Explain about the Vitamin D?

Vitamin D is a generic term and indicates a molecule of the general structure with  rings (A, B, C, D), as you may have noticed in Figure B. The ring structure is  derived from the cyclopentanoper hyclrophenenthrene ring structure for steroids. Ergocalsiferol (D₂), cholecaliferol (D₃) are the forms of vitamin D.

Vitamin D, can either be made in the skin from a cholesterol-like precursor (7-dehydrocholesterol) by exposure to sunlight or can be provided pre-formed in the diet. The version made in the skin is referred to as vitamin D₃ whereas the dietary form can be either vitamin D3 or a closely-related molecule of plant origin known as vitamin D2. Figure m shows the formation of Vitamin D₃ from its precursor 7-dehydrocholesterol. The synthesis of vitamin D₃ from its provitamin, 7-dehydrocholesterol, occurs by UV irradiation and proceeds from the provitamin to the previtamin and finally to the vitamin. The first main step of the reaction is the photochemical conversion of 7-dehydrocolesterol to previtamin D₃. This reaction yields upto 85% of previtamin D₃. The next and final step is accomplished by the thermal conversion of previtamin D, to vitamin D₃.