(a) What are intestinal glands? Describe the roles of intestinal juices in digestion and absorption of food. 20

(b) Define thermoregulation. How do homeotherms regulate body temperature in hot and cold climates? 15

(c) Draw the structure of retina

Question

(a) What are intestinal glands? Describe the roles of intestinal juices in digestion and absorption of food. 20

(b) Define thermoregulation. How do homeotherms regulate body temperature in hot and cold climates? 15

(c) Draw the structure of retina and explain the mechanism of vision in mammalian eye. 15

UPSC Answer Check · Accepted Answer

The directive 'describe' demands detailed, systematic exposition of structures and processes across all three sub-parts. Allocate approximately 40% of time/words to part (a) [20 marks], and 30% each to parts (b) and (c) [15 marks each]. Structure: brief introduction acknowledging the interconnected nature of physiological systems; body with three clearly demarcated sections addressing each sub-part sequentially; no separate conclusion needed but ensure cross-references where relevant (e.g., neural control linking thermoregulation and vision). For part (c), the diagram must be drawn first, then explained.
- Part (a): Crypts of Lieberkühn as simple tubular glands; distinction between duodenal (Brunner's) glands and intestinal glands; composition of intestinal juice (succus entericus) including peptidases, saccharases, lipase, nucleosidases; role in final carbohydrate and protein digestion; absorption mechanisms via villi microstructure
- Part (b): Definition of thermoregulation as maintenance of thermal homeostasis; distinction between poikilothermy, homeothermy, and heterothermy; heat gain mechanisms in cold climates (vasoconstriction, piloerection, shivering thermogenesis, non-shivering thermogenesis via brown adipose tissue in Indian Himalayan mammals like Himalayan tahr)
- Part (b continued): Heat loss mechanisms in hot climates (vasodilation, sweating, panting, postural adjustments); hypothalamic set point and negative feedback control; specific Indian examples (desert fox, blackbuck adaptations)
- Part (c): Retinal layers correctly sequenced from choroid outward: pigment epithelium, photoreceptor layer (rods and cones), external limiting membrane, outer nuclear layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer, nerve fiber layer, internal limiting membrane; fovea centralis and optic disc identification
- Part (c continued): Phototransduction cascade: photon absorption by rhodopsin/iodopsin → conformational change → transducin activation → phosphodiesterase → cGMP reduction → sodium channel closure → hyperpolarization → glutamate release modulation → bipolar cell signaling → ganglion cell action potential → optic nerve transmission; dark and light adaptation mechanisms

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	25%	12.5	Demonstrates precise anatomical knowledge: for (a) correctly identifies intestinal glands as crypts of Lieberkühn with accurate histological classification; for (b) distinguishes homeothermy from endothermy and provides accurate hypothalamic set point values; for (c) correctly orders all ten retinal layers without inversion and accurately describes phototransduction biochemistry including cGMP dynamics	Identifies major structures correctly but makes minor errors: confuses Brunner's glands with intestinal glands in (a); omits brown adipose tissue in (b); inverts some retinal layers in (c) or omits one layer; describes phototransduction in general terms without specific second messengers	Fundamental conceptual errors: describes intestinal glands as salivary gland analogues; confuses homeotherms with poikilotherms; draws retina as single layer or reverses entire layer sequence; describes vision as 'image projection' rather than phototransduction
Diagram / labelling	15%	7.5	For part (c), produces a large, neat cross-sectional diagram of retina occupying at least half page; all ten layers clearly demarcated with boundary lines; labels include directional markers (choroid side vs. vitreous side); photoreceptor ultrastructure shows outer/inner segments; fovea and optic disc indicated; no line crowding; uses pencil for construction lines	Diagram present but incomplete: shows 6-8 layers with some omissions; labels crowded or arrows unclear; lacks directional orientation; scale disproportionate; drawn in pen with errors uncorrected; fovea or optic disc missing	No diagram attempted, or diagram is schematic/wrong structure (e.g., whole eye section instead of retina); labels point to wrong structures; diagram too small (<1/4 page); illegible handwriting; no attempt at layered organization
Examples & nomenclature	15%	7.5	Uses precise terminology throughout: 'crypts of Lieberkühn,' 'enterokinase,' 'succus entericus,' 'non-shivering thermogenesis,' 'brown adipose tissue,' 'rhodopsin,' 'iodopsin,' 'transducin,' 'bipolar cells'; cites Indian fauna for thermoregulation (Himalayan tahr, Nilgiri tahr, desert cat); mentions specific enzymes (erepsin, maltase, sucrase, lactase, enterokinase)	Uses generic terms: 'intestinal juice' without 'succus entericus,' 'digestive enzymes' without specificity; examples limited to common laboratory animals (rat, rabbit) or no Indian examples; mixes rod/cone function; uses 'vitamin A' without retinal/retinol distinction	Incorrect or invented terminology: 'intestinal acid,' 'thermal glands,' 'eye nerves'; no specific examples; confuses anatomical terms (e.g., 'cornea' for 'retina'); misspells critical terms repeatedly
Process explanation	25%	12.5	For (a), explains enzyme activation cascade (enterokinase → trypsinogen → trypsin → other zymogens) and membrane transport mechanisms (active transport, facilitated diffusion, pinocytosis); for (b), describes hypothalamic negative feedback loop with preoptic anterior hypothalamus and posterior hypothalamus roles; for (c), details phototransduction as G-protein coupled receptor pathway with specific ion channel dynamics and signal convergence in retina	Describes processes in correct sequence but lacks mechanistic detail: states 'enzymes digest food' without activation cascade; mentions 'hypothalamus controls temperature' without set point mechanism; describes 'light hits retina and signal goes to brain' without phototransduction biochemistry	Processes described backwards or with fundamental errors: absorption precedes digestion; heat loss mechanisms described for cold adaptation; 'rods see color, cones see black and white'; no understanding of neural signal generation
Evolutionary / applied context	20%	10	For (a), mentions clinical relevance: celiac disease villous atrophy, lactose intolerance enzyme deficiency; for (b), discusses Bergmann's and Allen's rules with Indian subspecies examples; evolutionary significance of endothermy in mammals; for (c), explains fovea evolution in primates for visual acuity; mentions retinitis pigmentosa or age-related macular degeneration; connects nocturnal vs. diurnal eye structure differences	Brief mention of applied aspects without elaboration: 'diarrhea affects absorption' for (a); 'animals adapt to climate' for (b); 'blindness' for (c) without specific condition; no evolutionary perspective	No applied or evolutionary context provided; or completely irrelevant connections (e.g., Darwin's finches for intestinal glands); confuses adaptation with acclimatization

Q7

Directive word: Describe

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