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Q8:
What are the conditions necessary for fixation of atmospheric nitrogen by Rhizobium. What is their role in $N_2$-fixation?
Solution :
1. Physiological Conditions for Nitrogen Fixation by Rhizobium
The symbiotic nitrogen fixation process, mediated by the association between the bacterium Rhizobium and the roots of leguminous plants, requires a highly regulated micro-environment. The necessary conditions are:
- Anaerobic Environment: The enzyme nitrogenase is highly sensitive to molecular oxygen ($O_2$), which irreversibly inactivates the catalytic iron-molybdenum protein complex [Per the kinetics of the Nitrogenase complex].
- Presence of Leghaemoglobin: An oxygen-scavenging pigment is required to maintain a low $O_2$ tension while simultaneously supplying oxygen for bacterial respiration.
- ATP Supply: Nitrogen fixation is an energetically expensive process requiring significant chemical energy [Per the stoichiometry of the nitrogenase reaction: $N_2 + 8e^- + 8H^+ + 16ATP \rightarrow 2NH_3 + H_2 + 16ADP + 16Pi$].
- Presence of Nitrogenase Enzyme: The specific biological catalyst (dinitrogenase) containing Mo-Fe (Molybdenum-Iron) cofactor must be present.
- Reducing Environment: A steady supply of reductants (electrons) typically derived from ferredoxin is essential for the reduction of atmospheric nitrogen to ammonia.
2. Role of Rhizobium in $N_2$-Fixation
The role of Rhizobium is multifaceted, involving structural, enzymatic, and biochemical coordination within the root nodule:
| Role Component | Biological Function |
|---|---|
| Nod Factor Secretion | Bacterial signaling molecules trigger root hair curling and the initiation of nodule primordia development [By chemo-attraction and signal transduction pathways]. |
| Nitrogenase Synthesis | The bacteria synthesize the nitrogenase complex, which reduces atmospheric $N_2$ into the plant-assimilable form, ammonia ($NH_3$). |
| Leghaemoglobin Production | The bacterium contributes to the synthesis of the apoprotein part of leghaemoglobin, which acts as an "$O_2$ buffer" to protect nitrogenase from inhibition. |
| Ammonia Translocation | The bacteria export the synthesized $NH_3$ to the host plant cytosol, where it is promptly protonated to $NH_4^+$ to avoid toxicity, and incorporated into amino acids via the GS-GOGAT pathway. |
Visual Representation of the Nodule Environment
Final Solution: The primary conditions for Rhizobium-mediated nitrogen fixation include the maintenance of a low oxygen tension via leghaemoglobin, an ample supply of ATP, and the presence of the nitrogenase enzyme. Rhizobium acts by providing the metabolic machinery (enzymes) for the reduction of inert $N_2$ into ammonia, which is then utilized by the host plant for protein synthesis and developmental growth.
More Questions from Class 11 Biology Mineral Nutrition EXERCISES
- Q1: ‘All elements that are present in a plant need not be essential to its survival’. Comment.
- Q2: Why is purification of water and nutrient salts so important in studies involving mineral nutrition using hydroponics?
- Q3: Explain with examples: macronutrients, micronutrients, beneficial nutrients, toxic elements and essential elements.
- Q4: Name at least five different deficiency symptoms in plants. Describe them and correlate them with the concerned mineral deficiency.
- Q5: If a plant shows a symptom which could develop due to deficiency of more than one nutrient, how would you find out experimentally, the real deficient mineral element?
- Q6: Why is that in certain plants deficiency symptoms appear first in younger parts of the plant while in others they do so in mature organs?
- Q7: How are the minerals absorbed by the plants?
- Q9: What are the steps involved in formation of a root nodule?
- Q10(a): Which of the following statements are true? If false, correct them: (a) Boron deficiency leads to stout axis.
- Q10(b): Which of the following statements are true? If false, correct them: (b) Every mineral element that is present in a cell is needed by the cell.
- Q10(c): Which of the following statements are true? If false, correct them: (c) Nitrogen as a nutrient element, is highly immobile in the plants.
- Q10(d): Which of the following statements are true? If false, correct them: (d) It is very easy to establish the essentiality of micronutrients because they are required only in trace quantities.
CBSE Solutions for Class 11 Biology Mineral Nutrition
Chapters in CBSE - Class 11 Biology
- The Living World
- Biological Classification
- Plant Kingdom
- Animal Kingdom
- Morphology of Flowering Plants
- Anatomy of Flowering Plants
- Structural Organisation in Animals
- Cell – The Unit of Life
- Biomolecules
- Cell Cycle and Division
- Transport in Plants
- Photosynthesis
- Respiration in Plants
- Plant Growth and Development
- Digestion and Absorption
- Breathing and Exchange of Gases
- Body Fluids and Circulation
- Excretory Products and Elimination
- Locomotion and Movement
- Neural Control and Coordination
- Chemical Coordination and Integration
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