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Questions and Answers
What is the chief form of nitrogen utilized by plants?
Which of the following are components that contain nitrogen in plants?
What process helps convert atmospheric nitrogen into usable nitrogenous salts for plants?
Which type of nitrogen fixation is performed by free-living microorganisms?
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What is the primary function of lectins in the interaction between Rhizobium and legumes?
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What role does nitrogen play in plants?
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How do Rhizobium bacteria enter the roots of legumes?
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Which of the following forms of nitrogen can be absorbed by higher plants directly from the soil?
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Which of the following bacteria is considered free-living aerobic nitrogen-fixing bacteria?
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What happens immediately after Rhizobium enters the root hair?
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What is the role of growth factors secreted by legume roots in relation to Rhizobium?
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What is a major nitrogenous compound produced as a result of lightning?
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What is formed as a result of the interaction between Rhizobium and the cortical cells of legumes?
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Which of the following fungi are categorized as free-living?
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What type of nitrogen fixation occurs specifically in the root nodules of leguminous plants?
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Which of the following is a characteristic of Rhizobium species in leguminous plants?
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Which of the following plants has been identified for nodule formation with Frankia?
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Which feature of root nodules can vary in size and shape?
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Which type of Rhizobium is characterized as fast-growing?
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Which plant is known for having leaf nodules?
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What distinguishes non-nodulation symbiosis from others?
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What condition is necessary for nitrogenase enzyme to function effectively?
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Which of the following elements is critical for the structure of nitrogenase?
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What is the role of leghaemoglobin in nitrogen-fixing bacteria?
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What role does reduced ferrodoxin play in the process described?
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Which process reduces N2 into ammonia (NH3) during nitrogen fixation?
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Which process begins with the uptake of nitrate by plant roots?
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What is the primary function of ferrodoxin in nitrogen fixation?
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In the nitrogen assimilation pathway, what is the first product formed from nitrate reduction?
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Which molecule acts as the electron donor starting from sucrose in root nodules?
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What happens to ammonium after it is produced through nitrate reduction?
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What does the nitrogen fixation equation, $N_2 + 8H^+ + 8e^- + 16 ATP = 2NH3 + H2 + 16ADP + 16 Pi$ signify?
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How many moles of ATP are required to convert nitrogen gas (N2) into ammonia (NH3)?
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What enzyme is responsible for reducing nitrite to ammonium in the chloroplast?
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Study Notes
Nitrogen in Plants
- Nitrogen is vital for plant growth and development, second only to water.
- Essential component of chlorophyll, cytochromes, alkaloids, and many vitamins.
- Plays a crucial role in metabolism, growth, reproduction, and heredity.
Nitrogen Sources
- Atmospheric nitrogen (78%) is unusable by plants.
- Nitrogen-fixing bacteria, blue-green algae, and leguminous plants convert atmospheric nitrogen into usable forms.
- Nitrates are the primary source of nitrogen for plants, followed by nitrites and ammonia.
- Soil organisms utilize amino acids as a nitrogen source, which can also be absorbed by plants.
- Insectivorous plants obtain nitrogen from organic nitrogenous compounds in insects.
Nitrogen Fixation
- The conversion of free nitrogen (N2) into usable nitrogenous salts for plant absorption.
Types of Nitrogen Fixation
- Non-biological fixation: Occurs primarily through lightning, a process involving several chemical reaction steps.
- Biological fixation: Microorganisms convert atmospheric nitrogen into usable forms.
Non-Biological Fixation
- Occurs without the involvement of microorganisms.
- Primarily occurs during lightning in rainy seasons.
- Involves a series of chemical reactions leading to the formation of nitric oxide (NO), nitrogen peroxide (NO2), nitric acid (HNO3), and ultimately calcium nitrate (Ca(NO3)2).
Biological Fixation
- Microorganisms fix nitrogen into nitrogenous salts.
- Two main types: Symbiotic and Non-symbiotic.
Non-Symbiotic Fixation
- Free-living microorganisms carry out nitrogen fixation.
- Includes aerobic, anaerobic, and blue-green algae.
- Bacteria involved:
- Free-living aerobic: Azotobacter, Beijerenckia
- Free-living anaerobic: Clostridium
- Free-living photosynthetic: Chlorobium, Rhodopseudomonas
- Free-living chemosynthetic: Desulfovibro, Thiobacillus
- Also includes some free-living fungi (yeasts and Pillularia).
- Blue-green algae:
- Unicellular: Gloeothece, Synechococcus
- Filamentous (non-heterocystous): Oscillatoria
- Filamentous (heterocystous): Tolypothrix, Nostoc, Anabaena
Symbiotic Fixation
- Microorganisms fix nitrogen while living symbiotically inside plants.
- Three categories:
- Nodule formation in leguminous plants
- Nodule formation in non-leguminous plants
- Non-nodulation
Nodule Formation in Leguminous Plants
- About 2500 species in the Leguminosae family (e.g., Cicer arientium, Pisum, Cajanus, Arachis) form root nodules with Rhizobium species.
- Nitrogen fixation occurs strictly within the root nodules.
- Symbiotic relationship offers food and shelter to bacteria, while bacteria provide fixed nitrogen to the plant.
- Nodules can remain active and fix nitrogen even after harvesting.
Nodule Formation in Non-Leguminous Plants
- Some non-leguminous plants also form root nodules.
- Causuarina equisetifolia, Alnus, Myrica gale, Parasponia with Frankia or Rhizobium.
- Leaf nodules are also observed in some plants (e.g., Dioscorea, Psychotria).
- Certain gymnosperms produce nodules in roots (e.g., Podocarpus) or leaves (e.g., Pavetta zinumermanniana, Chomelia).
Non-Nodulation
- Nitrogen fixation occurs in associations with organisms other than root nodules.
- Lichens with cyanobacteria
- Anthoceros with Nostoc
- Azolla with Anabaena azollae
- Cycas with Nostoc and Anabaena
- Gunnera macrophylla with Nostoc
- Digitaria, Maize, and Sorghum with Spirillum notatum
- Paspalum notatum with Azotobacter paspali
Symbiotic Nitrogen Fixation
- Small, knob-like protuberances called root nodules are formed.
- Nodules vary in size and shape (spherical, flat, finger-like, or elongated).
- Size ranges from pinhead to one centimeter.
- Various species of Rhizobium are associated with different host plants.
- Named after the host plant:
- Pea: Rhizobium leguminosarum
- Beans: R. phaseoli
- Soybeans: R. japonicum
- Lupins: R. lupini
- Two types of Rhizobium:
- Bradyrhizobium: slow-growing species.
- Rhizobium: fast-growing species.
- Rhizobium bacteria:
- Gram-negative
- Non-spore forming
- Micro-aerobic
- Exhibit some degree of host specificity.
- Recognition between bacteria and host occurs due to chemical substances called lectins (phytoagglutinins, carbohydrate-containing plant proteins).
Formation of Root Nodules in Legumes
- Root nodules are formed due to Rhizobium infection.
- Free-living Rhizobium bacteria near legume roots do not fix nitrogen independently.
- Root secretions from legumes promote bacterial multiplication.
- (E.g.,) Pisum sativum secretes homoserine and carbohydrate-containing proteins (lectins).
- Lectins on root hairs bind with carbohydrate receptors on rhizobial cells facilitating recognition and attachment.
- Bacterially-infected root hairs deform and curve.
- A tubular infection thread forms within the root hair, allowing bacteria to enter.
- A new cell wall forms around the infection thread.
- The infection thread contains mucopolysaccharides, embedding bacteria and supporting multiplication.
- The thread extends to the inner cortical layers, releasing bacteria.
- Bacterial cells multiply and colonize multiplying cortical cells, leading to nodule formation.
- Bacterial cells ultimately become dormant (bacteroids).
- Bacteroids float in leghaemoglobin, a reddish pigment in host cell cytoplasm.
- Functions as an efficient oxygen scavenger.
- Maintains steady-state oxygen levels.
- Stimulates ATP production.
- Synthesized nitrogenous compounds are transported through vascular tissues.
- Bacteroids are enclosed by a double membrane derived from the host cell wall.
- Bacteroids lack a firm wall (osmotically liable).
Biochemistry of Nitrogen Fixation
- Key requirements for nitrogen fixation:
- Nitrogenase enzyme
- Protective mechanism against oxygen
- Ferrodoxin
- Hydrogen-releasing system/electron donor (pyruvic acid or glucose/sucrose)
- Constant supply of ATP
- Coenzymes and cofactors: TPP, CoA, inorganic phosphate, and Mg+2
- Cobalt and molybdenum
- A carbon compound
Nitrogenase Enzyme
- Plays a pivotal role in nitrogen fixation.
- Active under anaerobic conditions.
- Consists of two protein subunits:
- Non-heme iron protein (Fe-protein or nitrogen reductase):
- 60,000 Daltons.
- Two identical subunits.
- Each subunit contains 4Fe and 4S atoms.
- Iron-molybdenum protein (Mo Fe-protein or nitrogenase):
- Larger subunit – 200,000 Daltons.
- 4 subunits.
- 1-2 Mo, 12-32 Fe, and 24 S atoms in each subunit.
- Non-heme iron protein (Fe-protein or nitrogen reductase):
- Fe-protein reacts with ATP and reduces the second subunit (Mo Fe-protein), ultimately reducing N2 into ammonia.
- The overall reaction is: N2 + 6H+ + 6e- → 2NH3
Nitrogen Fixation Process
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The complete nitrogen fixation process can be summarized as: N2 + 8H+ + 8e- + 16 ATP = 2NH3 + H2 + 16ADP + 16 Pi
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The reduction of N2 into NH3 requires 6 protons and 6 electrons.
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12 moles of ATP are required, with each electron pair requiring 4 ATP.
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A modified equation highlighting the key reactants: N2 + 8H+ + 8e- → 2NH3 + H2
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Hydrogen produced is catalyzed into protons and electrons by hydrogenase. Hydrogenase H2 → 2H+ + 2e-
Pathway of Nitrogen Fixation in Root Nodules
- Glucose-6-phosphate acts as an electron donor.
- Sucrose (synthesized in leaves) is transported to roots and converted into glucose and fructose.
- Glucose is further converted to glucose-6-phosphate.
- Glucose-6-phosphate is oxidized to 6-phosphogluconic acid. Glucose-6-phosphate + NADP+ + H2O → 6-phosphogluconic acid + NADPH + H+
- NADPH donates electrons to ferrodoxin, reducing ferrodoxin and releasing protons.
- Reduced ferrodoxin acts as an electron carrier, transferring electrons to Fe-protein, reducing it and becoming oxidized.
- Reduced Fe-protein combines with ATP in the presence of Mg+2.
- The Mo Fe-protein is activated and reduced, donating electrons to N2 for conversion to NH3.
- Nitrogenase is released after the complete reduction of N2 to NH3.
- The detailed steps involving Mo-NΞN, Mo – N=NH, NΞN, Mo=N-NH2, Mo + NH3 and MoΞN+NH3 illustrate the progressive reduction of nitrogen.
Ammonification
- The process of converting organic nitrogen compounds (e.g., proteins, amino acids) into ammonia (NH3) by soil microorganisms.
Nitrification
- The oxidation of ammonia (NH3) into nitrite (NO2-) by Nitrosomonas bacteria and then into nitrate (NO3-) by Nitrobacter bacteria in the soil.
Nitrogen Assimilation
- Plants take up nitrate (NO3-) and ammonium (NH4+) as their main nitrogen sources.
- Nitrate assimilation starts with uptake followed by nitrate reduction to ammonium, which is then incorporated into glutamine and glutamate.
- Nitrate uptake occurs through the roots and is either reduced, stored in the vacuoles, or transported to the shoot for reduction and vacuolar storage (also for osmoregulation).
- Nitrate reductase (NR) in the cytosol reduces nitrate to nitrite.
- Nitrite enters the plastids and is reduced to ammonium by nitrite reductase (NIR).
- Ammonium is assimilated into amino acids (glutamine/glutamate) by the GS/GOGAT pathway.
- These amino acids serve as substrates for transamination reactions to produce all other proteinous amino acids.
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Description
Explore the essential role of nitrogen in plant growth and development, examining its sources and the process of nitrogen fixation. This quiz covers how nitrogen is utilized by plants and the various forms it takes in the ecosystem. Test your knowledge on nitrogen's significance for metabolism, growth, and heredity.
