Nitrogen Fixation | Biology for Grade 11 PDF Download

BIOLOGICAL N₂ FIXATION

It is done by symbiotic bacteria & free living bacteria.
Symbiotic N₂ fixation (Diazotrophy) : In leguminous plants (Fabaceae) by symbiotic bacterium Rhizobium, which form nodules in their roots.

• N₂ directly convert into NH₄+ ion, which is used in plant metabolism.

• Root nodules act as site for N₂ fixation. It contains all necessary biochemical components like enzyme Nitrogenase, Leghaemoglobin, required in N₂ fixation.

• Enzyme nitrogenase is a Mo–Fe protein & catalyse the conversion of atm. N₂ to NH₃.

It posses two units unit-Ist is Mo-Fe protein & unit-IInd is Ferredoxin (fe-s protein).

• Nitrogenase is extremely sensitive to oxygen. So to protect it from oxygen, nodules contains an O₂ scavanger called leghaemoglobin (Lhb) and combined with O₂ to form oxyleghaemo-globin(Olhb)

• Leghaemoglobin is pink in colour & similar to haemoglobin of vertebrates. It is synthesised by plant gene (Globin part by plant and heam part given by bacteria).  

(A) NODULE FORMATION : It is due to interactions between bacteria and host root. It occurs in following steps :

(i) Multiplication & colonization of Rhizobia at Rhizosphere and attachment to epidermal root hair cells. Initial attraction of Rhizobia to host root is chemotactic (Rhicadhesin protein of bacterial cell identify host root) as root exude amino acids, sugars, organic acids and flavonoids.

(ii) Characterstic curling of root hairs and invasion of the bacteria to form an infection thread, by the invegination of plasma membrane of root hair cells and it reaches up to the cortex of roots.
Curling of root hairs is stimulated by specific complex polysaccharides found on the surface of rhizobia, recognised by Lectins (small proteins of host plant root).

(iii) Nodule initiation & development in root cortex. Mitogenic agents secreted (Kinetin) by bacteria & auxin produced by plant cell promotes cell division & extension leading to nodule formation.
Nodule establishes direct vascular connection with host for exchange of nutrients. Root nudule cells have chromosome in double  to other somatic cells. Thus nodule cells are polyploid specially Tetraploid.

Development of root nodules in soyabean.

(a) Rhizobium bacteria contact a susceptible root hair, div ide near it, and upon successful infection of the root hair cause it to curl.

(b) Infection thread carrying dividing bacteria, now modified and apparent as bacteriods. Bacteroids cause inner cortical and pericycle cells to divide. Division and growth of cortical and pericycle cells lead to nodule formation.

(c) A mature nodule complete with v ascular tissues continuous with those of the root.

(iv) Release of bacteria from infection thread and they differentiate as specilized nitrogen fixing cell.

• Bacteria continue to multiply during it's path in root hair cells & bacteria distribute in most of cells.

• The membrane of infection thread bud off to form small vesicles which containing one or more bacteria. Then bacteria stop dividing & enlarge & differentiate in group of nitrogen fixing cells and called as bacteroid & it's membrane is called as peribacteroid membrane.

(B) MECHANISM OF BIOLOGICAL N₂ FIXATION :
By Burris. The atm. N₂ is reduce by the addition of hydrogen atoms.

• The three bonds between two nitrogen atoms N ≡ N or dinitrogen are broken & ammonia (NH₃) is formed by reduction of N ≡ N and then reduction of ammonia (NH₃) to form ammonium ions (NH₄+).

N₂ fixation requires 3 components :

(i) A strong reducing agent - NADPH₂/FADH₂/NADH₂ – from photosynthesis & respiration.

(ii) ATP to transfer hydrogen atom to dinitrogen – from respiration & photosynthesis.

(iii) Nitrogenase enzyme.

• Steps of N₂ fixation :

(a) Reduction of N₂ to NH₃ :

H = N + 8e^- + 8H⁺ + 16 ATP →  2NH₃ +  H₂ + 16 ADP + 16ip

(b) Reduction of NH₃ to NH₄*:

2NH₃ + H₂ → 2NH₄* + 2e^-

Overall Reaction:

Biological N₂ fixation is controlled mainly by four genes :

(i) NOD gene of host plant : These encodes no. of nodule specific protein called nodulins, synthesis, requires for active nodule devlopment.

(ii) nod, nif and fix gene of bacteria.

SYNTHESIS OF AMINO ACIDS & NITROGEN ASSIMILATION

Nitrogen assimilation :– Inorganic NH₃ (Produced by nitrate reduction or biological fixation or obtained from soil as NH₄+) reacts with a TCA cycle intermediate – a–ketoglutaric acid to form an amino acid glutamic acid. This process known as Reductive amination or Amino acid Biosynthesis.

Transamination :– Transfer of Amino group from glutamic acid to other keto acid is known as transamination. This is a process of formation of ot her am ino ac ids in plant s. (transaminase enzyme ) Ex.

Glutamic acid is first formed amino  acid in plants & can synthesize different amino acids by transamination.

Mechanism of nitrogen fixation

(a) The 2nd unit (ferredoxin) of nitrogenase, receiv e electrons from e– donar (FADH₂/NADH₂NADPH₂) and become reduced.

(b) This reduced 2nd unit is now activated by ATP and form a complex called ferredoxin ATP complex.

(c) On other side unit 1st (Fe-Mo protein) of nitrogenase, reacts with molecular nitrogen to form nitrogenase-nitrogen complex.

(d) Ferredoxin ATP complex then transfer electron to nitrogenase-nitrogen complex, so that the later gets reduced. This reaction is catalysed by hydrolysis of ATP.

(e) The reduced nitrogenase-nitrogen complex now receives proton (H⁺) resulting in formation of ammonium (NH₄⁺) ion.

Catalytic Amidation :

Transportation of fixed N₂/Assimilated N₂ in plants occurs mainly in form of amides especially in leguminous plants as amides are more stable than amino acids and posses high Nitrogen to Carbon ratio (2N to 4C - in

Asparagine, 2N to 5C in glutamine (as glutamate posses 1N to 5C) Formation of amides from amino acids catalysed by enzymes called as catalytic amidation.

In legumes of temperate origine like pea and clover-Asparagine is translocated in non nodulated plant parts.

In legumes of tropical origin like soyabean and cowpea-ureides are translocated in non nodulated plant parts.

Glutamine synthesis :

Glutamic acid/Glutamate + NH₄ + ATP     Glutairiae + ADP + iP

Asparagine syntnesis:

Glutamic acid/Glutamate  + Aspartate + ATP    Aspartate  + ADP + ip +  Glutamate / a- Ketoglutaric acid