Botany 2021 Paper II 50 marks Describe

Q6

(a) Describe C₄ cycle of photosynthesis and differentiate amongst C₃, C₄ and CAM plants. 10+10=20 (b) Define Vernalization. Discuss its mechanism and importance. 5+10=15 (c) What is biological nitrogen fixation ? Describe root nodule formation and role of nitrogenase complex in fixing of nitrogen. 5+5+5=15

हिंदी में प्रश्न पढ़ें

(a) प्रकाश-संश्लेषण के C₄ चक्र का वर्णन कीजिए और C₃, C₄ तथा CAM पौधों के बीच अंतर स्पष्ट कीजिए । 10+10=20 (b) वसंतीकरण की परिभाषा दीजिए । इसकी क्रियाविधि एवं महत्व की विवेचना कीजिए । 5+10=15 (c) जैविक नाइट्रोजन स्थिरीकरण क्या है ? जड़ों पर प्रथिका निर्माण तथा नाइट्रोजन स्थिरीकरण में नाइट्रोजिनेस कॉम्प्लेक्स की भूमिका का वर्णन कीजिए । 5+5+5=15

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Directive word: Describe

This question asks you to describe. 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 'describe' demands detailed, systematic exposition of processes and phenomena. Allocate approximately 40% of time/words to part (a) given its 20 marks, with 30% each to parts (b) and (c). Structure: begin with concise definitions, proceed to detailed biochemical/physiological mechanisms for each part, integrate relevant diagrams, and conclude with agricultural/ecological significance. For (a), emphasize the Hatch-Slack pathway and comparative table; for (b), focus on the vernalin/FT protein mechanism; for (c), detail the infection thread-nodule ontogeny sequence.

Key points expected

Part (a): C₄ cycle with correct identification of mesophyll and bundle-sheath cell reactions, PEP carboxylase and RuBisCO localization, and clear distinction of Kranz anatomy
Part (a): Systematic differentiation of C₃, C₄ and CAM plants across 6-8 parameters (CO₂ acceptor, first stable product, photorespiration, water use efficiency, stomatal behavior, geographical distribution)
Part (b): Precise definition of vernalization as low-temperature induction of flowering; molecular mechanism involving VIN3, FLC repression and FT activation; agricultural importance in wheat and sugar beet breeding
Part (c): Definition of BNF distinguishing symbiotic vs. asymbiotic; detailed root nodule formation sequence from rhizobial chemotaxis to nodule organogenesis via infection thread
Part (c): Nitrogenase complex structure (Mo-Fe protein and Fe protein), 16 ATP per N₂ reduced, anaerobic protection via leghemoglobin, and specific nod/nif/fix gene functions

Evaluation rubric

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	20%	10	Accurately states that C₄ plants use PEP carboxylase in mesophyll and RuBisCO in bundle-sheath cells; correctly identifies OAA→malate/aspartate shuttle; defines vernalization as quantitative/qualitative low-temperature requirement; distinguishes BNF from industrial fixation; no confusion between photorespiration and nitrogenase oxygen sensitivity	Generally correct but conflates C₄ and CAM as 'both use PEP carboxylase' without temporal/spatial distinction; vague on vernalization temperature ranges; mentions nitrogenase but unclear on ATP requirement or anaerobic conditions	Fundamental errors: states C₄ plants lack RuBisCO; confuses vernalization with stratification; describes nodule formation without mentioning infection thread or believes nitrogenase functions in aerobic conditions
Diagram / labelling	20%	10	Three distinct, well-labelled diagrams: (a) Kranz anatomy cross-section with Calvin and Hatch-Slack cycles spatially separated; (b) vernalization pathway showing shoot apex transition; (c) root nodule development stages with infection thread and bacteroid zone; all enzymes and cell types clearly indicated	Two adequate diagrams with minor labelling omissions; or one excellent diagram with others described textually only; diagrams present but missing key labels (e.g., PEP carboxylase location, leghemoglobin)	No diagrams or schematic sketches without structural accuracy; diagrams copied without relevance to question; misplaces mesophyll/bundle-sheath functions in C₄ diagram
Examples & nomenclature	20%	10	Indian examples: C₄—Sorghum, Pearl millet (bajra), Maize; CAM—Kalanchoe, Agave; vernalization—wheat varieties (e.g., Sonalika vs. Kalyan Sona), sugar beet; BNF—Rhizobium-legume symbiosis (Cicer arietinum, Cajanus cajan), Frankia-Casuarina; correct gene nomenclature (VIN3, FLC, FT, nodABC, nifHDK)	Generic examples (maize, cactus, clover) without Indian specificity; mentions rhizobia but not specific host; some gene names mentioned but confused (e.g., nif vs. nod functions)	No specific examples or incorrect assignments (e.g., rice as C₄; pineapple as C₃); confuses Azotobacter with Rhizobium; no mention of any genes or enzymes by name
Process explanation	20%	10	Sequential, mechanistic detail: (a) CO₂→HCO₃⁻ fixation by PEPcase, 4-carbon acid transport, decarboxylation releasing CO₂ for Calvin cycle, pyruvate return with ATP expenditure; (b) epigenetic silencing of FLC by VIN3-PRC2 complex, photoperiod pathway integration; (c) flavonoid induction of nod genes, Nod factor perception, cortical cell dedifferentiation, bacteroid differentiation, ammonium export via GS-GOGAT	Broadly correct sequence but missing key intermediates (e.g., no mention of malate/pyruvate shuttle; vernalization described as 'cold treatment' without molecular mechanism; nodule formation as 'bacteria enter roots' without infection thread detail)	Descriptive without mechanism; lists steps without causal connections; significant gaps in sequence (e.g., describes C₄ as 'just different pathway' with no spatial separation; BNF as 'bacteria fix nitrogen' with no plant involvement)
Application / ecology	20%	10	Links C₄ adaptation to tropical Indian agriculture and climate change resilience; vernalization to wheat breeding programmes and double-cropping systems; BNF to organic farming, reduced urea dependency, and Mycorrhiza-Rhizobium co-inoculation in pulses; mentions C₄ rice project and biofertilizer industry (e.g., Rhizobium inoculants for groundnut in Gujarat)	Generic agricultural importance stated without Indian context; mentions 'biofertilizers' or 'crop improvement' without specific programmes; ecological significance of CAM in arid zones mentioned superficially	No application or ecological discussion; or irrelevant applications (e.g., genetic engineering not asked); concludes with 'important for agriculture' without elaboration

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