Assertion (A): Mycorrhizae are symbiotic associations between fungi and plant roots. Reason (R): The fungus provides water and minerals to the plant and receives sugars in return. MCQ with Answer and Explanation

Explanation:
Vitamin B₁₂ (cobalamin) contains a cobalt ion at the center of its corrin ring. It is the only vitamin that contains a metal. Iron is in heme, zinc in enzymes, magnesium in chlorophyll.

Explanation:
The proximal convoluted tubule reabsorbs nearly 100% of filtered glucose, amino acids, and vitamins via active and secondary active transport.

Explanation:
Cyclic adenosine monophosphate (cAMP) is a common intracellular second messenger produced by adenylyl cyclase upon hormone receptor activation (e.g., glucagon, adrenaline). It activates protein kinase A, triggering a cascade of cellular responses. Other second messengers include IP₃, DAG, Ca²⁺.