Unit 4

• Stepwise Biosynthesis of Active Vitamin D:

• Skin: 7-dehydrocholesterol is converted by solar UV light into cholecalciferol (Vitamin $D_3$).
• Liver: Cholecalciferol undergoes 25-hydroxylation by hepatic 25-hydroxylase to form 25-hydroxyvitamin $D_3$ [calcidiol, $25(OH)D_3$].
• Kidney: Calcidiol is transported to the proximal convoluted tubules of the Kidney, where the mitochondrial enzyme $1\alpha$-hydroxylase (stimulated by parathyroid hormone) adds a second hydroxyl group to produce 1,25-dihydroxyvitamin $D_3$ [Calcitriol, $1,25(OH)_2D_3$].
  • Calcitriol is the biologically active hormonal form released by the kidney into the systemic circulation to regulate calcium and phosphate homeostasis.

• nalysis:
• (Kidney): Correct organ releasing active calcitriol.

• Thermoregulatory Thermal Comfort Zones in Cattle:
• Upper Critical Temperature (UCT) is the ambient environmental temperature above which an animal must actively expend metabolic energy (sweating, panting) to dissipate heat and prevent lethal hyperthermia.
• Bos indicus (Zebu cattle - Hariana breed):
• Possess numerous large, functional sweat glands, sleek reflective coats, loose vascular skin with extensive dewlap/umbilical fold surface area, and lower basal metabolic rates.
• They have the highest UCT ($\approx 35 - 38^\circ\text{C}$) among cattle breeds.
• Bos taurus (Holstein Friesian, Jersey):
• Adapted to temperate climates; exhibit heavy heat stress with a lower UCT of $\approx 24 - 27^\circ\text{C}$.
• nalysis:
• (Hariana): Correct breed with the highest UCT.

Correct option: (2) Hariana

• Aromatic Amino Acids:
• Amino acids containing an aromatic ring in their side chain:
• Tyrosine: Phenolic ring.
• Tryptophan: Indole ring.
• Phenylalanine: Benzyl ring.
• Aspartic acid and (B) Glutamic acid are acidic dicarboxylic amino acids.
• nalysis:
• ((C) and (D) only): Correct.

Correct option: (4) (C) and (D) only.