Assertion (A): AIDS is caused by a retrovirus. Reason (R): HIV contains RNA and the enzyme reverse transcriptase. MCQ with Answer and Explanation

Explanation:
Biotin (vitamin B₇) is a water-soluble B vitamin that functions as a coenzyme for carboxylase enzymes, important in fatty acid synthesis and gluconeogenesis. Vitamin B₁ acts in decarboxylation, A and K are fat-soluble. Biotin is widely distributed in foods and also synthesized by gut bacteria.

Explanation:
Vitamin D3 (cholecalciferol) is synthesized in the epidermis of the skin when 7-dehydrocholesterol is exposed to ultraviolet B (UVB) radiation from sunlight. Vitamin B12 (cobalamin) is unique among vitamins as it contains the metal ion cobalt in its structure. Vitamin K, not Vitamin A, is essential for the synthesis of blood clotting factors. Vitamin A is crucial for vision and immunity. Thus, statements 1 and 2 are correct.

Explanation:
The tonoplast is the single membrane enclosing the central vacuole in plant cells. It regulates the movement of ions, sugars, and waste products into and out of the vacuole, maintaining turgor pressure. The nuclear membrane is the nuclear envelope, mitochondria have inner/outer membranes, chloroplasts have a double membrane.