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.