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Questions and Answers
What role do lectins play in the relationship between Rhizobium and legumes?
What role do lectins play in the relationship between Rhizobium and legumes?
Which of the following is true about the formation of root nodules in legumes?
Which of the following is true about the formation of root nodules in legumes?
What occurs after Rhizobium enters the root hair of legumes?
What occurs after Rhizobium enters the root hair of legumes?
Which characteristic of Rhizobium aids in its specificity towards legumes?
Which characteristic of Rhizobium aids in its specificity towards legumes?
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What is the primary function of the mucopolysaccharides found in the tubular infection thread?
What is the primary function of the mucopolysaccharides found in the tubular infection thread?
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What is the primary function of leghaemoglobin in host cells?
What is the primary function of leghaemoglobin in host cells?
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Which component is essential for the nitrogen fixation process?
Which component is essential for the nitrogen fixation process?
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What reaction occurs during nitrogen fixation involving nitrogenase?
What reaction occurs during nitrogen fixation involving nitrogenase?
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How many ATP molecules are required to convert N2 into ammonia in the nitrogen fixation process?
How many ATP molecules are required to convert N2 into ammonia in the nitrogen fixation process?
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Which enzyme is responsible for the conversion of hydrogen gas into protons and electrons?
Which enzyme is responsible for the conversion of hydrogen gas into protons and electrons?
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What is the role of ferrodoxin in nitrogen fixation?
What is the role of ferrodoxin in nitrogen fixation?
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Which of the following compounds is synthesized in the root nodules and acts as an electron donor?
Which of the following compounds is synthesized in the root nodules and acts as an electron donor?
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What is a characteristic of bacteroids in host cells?
What is a characteristic of bacteroids in host cells?
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What is the role of reduced ferrodoxin in the nitrogen assimilation process?
What is the role of reduced ferrodoxin in the nitrogen assimilation process?
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Which step occurs first in the nitrate assimilation pathway?
Which step occurs first in the nitrate assimilation pathway?
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What is the primary enzyme responsible for the reduction of nitrate to nitrite?
What is the primary enzyme responsible for the reduction of nitrate to nitrite?
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In the GS/GOGAT pathway, what is ammonium converted into?
In the GS/GOGAT pathway, what is ammonium converted into?
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What ultimately happens to the enzyme after complete reduction of N2 to NH3?
What ultimately happens to the enzyme after complete reduction of N2 to NH3?
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Which of the following is NOT a major role of nitrogen in plants?
Which of the following is NOT a major role of nitrogen in plants?
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What is the chief form of nitrogen that plants utilize from the soil?
What is the chief form of nitrogen that plants utilize from the soil?
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Which process describes the conversion of atmospheric nitrogen into nitrogenous salts?
Which process describes the conversion of atmospheric nitrogen into nitrogenous salts?
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Which of the following is a characteristic of non-symbiotic nitrogen fixation?
Which of the following is a characteristic of non-symbiotic nitrogen fixation?
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Which statement is true regarding biological nitrogen fixation?
Which statement is true regarding biological nitrogen fixation?
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What is a product of non-biological nitrogen fixation during a lightning strike?
What is a product of non-biological nitrogen fixation during a lightning strike?
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Which free-living bacteria are known for nitrogen fixation?
Which free-living bacteria are known for nitrogen fixation?
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What is the primary disadvantage of atmospheric nitrogen for plants?
What is the primary disadvantage of atmospheric nitrogen for plants?
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What is the primary role of microorganisms in the root nodules of leguminous plants?
What is the primary role of microorganisms in the root nodules of leguminous plants?
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Which of the following genera is associated with root nodules in non-leguminous plants?
Which of the following genera is associated with root nodules in non-leguminous plants?
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What term did DeBary coin to describe the interaction between different organisms living together?
What term did DeBary coin to describe the interaction between different organisms living together?
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Which type of Rhizobium is characterized as a slow-growing species?
Which type of Rhizobium is characterized as a slow-growing species?
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Which of the following plants is known for having an association with Rhizobium?
Which of the following plants is known for having an association with Rhizobium?
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What type of nodulation can occur in Gymnosperms?
What type of nodulation can occur in Gymnosperms?
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Which organism is associated with the symbiotic nitrogen fixation in Azolla?
Which organism is associated with the symbiotic nitrogen fixation in Azolla?
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What type of nodules can be formed by Dioscorea and Psychotria?
What type of nodules can be formed by Dioscorea and Psychotria?
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Study Notes
Role of Nitrogen in Plants
- Nitrogen is a major substance in plants, second only to water.
- It serves as a building block for essential components like chlorophyll, cytochromes, alkaloids, and many vitamins.
- Plays a crucial role in plant metabolism, growth, reproduction, and heredity.
Sources of Nitrogen
- Atmospheric nitrogen makes up 78% of the atmosphere but is unavailable to plants directly.
- Nitrogen-fixing bacteria, blue-green algae, and leguminous plants can utilize atmospheric nitrogen.
- Nitrates, nitrites, and ammonia are readily available sources of nitrogen.
- Amino acids present in soil are utilized by many soil organisms and can also be taken up by plants.
- Insectivorous plants obtain organic nitrogenous compounds from insects.
Nitrogen Fixation
- The process of converting free nitrogen into usable nitrogenous salts for plant absorption.
Types of Nitrogen Fixation
- Non-biological fixation: Occurs primarily through lightning, creating nitrogen oxides that are eventually converted into nitrates.
- Biological fixation: Involves the use of microorganisms to convert nitrogen into usable forms. This can be symbiotic or non-symbiotic.
Non-biological Fixation
- Occurs without the involvement of microorganisms.
- Common during the rainy season due to lightning strikes.
- Nitrogen gas reacts with oxygen to produce nitric oxide, which is then oxidized to nitrogen dioxide.
- Nitrogen dioxide reacts with water to form nitric acid, which can then combine with calcium oxide to form calcium nitrate.
Biological Fixation
- Microorganisms fix atmospheric nitrogen into nitrogenous salts.
- Two main types: symbiotic and non-symbiotic fixation.
Non-Symbiotic Fixation
- Carried out by free-living microorganisms, including aerobic, anaerobic bacteria, and blue-green algae.
- Examples of nitrogen-fixing bacteria include Azotobacter, Beijerenckia (aerobic), Clostridium (anaerobic), Chlorobium and Rhodopseudomonas (photosynthetic), and Desulfovibrio and Thiobacillus (chemosynthetic).
- Some free-living fungi like yeasts and Pillularia also fix nitrogen.
Symbiotic Fixation
- Microorganisms live in symbiotic relationships with plants, fixing nitrogen inside them.
- Three categories: nodule formation in leguminous and non-leguminous plants, and non-nodulation.
Nodule Formation in Leguminous Plants
- About 2500 species in the family Leguminosae form root nodules containing Rhizobium species.
- Nitrogen fixation occurs solely within these root nodules.
- The symbiotic relationship benefits both partners: bacteria receive food and shelter, while the plant receives fixed nitrogen.
- Nodules can continue nitrogen fixation even after harvesting.
Nodule Formation in Non-Leguminous Plants
- Some non-leguminous plants also form root nodules.
- Examples include Causuarina equisetifolia, Alnus, Myrica gale, and Parasponia, with Frankia or Rhizobium species responsible for nitrogen fixation.
- Leaf nodules are also observed in some plants like Dioscorea and Psychotria.
- Some gymnosperms possess root nodules (e.g., Podocarpus) or leaf nodules (e.g., Pavetta zinumermanniana and Chomelia).
Non-Nodulation
- Nitrogen fixation can occur without nodule formation in certain partnerships.
- Examples include 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, and Paspalum notatum with Azotobacter paspali.
Symbiotic Nitrogen Fixation
- Root nodules are small, knob-like structures that vary in size and shape.
- Various species of Rhizobium are associated with different legumes.
- The Rhizobium species is named after the host plant, e.g., Rhizobium leguminosarum in pea, R. phaseoli in beans, R. japonicum in soybeans, and R. lupini in lupins.
- Two types of Rhizobium: Bradyrhizobium (slow-growing) and Rhizobium (fast-growing).
- Rhizobium bacteria are gram-negative, non-spore-forming, micro-aerobic, and exhibit specificity towards their host plants.
- The host and bacteria recognize each other through chemical substances called lectins (phytoagglutinins).
Formation of Root Nodules in Legumes
- Root nodules form due to Rhizobium infection.
- Free-living Rhizobium bacteria cannot fix nitrogen independently.
- Legumes secrete growth factors that stimulate bacteria multiplication.
- Lectins on the root surface interact with receptors on Rhizobium cells, facilitating attachment.
- Rhizobia enter the roots through infected root hairs.
- Bacteria multiply within an infection thread formed in the root hair cell.
- The infection thread grows and releases bacteria into the cortex.
- Cortical cells divide, forming a nodule on the root surface.
- Bacteria colonize the host cells and become dormant bacteroids.
- Leghaemoglobin, a reddish pigment in host cells, scavenges oxygen and maintains optimal levels.
- Fixed nitrogen compounds are transported through the vascular tissues.
Biochemistry of Nitrogen Fixation
- Nitrogen fixation requires specific components:
- Nitrogenase enzyme
- Mechanism to protect against oxygen
- Ferrodoxin
- Electron donor (e.g., pyruvic acid or glucose/sucrose)
- ATP supply
- Coenzymes & cofactors (TPP, CoA, inorganic phosphate, Mg+2)
- Minerals: cobalt and molybdenum
- A carbon compound
Nitrogenase Enzyme
- Crucial for nitrogen fixation.
- Active in anaerobic conditions.
- Consists of two protein subunits:
- Non-heme iron protein (Fe-protein): Contains 4Fe and 4S atoms.
- Iron-molybdenum protein (Mo-Fe-protein): Contains 1-2 Mo, 12-32 Fe, and 24 S atoms.
- Fe-protein reacts with ATP and reduces the Mo-Fe-protein, which ultimately reduces N2 to ammonia.
Nitrogen Fixation Process
- The overall reaction is: N2 + 8H+ + 8e- + 16 ATP = 2NH3 + H2 + 16ADP + 16 Pi
- Reduction of N2 to NH3 requires 6 protons and 6 electrons, with 12 ATP molecules needed.
- One electron pair requires 4 ATP.
- The simplified equation is: N2 + 8H+ + 8e- = 2NH3 + H2
- Hydrogen produced is converted to protons and electrons by hydrogenase.
Pathway of Nitrogen Fixation in Root Nodules
- Glucose-6-phosphate serves as an electron donor.
- It is converted to 6-phosphogluconic acid, releasing protons and reducing NADP+ to NADPH.
- NADPH donates electrons to ferrodoxin.
- Reduced ferrodoxin acts as an electron carrier, transferring electrons to the Fe-protein, which is then reduced.
- Reduced Fe-protein combines with ATP in the presence of Mg+2, activating and reducing the Mo-Fe-protein.
- Mo-Fe-protein donates electrons to N2, reducing it to NH3.
Ammonification
- The process of converting organic nitrogen into ammonia.
Nitrification
- The oxidation of ammonia to nitrate by nitrifying bacteria.
- Nitrosomonas bacteria oxidize ammonia to nitrite, which is then oxidized to nitrate by Nitrobacter bacteria.
Nitrogen Assimilation
- Plants take up nitrate and ammonium.
- Nitrate assimilation begins with nitrate uptake, followed by reduction to nitrite.
- Nitrite is further reduced to ammonium.
- Ammonium is fixed into glutamine and glutamate by the GS/GOGAT pathway.
- These amino acids serve as precursors for other amino acids through transamination reactions.
- Nitrate is either reduced, stored in vacuoles, or translocated to the shoot for reduction and storage.
- Nitrate reductase (NR) in the cytosol reduces nitrate to nitrite.
- Nitrite reductase (NIR) in the plastid converts nitrite to ammonium.
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Description
This quiz explores the vital role of nitrogen in plant life, emphasizing its significance in key biological processes such as metabolism and growth. Additionally, it covers various nitrogen sources and the essential concept of nitrogen fixation, including both biological and non-biological methods. Test your knowledge on how plants utilize nitrogen for their development.
