This document discusses vitamin D3 (cholecalciferol). It describes the biochemical structure of vitamin D3, including its stable ring A conformation. It highlights the functional groups of vitamin D3, including the triene functional group and hydroxyl functional group. It then relates the functions of these groups to vitamin D3's role in calcium metabolism and its ability to act as a steroid hormone. Specifically, the hydroxyl group allows it to interact with ligands and regulate calcium metabolism, while the triene group gives it a non-polar structure allowing it to be fat-soluble and act as a steroid.