Nitrogen Fixation And Nitrogen Metabolism - Practice Questions & MCQ

• Higher plants generally utilize the oxidized forms such as nitrate (NO3-) and nitrite (NO2-) or the reduced form (N) of nitrogen which is made available by a variety of nitrogen fixers.
• 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 is termed as nitrogen fixation.
• Only certain prokaryotic systems can use atmospheric nitrogen directly.
• In a stable ecological system, the atmospheric nitrogen is converted into a metabolically useful form by a few prokaryotic life styles for supply to higher plants and animals.
• In nature, lightning and ultraviolet radiation provide enough energy to convert nitrogen to nitrogen oxides (NO, NO2 N2O). 
• Industrial combustions, forest fires, automobile exhaust and power -generating stations are also sources of atmospheric nitrogen oxides.
• Decomposition of organic nitrogen of dead plants and animals into ammonia is called ammonification.
• Some of this ammonia volatilises and re-enters the atmosphere but most of it is converted into nitrate by soil bacteria.

• During the rains, NO2 combines with rain water to form nitrous acid (HNO2) and nitric acid (HNO3).
• The acids fall on the soil along with rain water and react with the alkaline radicals to form water soluble nitrates (NO3-) and nitrites (N02-).

• The nitrates are soluble in water and are directly absorbed by the roots of the plants.

• 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. 
• Such microbes are called N2 - fixers.
• Asymbiotic biological nitrogen fixation: This is done by many aerobic and anaerobic bacteria, cyanobacteria (blue green algae) and. some fungi.
•  Free living N2 fixing bacteria:
  • Aerobic and saprotrophic  : Azotobacter
  • Anaerobic :  Clostridium
  • Photosynthetic : Chlorobium
  • Chemosynthetic : Thiobacillis 
• Cyanobacteria (blue-green algae) e.g., Anabaena, Nostoc, Tolypothrix cylindrospermum, Calotherix and Aulosira etc.
• Free living fungi e.g., Yeast cells and Pullularia.
   

From an ecological perspective, the nitrogen cycle consists of the following stages:

1. Ammonification

• Ammonification is a process in which the organic nitrogen of plants and animals after their death is converted to ammonium ions (NH4) by the action of saprotrophic fungi and bacteria. 
• The saprotrophs use the ammonia (NH3) to synthesize their own proteins and other nitrogen-containing organic compounds.

2. Nitrification:

• Ammonium ions added to the soil by ammonification, are soon oxidized by a process known as nitrification. 
• It takes place in two stages. 
• In the first stage, ammonium (NH4+) is converted to nitrite (NO2-). 
• This reaction involves the addition of oxygen to ammonia, giving rise to hydroxylamine (NH2OH), which is further oxidized to nitrite. 
• This reaction is completed by bacteria such as  
• The second stage of nitrification involves the oxidation of nitrite (NO2) to nitrate (NO3) by bacteria like Nitrobacter, Nitrospira and Nitrococcus. 
• The reaction proceeds by the addition of water followed by the removal of hydrogen.

TIP: 

• Nitrate (NO3–) formed in the process of nitrification is used by most plants as a mineral metabolite and may be converted by them into amino groups and other nitrogen- containing compounds. 
• Nitrates are also added to the soil through rock dissolution and combination of atmospheric nitrogen with oxygen by lightning (nitrates so formed reach the soil by rain).
• However, many plants also absorb ammonia from the soil.

3. Denitrification:

• It is a process in which the nitrate ion (NO3) is reduced to nitrogen dioxide (NO2), di-nitrogen oxide (N2O), nitrogen monoxide (NO) or nitrogen (N2) by certain soil bacteria like Pseudomonas denitrificans. 
• Thus, nitrogen is liberated into the atmosphere. 
• Plants also lose small amounts of nitrogen to the atmosphere as gaseous ammonia, N2O, NO2 and NO especially when well fertilized with nitrogen

• Both Rhizobium sp. and Frankia are free living in soil, but only as symbionts, can fix atmospheric di-nitrogen.
• Nodule formation involves multiple interactions between free-living soil Rhizobium and roots of the host plant (the legumes).

The important stages involved in nodule formation are as follows:

• A variety of microorganisms exist in the rhizosphere (i.e. immediate vicinity of roots) of host root.
• The roots of young leguminous plants secrete a group of chemical attractants like flavonoids and betaines.
• In response to these chemical attractants, specific rhizobial cells migrate towards the root hairs and produce nod (nodulation) factors. 
• The nod factors found on the bacterial surface bind to the lectin proteins present on the surface of root hairs. This lectin-nod factor interaction induces growth and curling of root hairs around Rhizobia.
• At these regions, wall degrades in response to nod-factors and Rhizobia enter the root hair invagination of the plasma membrane called infection thread.
• The infection thread filled with dividing Rhizobia elongate through the root hair and later branched to reach different cortical cells.
• The Rhizobia are released into the cortical cells either single or in groups enclosed by a membrane. 
• The Rhizobia stop dividing, loose cell wall and become nitrogen-fixing cells as led bacteroids.
• The membrane surrounding the bacteroids is called peribacteroid membrane. The infected cortical cells divide to form nodules.

• The nodule serves as a site for N2 fixation. 
• It contains all the necessary bio-chemicals such as the enzyme complex called nitrogenase and leghaemoglobin (leguminous haemoglobin). 
• The nitrogenase has 2 components i.e. Mo-Fe protein (molybdo ferredoxin) and Fe-protein (azo ferredoxin).
• The nitrogenase catalyzes the conversion of atmosphere di-nitrogen (N2) to 2NH3. 
• The ammonia is the first stable product of nitrogen fixation.
• The nitrogenase is extremely sensitive to oxygen. 
• To protect these enzymes, nodule contains an oxygen scavenger called leghaemoglobin (Lb), which is a reddish-pink pigment. 
• It is interesting to note that these microbes live as aerobes under free-living conditions (where nitrogenase is not operational), but during nitrogen-fixing events, they become anaerobic (thus protecting the nitrogenase enzyme).
•  The ammonia synthesis by nitrogenase requires a very high input of energy (16 ATP for each NH3 produced).
• The energy required, thus, is obtained from the respiration of the host cells.