Nitrogen Metabolism | Biology for JAMB PDF Download

NITROGEN METABOLISM

Role of Nitrogen in Plants :– Constituent of proteins, nucleic acids ATP, GTP, Chitin, Vitamins, chlorophyll, alkaloids, cytochromes, hormones. Nitrogen is necessary to plants for heridity, reproduction, growth metabolism and development.

 Sources of Nitrogen to plants

:(1) Atmospheric nitrogen : N ≡ N (Molecular, inert or elemental form) used by Rhizobium(Legumes), BGA, Lichens, Yeast, Pullularia.

• These converts atm. N₂ into metabolically usefull ammonaium ions (NH₄+). This process is called as biological nitrogen fixation.

(2) NO₃^–, NO₂^–, NH₄⁺ in soil : These are major source of nitrogen to plants.

• Nitrate ions (NO₃–) are cheif form of nitrogen used by majority of plants.

Plants grow in acidic soil & found in forest use ammonium ions (NH₄+) as major N₂ source.
Nitrate ions are cheif source of N₂ for plants but they can not be used directly in metabolic pathway in plant cells, as it is highly oxidised form. so NO₃– (Nitrate) first converted into NH₄⁺ (ammonium ions) called nitrate reduction. So NH₄+ ions enters in plant metabolism.

(3) Organic nitrogen in soil : as amino acids, protein body.
Due to death & decay of organisms. This is not a major source of N₂.

(4) Insect bodies : for some plants (insectivorous plants)

(5) Urea as chemical/artificial fertilizers  

Nitrogen (N₂) Cycle :

(1) Biological Nitrogen Fixation / Diazotrophy (N₂ → NH₃) :– Conversion of molecular or elemental nitrogen ( N≡N) into inorganic nitrogenous compounds (NH₄+) through agency of living organisms is called asbiological nitrogen fixation or Diazotrophy

Nitrogen Fixing organisms (Diazotrophs) :–

(A) Free living organisms or non–symbiotic organisms/bacteria :– Azotobacter,  Aerobacter, Beijerinckia, Heliobacterium (Clostridium, Rhodospirillum, Chromatium, Chlorobium, Rhodopseudomonas .

• Cyanobacteria (Blue = green algae) – Nostoc, Anabaena, Caulothrix, Cylindriospermum, Tolypothrix, Aulosira etc.

(B) Symbiotic Nitrogen fixers :– 

• Rhizobium leguminosarum (= Bacillus radicicola) is association with the root nodules of leguminous plants. None of these two partners alone can fix atm. nitrogen.

(i) Root nodules of non–legume (15–genera) Angiosperms :– Casuorina, Myrica, Alnus, Ceanothus, Elaiagnus, Hippophae, Coriaria.
Filamentous, Actinomycetous bacteria Frankia live in root nodules of these non-legume plants.
Both Rhizobium & Frankia live freely in soil but fix nitrogen only when in symbiotic association with host plant.

(ii) Root nodules of gymnosperms :– Cycadaceae, Podocarpus.

(iii) Leaf Nodules (Phyllosphere) :– Psychotria, Dioscoria (Klebsiella), Rubiaceae (Mycobacteria) .

(iv) Lichens :– Cyanobacteria (BGA) partner

(v) Non nodule forming associations : Between Azolla (aquatic fern) & Cyanobacteria Anabaena.

(vi) Paspalum-notatum grass have loose symbiosis (associative symbiosis) with Azotobacter pospali in roots and Azospirillum with wheat, maize, sorghum etc.

(2) Ammonification : Conversion of dead organic nitrogenous compounds into ammonia. Bacillus mycoides, B. ramosus.

(3) Nitrification : Oxidation of ammonia, produced by ammonification into nitrates by nitrifying bacteria is called asnitrification.

(4) Denitrification : Nitrates or nitrites converts back into molecular or atm. nitrogen by denitrifying bacteria is denitrification. Ex. Pseudomonas

(5) Nitrate reduction : 

• Plants take nitrogen from soil, chiefly in nitrate forms which is highly oxidised form.

• Nitrate reductase is Molybdo flavoprotein isolated by Evans and Nason 1953 from Neurospora and Glycin max leaves.