Q6
(a) Explain the compartmentation of biochemical reactions in photorespiration. Comment upon the significance of the process. 15+5=20 (b) Describe the structure of phytochrome. Explain its mode of action in flowering plants. 5+10=15 (c) Give a concise account on altitudinal zonation of vegetation with special reference to Himalayan Vegetation. 15
हिंदी में प्रश्न पढ़ें
(a) प्रकाश श्वसन में जैवरासायनिक क्रियाओं के विभागों को समझाइए । प्रक्रिया के महत्व पर टिप्पणी कीजिए । 15+5=20 (b) पादपवर्णक (फाइटोक्रोम) की संरचना का वर्णन कीजिए । फूल वाले पौधों में उनकी प्रक्रिया को स्पष्ट कीजिए । 5+10=15 (c) हिमालय के वनस्पतियों के विशेष संदर्भ के साथ वनस्पति के तुंगीय क्षेत्र वर्गीकरण का संक्षिप्त वर्णन कीजिए । 15
Directive word: Explain
This question asks you to explain. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.
See our UPSC directive words guide for a full breakdown of how to respond to each command word.
How this answer will be evaluated
Approach
The directive 'explain' demands clear causal reasoning and mechanistic detail across all three parts. Allocate approximately 40% of time/words to part (a) given its 20 marks, with 30% each to parts (b) and (c). Structure: begin with photorespiration's three-organellar compartmentation with a schematic diagram, followed by phytochrome's chromophore-apoprotein structure and signal transduction, concluding with Himalayan vegetation zones arranged altitudinally with characteristic species.
Key points expected
- Part (a): Three distinct compartments (chloroplast, peroxisome, mitochondrion) with specific enzymes—Rubisco oxygenase, phosphoglycolate phosphatase, glycolate oxidase, catalase, glutamate-glyoxylate aminotransferase, glycine decarboxylase complex, and serine hydroxymethyltransferase
- Part (a): Significance including carbon loss (25%), ammonia recycling, protection against photoinhibition, and evolutionary context of C3 vs C4 plants with reference to Hatch-Slack pathway
- Part (b): Phytochrome structure—homodimeric chromoprotein with linear tetrapyrrole chromophore (phytochromobilin) covalently linked to cysteine residue via thioether bond; N-terminal photosensory and C-terminal regulatory domains
- Part (b): Mode of action—Pr/Pfr photoconversion, nuclear translocation, interaction with PIFs (phytochrome interacting factors), ubiquitin-proteasome pathway, and specific roles in photoperiodism (SDP/LDP) with examples like Xanthium, Pharbitis
- Part (c): Himalayan altitudinal zones—Tropical (Tarai, Bhabhar, Shiwaliks: Shorea, Terminalia), Subtropical (Pinus roxburghii), Temperate (Quercus, Rhododendron), Subalpine (Abies, Betula), Alpine (Kobresia meadows), Alpine desert with specific elevation ranges
- Part (c): Ecological factors driving zonation—temperature lapse rate (6.5°C/1000m), rainfall patterns, aspect effects, and anthropogenic influences; mention of endemic species like Meconopsis, Saussurea obvallata (Brahma Kamal)
Evaluation rubric
| Dimension | Weight | Max marks | Excellent | Average | Poor |
|---|---|---|---|---|---|
| Concept correctness | 22% | 11 | Precise biochemical details for (a): correct stoichiometry (2 glycolate → 1 serine + 1 CO2 + 1 PGA), enzyme locations, and membrane permeability; for (b) accurate chromophore chemistry and photoreversibility; for (c) correct elevation ranges and zone boundaries with recognition of east-west Himalayan variation | Basic compartmentation identified but enzymes misplaced or stoichiometry errors; phytochrome described as 'light receptor' without structural detail; zones listed but elevations approximate or confused | Fundamental errors—photorespiration described as single compartment, phytochrome confused with cryptochrome, or tropical and alpine zones reversed in elevation |
| Diagram / labelling | 18% | 9 | For (a): clear three-compartment schematic showing metabolite flux (glycolate, glycine, serine, glycerate) with enzyme labels and membrane notation; for (b) phytochrome dimer structure with chromophore detail; for (c) altitudinal profile diagram with characteristic species and elevation markers | Single diagram for photorespiration with partial labelling; phytochrome mentioned without structural representation; zones described textually without visual representation | No diagrams despite clear visual demand; or diagrams with major errors (e.g., Calvin cycle mislabelled as photorespiration) |
| Examples & nomenclature | 18% | 9 | Specific C3/C4/CAM examples for photorespiration significance (rice, maize, pineapple); phytochrome gene families (PHYA-PHYE in Arabidopsis) and species-specific flowering responses; Himalayan endemics with scientific names (Quercus leucotrichophora, Pinus wallichiana, Abies spectabilis, Rhododendron arboreum) | Generic plant types mentioned without species names; phytochrome types (Type I/II) confused; common Himalayan species named but with spelling errors | No scientific nomenclature; or invented species names; confusion between Himalayan and Western Ghats vegetation |
| Process explanation | 22% | 11 | For (a): mechanistic explanation of oxygenase reaction, metabolite shuttling, and ammonia reassimilation cycle; for (b): detailed photoconversion mechanism, conformational change, kinase activity, and signal transduction cascade to gene expression; for (c): explanation of ecophysiological adaptations (xerophytism, phenology) across zones | Sequential description without mechanistic depth; phytochrome action described as 'absorbs red light and causes flowering' without downstream events; zonation described as 'due to temperature' without physiological explanation | Processes confused—photorespiration conflated with respiration, phytochrome with auxin action, or altitudinal zonation attributed solely to human activity |
| Application / ecology | 20% | 10 | Critical evaluation of photorespiration as evolutionary relic and target for bioengineering (RIPE project); agricultural implications of phytochrome manipulation (photoperiodism in rice cultivation); conservation significance of Himalayan zones (biodiversity hotspots, climate change vulnerability, Chipko movement relevance) | Brief mention of C4 engineering or crop improvement without specifics; flowering mentioned without agricultural context; general statement about Himalaya as biodiversity hotspot | No application or ecological context; or irrelevant applications (e.g., medicinal uses of plants without ecological link) |
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