NEET Biology Notes Mineral Nutrition Nitrogen Cycle
- Nitrogen is a constituent of amino acids, proteins, hormones, chlorophylls and many of the vitamins. Nitrogen exists as two nitrogen atoms joined by a very strong triple covalent bond (N=N).
- The process of conversion of nitrogen (N2) to ammonia NH3 is termed as nitrogen fixation.
- In nature, lightning and ultraviolet radiation provide enough energy to convert nitrogen to nitrogen oxides (NO, NOz andN20).
- Decomposition of organic nitrogen of dead plants and animals ammonification. into ammonia is called Ammonification.
- Ammonia is first oxidised to nitrite, by the action of the bacteria Nitrosom on as and/or Nitrococcus. The nitrite is further oxidised to nitrate, with the help of the bacterium Nitrobacter. These steps are called nitrification and the nitrifying bacteria are chemoautotrophs.
- The nitrate formed is absorbed by the plants and is transported to the leaves. In leaves, it is reduced to form ammonia that finally forms the amine group of amino acids.
- The nitrate present in the soil is also reduced to nitrogen by the process of denitrification, which is carried by bacteria Pseudomonas and Thiobacillus.
Biological Nitrogen Fixation
Reduction of nitrogen to ammonia by living organisms is called biological nitrogen fixation. The enzyme nitrogenase, which is capable of nitrogen reduction is present exclusively in prokaryotes.
Free-Living Nitrogen Fixing Organisms
Azotobacter, Beijerinckia and Derxia are aerobic free living, saprotrophic (heterotrophic) nitrogen fixing bacteria. Azotobacter species (aerobic) are the main nitrogen fixing free-living bacteria. Clostridium (anaerobic) stands next. These bacteria adds about 10-25 mg nitrogen/annum.
The nitrogen fixation is a reduction process and is independent of respiration. The free-living cyanobacteria are considered as major important nitrogen fixers. They can fix ten times as much nitrogen as the other free-living bacteria fix under suitable conditions.
Cyanobacteia are mainly responsible for maintaining the fertility and productivity for rice fields, e.g. Nostoc, Anabaena, Cylindrospermum are active in sugarcane and maize fields.
Symbiotic Nitrogen Fixation
The examples of symbiotic biological nitrogen fixing associations are
- Species of rod-shaped Rhizobium has such relationship with the roots of several legumes such as alfa-alfa, sweet clover, sweet pea, lentils, garden pea, broad bean, clover bean, etc.
- The microbe Frankia also produces nitrogen-fixing nodules on the roots of non-leguminous plants, e.g. Alnus.
- The presence of leguminous haemoglobin or leghaemoglobin makes the nodules pink in colour.
Ehizobia multiply and colonise the surroundings of roots and get attached to epidermal and root hair cells. The root hairs curl and the bacteria invade the root hair. An infection thread is produced carrying the bacteria into the cortex of the root, where they initiate the nodule formation in the cortex of the root. Then, the bacteria are released from the thread into the cells, which leads to the differentiation of specialised nitrogen fixing cells. The nodule thus formed, establishes a direct vascular connection with the host for the exchange of nutrients.
The nodule contains all the necessary biochemical components, such as the enzyme nitrogenase an Mo-Fe protein that is highly sensitive to oxygen and leghaemoglobin an pigment that protects nitrogenase enzyme from oxygen. Few plants are able to grow in even nitrogen deficient soils, without the association of nitrogen fixing organisms. These plants obtain or fulfil their nitrogen requirements by trapping insects. Therefore, are also called insectivorous plants, e.g. Nepenthes, Drosera, venus fly trap, Utricularia, etc.
Mechanism of N2 -Fixation
The fixation of nitrogen in root nodules of legumes takes place in the presence of enzyme nitrogenase. The active nitrogenase complex contains protein-1 and protein-2 components in the ratio of 1 : 2. Fixation of nitrogen, i.e. its reduction to NH3 is accomplished in three steps. In each step, two electrons (also 2 protons) are transferred from the reduced coenzyme NADPH to nitrogen. Di-imide and hydrazine are formed as intermediates. ATP, which comes through respiration provides energy during this reaction.
At each step, 2 electrons and 2 protons are transferred to nitrogen through the components of nitrogenase complex.
The ammonia synthesis by nitrogenase requires a very high input of energy (8ATP for each NH3 produced) which is obtained from the respiration of the host cells.
Ammonia thus produced and by the degradation of manures and organic matter may not be available to plants because it is readily leached from the soil. It is converted to nitrate with the help of certain microorganisms. This conversion (oxidation) of ammonia to nitrate is called nitrification. It is performed in two steps, i.e. nitrite formation and nitrate formation. In the first step, ammonium ions are oxidised to nitrites. Nitrosomonas are the most important agents of oxidation of ammonia to nitrite in soil. Certain other bacteria are Nitrosococcus, Nitrosolobus, Nitrosospira, Nocardia and Streptomyces.
In the second step, oxidation of nitrite to nitrate takes place and is dependent on the activities of bacteria belonging mainly to genera Nitrobacter. In addition, certain fungi.
Nitrate Assimilation in Plants
Nitrate cannot be utilised by plants as such. It is fust reduced to ammonia before being incorporated into organic compounds.
Reduction of nitrate occurs in following two steps :
- Step I
Reduction of nitrate to nitrite It is carried out by an inducible enzyme, nitrate reductase. The enzyme is a molybdoflavoprotein. It requires a reduced coenzyme NADH or NADPH for its activity, which is brought in contact with nitrate by FAD or FMN.
- Step II
Reduction of nitrite It is carried by the enzyme nitrite reductase. The enzyme is a metalloflavoprotein, which contains copper and iron. It occurs inside the chloroplast in leaf cells and leucoplast of other cells. Nitrite reductase requires reducing power. It is NADPH and NADH (NADPH in illuminated cells). Reduction process also requires ferredoxin, which occurs in green tissues of higher plants. It is presumed that in higher plants, either nitrite is translocated to leaf cells or some other electron donor (like FAD) operates in unilluminated cells. The product of nitrite reduction is ammonia.
Fate of Ammonia
At physiological pH, the ammonia is protonated to form ammonium (NH4) ion. This NH£ ion is used to synthesise amino acids in plants by two main ways :
- Reductive Animation
In this process, ammonia reacts with a-ketoglutaric acid and forms glutamic acid as indicated in the equation given below:
- Trans amination
- It involves transfer of amino group from one amino acid to the keto group of a keto acid.
- Glutamic acid is the main amino acid, from which the transfer of NH2 (the amino group) takes place and other amino acids are formed through transamination.
- The enzyme, transaminase catalyses all such reactions.
- Aspargine and glutamine are the two most important amides, found in plants that form structural part of proteins. Since, amides contain more nitrogen than the amino acids, they are transported to other parts of the plant via xylem vessels
- along with the transpiration. The nodules of some plants (e.g. soybean) also export the fixed nitrogen as ureides. These compounds also have a particularly high nitrogen to carbon ratio.
Proteins and nucleic acids of the dead remains of plants, animals and excretory products of animals are degraded by microorganisms (e.g. Bacillus ramosus, B. vulgaris, Clostridium, Actinomyces, etc.), with the liberation of ammonia. This process is called ammonification.
Proteins are first broken up into amino acids. The later are deaminated and form ammonia. Organic acids released during the process are used by microorganisms for their own metabolism.
Ammonia changes gaseous to ionic form in the soil and is utilised by the plants provided pH of the soil is more than six and plants contain abundant organic acids. Plant, e.g. Begonia and Oxalis can store ammonium ions.