Agriculture 2023 Paper II 50 marks Enumerate

Q7

(a) Enlist various photosynthetic pigments. Give a detailed account of chlorophyll synthesis along with the enzymes involved in the process. Also mention the role of carotenoids in photosynthesis. 20 (b) List the reactions of Calvin Cycle and Hatch-Slack Cycle for CO₂ fixation in plants along with enzymes involved. 20 (c) Briefly discuss the pathways of carbohydrate metabolism in plants. 10

हिंदी में प्रश्न पढ़ें

(a) विभिन्न प्रकाश-संश्लेषी वर्णकों को सूचीबद्ध कीजिए । पर्णहरित संश्लेषण का विस्तृत विवरण इस प्रक्रिया में शामिल प्रकिण्वों (एन्जाइम्स) सहित दीजिए । प्रकाश-संश्लेषण में कैरोटिनॉइडों की भूमिका का भी उल्लेख कीजिए । 20 (b) पौधों में कार्बन डाइऑक्साइड स्थिरीकरण में कैल्विन चक्र तथा हैच-स्लैक चक्र की अभिक्रियाओं में शामिल प्रकिण्वों (एन्जाइम्स) सहित सूचीबद्ध कीजिए । 20 (c) पौधों में कार्बोहाइड्रेट उपापचय के मार्गों की संक्षेप में चर्चा कीजिए । 10

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Directive word: Enumerate

This question asks you to enumerate. 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 'enumerate' demands systematic listing with detailed elaboration. Structure: brief introduction on photosynthesis significance → Part (a): classify pigments (chlorophylls, carotenoids, phycobilins), detail chlorophyll synthesis pathway (ALA to chlorophyll a/b with enzymes: ALA dehydratase, porphobilinogen deaminase, Mg-chelatase, chlorophyll synthase), explain carotenoid photoprotection and accessory pigment roles → Part (b): contrast C3 (Calvin cycle: 3 phases with RuBisCO, phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase, RuBisCO activase) and C4 (Hatch-Slack: mesophyll PEP carboxylase, bundle sheath RuBisCO) cycles with cellular localization → Part (c): outline glycolysis, TCA cycle, pentose phosphate pathway, gluconeogenesis with their metabolic significance. Allocate ~40% words to (a), 35% to (b), 25% to (c). Conclude with integration of these processes in crop productivity.

Key points expected

  • Part (a): Complete classification of photosynthetic pigments (chlorophyll a/b/c/d, bacteriochlorophylls, carotenoids: carotenes and xanthophylls, phycobilins: phycoerythrin and phycocyanin)
  • Part (a): Detailed chlorophyll biosynthesis pathway from glutamate/ALA to chlorophyll a and b, naming key enzymes at each step (ALA dehydratase, porphobilinogen deaminase, uroporphyrinogen III decarboxylase, coproporphyrinogen oxidase, protoporphyrinogen oxidase, Mg-chelatase, Mg-protoporphyrin IX methyltransferase, divinyl chlorophyllide a 4-vinyl reductase, chlorophyll synthase, chlorophyll a oxygenase for chlorophyll b)
  • Part (a): Carotenoid functions—accessory light harvesting (400-550 nm), photoprotection via xanthophyll cycle (violaxanthin→antheraxanthin→zeaxanthin), quenching triplet chlorophyll and singlet oxygen, structural stabilization of LHCII
  • Part (b): Calvin cycle three phases—carboxylation (RuBisCO: 3RuBP + 3CO₂ → 6PGA), reduction (PGA kinase + G3P dehydrogenase using ATP and NADPH to form G3P), regeneration (aldolase, FBPase, SBPase, phosphoribulokinase to regenerate RuBP); stoichiometry of 3CO₂ → 1G3P
  • Part (b): Hatch-Slack cycle spatial separation—mesophyll: PEP carboxylase (PEPC) fixes CO₂ to oxaloacetate → malate/aspartate; bundle sheath: malate decarboxylated to release CO₂ for Calvin cycle, pyruvate returned to mesophyll via pyruvate phosphate dikinase (PPDK); enzymes: NADP-ME or NAD-ME types
  • Part (c): Carbohydrate metabolism pathways—glycolysis (EMP pathway) to pyruvate, TCA cycle (Krebs cycle) for complete oxidation, oxidative pentose phosphate pathway (OPPP) for NADPH and ribose-5-phosphate, gluconeogenesis for sucrose/starch synthesis, glyoxylate cycle in oilseeds; integration with photosynthate partitioning

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Demonstrates precise biochemical knowledge: correct enzyme nomenclature (e.g., Mg-chelatase not Mg-chelatase complex generically), accurate stoichiometry (3ATP + 2NADPH per CO₂ in Calvin cycle), distinguishes C3 vs C4 carbon fixation mechanisms correctly, identifies correct cellular compartments (stroma vs cytosol vs bundle sheath)Basic pathways described but with minor errors: confuses chlorophyll synthesis steps, misidentifies some enzymes, general description of C3/C4 without precise enzymatic or stoichiometric details, occasional compartmentalization errorsMajor conceptual errors: confuses photosynthetic pigments categories, fundamentally wrong pathway sequences, incorrect enzyme-substrate relationships, conflates C3 and C4 cycles, significant misinformation about metabolic pathways
Quantitative reasoning15%7.5Provides accurate stoichiometric relationships: Calvin cycle (3CO₂:1G3P:9ATP:6NADPH), C4 energy cost (5ATP per CO₂ vs 3ATP in C3), quantum efficiency comparisons, numerical ratios in chlorophyll synthesis (e.g., 8 molecules ALA → 1 porphyrin ring), comparative yieldsMentions some quantitative aspects but inconsistently: rough ATP/NADPH mentions without precise ratios, general awareness of C4 being more efficient without numerical backing, incomplete stoichiometric treatmentAbsent or incorrect quantitative treatment: wrong stoichiometry, confuses energy requirements, no numerical data where essential for comparing pathways
Indian context examples15%7.5Integrates relevant Indian agricultural examples: C4 crops (maize, sorghum/jowar, pearl millet/bajra, sugarcane) vs C3 crops (rice, wheat, pulses) in Indian cropping systems; mentions IARI research on C4 photosynthesis improvement; references to Indian conditions (high temperature, light intensity favoring C4); examples of carotenoid-rich crops (carrot, mango, papaya) and their nutritional significanceGeneric mention of common crops without specific Indian context; may list rice/wheat without linking to C3 physiology relevant to Indian agriculture; superficial treatment of Indian relevanceNo Indian examples; entirely generic treatment ignoring subcontinental agricultural diversity, climate relevance, or research contributions
Diagram / process25%12.5Includes well-labeled diagrams: chlorophyll structure with porphyrin ring, phytol tail, Mg center; chlorophyll synthesis pathway flowchart; Calvin cycle with carbon numbering and phase demarcation; C4 pathway showing Kranz anatomy with mesophyll-bundle sheath spatial organization; carbohydrate metabolism integration diagram; clear carbon tracing through reactionsDescribes diagrams in text without drawing them, or sketches incomplete diagrams; mentions structures but without systematic labeling; describes processes sequentially without visual integrationNo diagrammatic representation; purely descriptive text without structural or process visualization; confusing sequential description without clarity
Policy / extension angle20%10Connects to agricultural applications: C4 rice project (IRRI-IARI collaboration) for improving photosynthetic efficiency; implications for climate-resilient agriculture; biofortification strategies for carotenoid enhancement (Golden Rice, orange-fleshed sweet potato); precision agriculture for optimizing light interception; crop breeding for improved RuBisCO efficiency; relevance to India's food security and climate adaptation policiesBrief mention of breeding or biotechnology without specific policy linkage; general statement on importance for yield improvement without concrete programs or strategiesNo application or policy dimension; purely academic treatment without relevance to agricultural improvement, extension, or national priorities

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