ammonia formed during nitrogen fixation?
Answer:
Plants convert ammonia into:
- Glutamine and glutamate via the GS-GOGAT pathway.
- Amino acids and proteins for metabolic functions.
19. What is the role of soil pH in nitrogen fixation?
Answer:
- Neutral to slightly acidic pH favors bacterial activity.
- Extreme pH levels inhibit microbial survival and enzyme function.
20. Why is nitrogen fixation considered energy-intensive?
Answer:
- Nitrogenase activity requires 16 ATP molecules to fix one molecule of nitrogen.
- High energy demand is necessary to overcome the triple bond of atmospheric nitrogen.
