Unit 6

• Occurrence of Vitamin B12 in Nature:
• Vitamin B12 (Cobalamin) is unique among all vitamins because it is synthesized exclusively by anaerobic bacteria and archaea.
• Higher plants do not synthesize and do not require Vitamin $B_{12}$, meaning it is completely absent from unfortified food of plant origin (grains, legumes, fruits, vegetables).
• Animals obtain Vitamin $B_{12}$ by consuming animal products (liver, meat, milk, eggs) or through symbiotic gastrointestinal/ruminal microbial synthesis.
• nalysis:
• (Vitamin B12): Correct vitamin absent in plant sources.

Correct option: (3) Vitamin B12

• Antioxidant Action of $\alpha$-Tocopherol:
• Assertion (A): Vitamin E is the primary lipid-soluble chain-breaking antioxidant protecting cellular and subcellular membranes against oxidative damage. (True).
• Reason (R): The chromanol ring of Vitamin E donates a phenolic hydrogen atom to lipid peroxyl radicals ($LOO^\bullet$), converting them into stable lipid hydroperoxides ($LOOH$) and quenching free radical propagation chains in lipid bilayers. (True).
• nalysis:
• Both (A) and (R) are correct, and (R) provides the definitive chemical explanation for (A). Hence, Option 1 is correct.

• Evaluation of Statements:
• Assertion (A): Hormones bind to receptors present on the cell surface or inside the cells.
• True. Hydrophilic hormones (peptides, catecholamines) bind to transmembrane cell-surface receptors (GPCRs, RTKs), while lipophilic hormones (steroids, thyroid hormones) cross the plasma membrane to bind intracellular cytoplasmic or nuclear receptors.
• Reason (R): All hormones are protein in nature.
• False. Hormones exhibit diverse chemical structures:

• Steroid hormones (estrogens, progesterone, testosterone, cortisol, aldosterone) are lipids derived from cholesterol.
• Amine hormones (epinephrine, norepinephrine, thyroxine, melatonin) are derived from single amino acids (tyrosine, tryptophan).
• Eicosanoids (prostaglandins) are derived from fatty acids.
  • Therefore, claiming all hormones are protein in nature is false.

• nalysis:
• ((A) is correct but (R) is not correct): Correct.

• Cellular Differentiation:
• The developmental process whereby a generic, unspecialized stem or progenitor cell undergoes selective, coordinated gene expression changes to develop stable morphological and biochemical attributes, adopting a specialized physiological role (e.g., neuron, hepatocyte, cardiomyocyte, erythrocyte).
• nalysis:
• (adoption of a specialized physiological role by a cell): Correct definition.

Correct option: (4) adoption of a specialized physiological role by a cell