Biological Nitrogen Fixation — Revision Notes

⚡ 30-Second Revision

BNF: — $N_2 \rightarrow NH_3$ by diazotrophs.
Diazotrophs: — Prokaryotes (bacteria, archaea) with nitrogenase.
Nitrogenase: — Fe-protein + MoFe-protein. Highly oxygen-sensitive. Requires 16 ATP per $N_2$.
Oxygen Protection:

- Rhizobium (symbiotic): Leghemoglobin (plant-derived) in nodules. - Azotobacter (free-living aerobic): High respiration rate. - Clostridium (free-living anaerobic): Anaerobic environment. - Anabaena (cyanobacteria): Heterocysts.

Symbiotic Fixers: — Rhizobium (legumes), Frankia (actinorhizal plants), Anabaena azollae (Azolla).
Nodule Formation: — Flavonoids (plant) $\rightarrow$ Nod factors (Rhizobium) $\rightarrow$ Root hair curling $\rightarrow$ Infection thread $\rightarrow$ Bacteroids.
Product: — Ammonia ($NH_3$), assimilated as $NH_4^+$.

2-Minute Revision

Biological Nitrogen Fixation (BNF) is the conversion of atmospheric nitrogen gas ($N_2$) into ammonia ($NH_3$) by specialized prokaryotes called diazotrophs. This process is crucial because $N_2$ is inert, but nitrogen is vital for biomolecules like proteins and DNA.

The key enzyme, nitrogenase, is extremely sensitive to oxygen and requires a substantial 16 ATP molecules per $N_2$ fixed. To protect nitrogenase, organisms have evolved various strategies: symbiotic bacteria like Rhizobium in legume root nodules utilize leghemoglobin (produced by the plant) to maintain a microaerobic environment.

Free-living aerobic bacteria such as Azotobacter achieve this through high respiration rates, while anaerobic bacteria like Clostridium thrive in oxygen-free conditions. Cyanobacteria like Anabaena form specialized cells called heterocysts for nitrogen fixation.

The ammonia produced is then assimilated by plants, forming the base of the nitrogen cycle and supporting agricultural productivity.

5-Minute Revision

Biological Nitrogen Fixation (BNF) is the biological process of converting inert atmospheric dinitrogen ($N_2$) into biologically usable ammonia ($NH_3$). This transformation is exclusively carried out by prokaryotic microorganisms known as diazotrophs, which possess the unique enzyme complex called nitrogenase.

Nitrogenase is composed of two metalloproteins: the Fe-protein (dinitrogenase reductase) and the MoFe-protein (dinitrogenase). It is highly susceptible to inactivation by oxygen, necessitating specific protective mechanisms.

The overall reaction is energetically demanding, requiring 16 ATP molecules, 8 electrons, and 8 protons for each $N_2$ molecule reduced to $2NH_3$ and $1H_2$.

BNF can be broadly categorized into two types:

1
Non-symbiotic (Free-living): — These diazotrophs live independently. Examples include aerobic bacteria (Azotobacter, Beijerinckia) which protect nitrogenase via high respiration rates; anaerobic bacteria (Clostridium) which live in oxygen-free environments; and photosynthetic cyanobacteria (Anabaena, Nostoc) which fix nitrogen in specialized, oxygen-deprived cells called heterocysts.
2
Symbiotic: — These involve a mutualistic association with a host plant. The most prominent example is the Rhizobium-legume symbiosis. Rhizobium bacteria infect legume roots, leading to the formation of root nodules. Within these nodules, the plant produces leghemoglobin, an oxygen-binding protein that maintains a microaerobic environment, protecting nitrogenase while allowing bacterial respiration for ATP generation. Other symbiotic associations include Frankia with actinorhizal plants and Anabaena azollae with the Azolla fern.

Nodule formation in legumes is a complex process initiated by chemical signaling: plant roots release flavonoids, which induce Rhizobium to produce Nod factors. These Nod factors trigger root hair curling, infection thread formation, and ultimately, the differentiation of bacteria into nitrogen-fixing bacteroids within the nodule.

The fixed ammonia is then rapidly assimilated by the plant, primarily via the GS-GOGAT pathway, into amino acids. BNF is vital for sustainable agriculture, reducing the need for synthetic nitrogen fertilizers.

Prelims Revision Notes

Biological Nitrogen Fixation (BNF) - NEET Revision Notes

1. Definition: Conversion of atmospheric $N_2$ into ammonia ($NH_3$) by prokaryotes (diazotrophs).

2. Significance: $N_2$ is inert; nitrogen is essential for proteins, nucleic acids, chlorophyll. BNF makes nitrogen available to the biosphere.

3. Key Enzyme: Nitrogenase Complex

* Components: Two metalloproteins: Fe-protein (dinitrogenase reductase) and MoFe-protein (dinitrogenase). * Oxygen Sensitivity: Extremely sensitive to oxygen; irreversibly inactivated. Requires anaerobic/microaerobic conditions. * Energy Requirement: Highly energy-intensive. Requires 16 ATP, 8 electrons, 8 protons per $N_2$ molecule fixed. * Reaction: $N_2 + 8e^- + 8H^+ + 16ATP \rightarrow 2NH_3 + H_2 + 16ADP + 16P_i$

4. Types of Nitrogen Fixation & Organisms

* Non-symbiotic (Free-living): * Aerobic: Azotobacter, Beijerinckia (soil bacteria). Protect nitrogenase by high respiration rates. * Anaerobic: Clostridium (soil bacteria). Live in oxygen-free environments.

* Photosynthetic: Cyanobacteria (Anabaena, Nostoc, Oscillatoria). Fix nitrogen in heterocysts (specialized, oxygen-deprived cells). * Symbiotic: * Legume-Rhizobium Symbiosis: Most common.

Rhizobium species (e.g., Rhizobium, Bradyrhizobium) infect roots of legumes (peas, beans, clover). * Root Nodules: Specialized structures on roots where fixation occurs. * Leghemoglobin: Plant-produced oxygen-binding protein in nodules.

Maintains microaerobic conditions (low $O_2$) for nitrogenase protection and bacterial respiration. * Non-legume Symbiosis: * Frankia (actinomycete) with actinorhizal plants (Alnus, Casuarina).

* Anabaena azollae with Azolla fern.

5. Nodule Formation (Legume-Rhizobium)

* Step 1: Plant roots release flavonoids (chemical attractants). * Step 2: Rhizobium produces Nod factors (lipo-chitooligosaccharides) in response. * Step 3: Nod factors induce root hair curling. * Step 4: Bacteria enter through infection thread. * Step 5: Bacteria released into cortical cells, differentiate into bacteroids (nitrogen-fixing form). * Step 6: Cortical cell proliferation forms the nodule.

6. Ammonia Assimilation: $NH_3$ is protonated to $NH_4^+$ and assimilated by plants, primarily via Glutamine Synthetase-Glutamate Synthase (GS-GOGAT) pathway, forming amino acids.

Vyyuha Quick Recall

Nice Fixers Love Anaerobic Roots, Always Creating Ammonia.

Nice Fixers: Nitrogen Fixation
Love Anaerobic: Leghemoglobin creates Anaerobic conditions
Roots: Rhizobium in root nodules
Always Creating Ammonia: Azotobacter, Clostridium, Anabaena (examples of fixers) all produce Ammonia.