(a) How can the reaction equilibria and reaction rates be explained by using free energy diagram in a simple enzymatic reaction ? 20

(b) Explain the following : 5+5=10

(i) Nitrate and nitrite reduction in the leaves of higher plants. 5

(ii) Protec

Question

(a) How can the reaction equilibria and reaction rates be explained by using free energy diagram in a simple enzymatic reaction ? 20

(b) Explain the following : 5+5=10

(i) Nitrate and nitrite reduction in the leaves of higher plants. 5

(ii) Protection of enzyme nitrogenase against oxygen and hydrogen damage. 5

(c) Comment on the following : 10+10=20

(i) Biosphere reserves 10

(ii) Red Data Book 10

UPSC Answer Check · Accepted Answer

Enzymatic Kinetics and Nitrogen Metabolism

Enzymes are biological catalysts that accelerate reaction rates without altering the thermodynamic equilibrium. In a simple enzymatic reaction, the free energy diagram illustrates the energy profile from substrate (S) to product (P). The uncatalyzed reaction follows a high-energy path with a large activation energy barrier (Δ G^ddagger). The catalyzed pathway involves the formation of an enzyme-substrate complex (ES), which stabilizes the transition state, denoted as ES‡. By lowering the activation energy (Δ G^ddagger) required to reach ES‡, the enzyme increases the rate of reaction. Crucially, the overall free energy change (Δ G) between S and P remains constant, meaning the equilibrium constant (K_eq) is unaffected; enzymes only hasten the approach to equilibrium.

Nitrate and Nitrite Reduction in Leaves

Nitrogen assimilation in higher plants occurs in two distinct steps. First, nitrate (NO₃⁻) is reduced to nitrite (NO₂⁻) in the cytosol by nitrate reductase (NR). NR is a molybdenum-iron enzyme that uses NADH as an electron donor. Second, nitrite is transported into the chloroplasts, where nitrite reductase (NiR) reduces it to ammonium (NH₄⁺). NiR contains an iron-sulfur center and siroheme, and it utilizes reduced ferredoxin generated by photosynthesis as its electron donor. This spatial separation ensures that toxic nitrite is rapidly converted to ammonium, which is then assimilated into glutamine via the GS-GOGAT cycle.

Protection of Nitrogenase

Nitrogenase, the enzyme responsible for N₂ fixation, is highly sensitive to both oxygen and hydrogen. Protection against oxygen damage is achieved through multiple mechanisms. In legume root nodules, leghemoglobin binds oxygen, maintaining low free O₂ levels while allowing diffusion to bacteroids for respiration. High respiratory rates in bacteroids further consume oxygen. Additionally, conformational protection occurs when the Fe-protein of nitrogenase undergoes a conformational change in the presence of oxygen, shielding the active site. In cyanobacteria, heterocysts provide physical protection via thick cell walls and lack Photosystem II, creating an anaerobic microenvironment.

Protection against hydrogen damage is equally critical. Nitrogenase produces hydrogen (H₂) as a byproduct of N₂ reduction. Accumulation of H₂ can inhibit the enzyme by competing for the active site. To prevent this, many nitrogen-fixing bacteria and cyanobacteria possess an uptake hydrogenase. This enzyme catalyzes the oxidation of H₂ back to protons and electrons, which are fed into the electron transport chain. This recycling mechanism prevents H₂ buildup, thereby avoiding product inhibition and ensuring the efficiency of nitrogen fixation.

Biosphere Reserves

Biosphere reserves are designated under the UNESCO Man and the Biosphere (MAB) programme. They follow a zonation system comprising three zones: the core zone, which is strictly protected for conservation; the buffer zone, surrounding the core, where only research and education are permitted; and the transition zone, where sustainable development activities like agriculture and tourism are allowed. India has several such reserves, including Nilgiri, Nanda Devi, and Sundarbans. These reserves serve as models for reconciling the conservation of biodiversity with the sustainable use of natural resources, providing areas for monitoring, research, and environmental education.

Red Data Book

The Red Data Book, published by the IUCN, is a comprehensive list of threatened species. It categorizes species based on their risk of extinction, including Extinct (EX), Extinct in the Wild (EW), Critically Endangered (CR), Endangered (EN), and Vulnerable (VU). This classification is vital for conservation prioritization and policy formulation. In India, several plant species are listed under these categories. For instance, Santalum album (Sandalwood) is Endangered due to over-exploitation, while Nepenthes khasiana (Pitcher Plant) is Critically Endangered due to habitat loss. Saussurea obvallata (Brahma Kamal) is also threatened. The Red Data Book provides the scientific basis for legal protection under the Wildlife Protection Act and guides conservation efforts to prevent further biodiversity loss.

In conclusion, understanding the enzymatic mechanisms of nitrogen metabolism and the protective strategies of nitrogenase is fundamental to enhancing agricultural productivity. Simultaneously, the establishment of biosphere reserves and the monitoring of threatened species through the Red Data Book are essential for preserving the ecological balance. Together, these biological and conservation strategies address the dual challenges of food security and biodiversity conservation.

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