Only UVB sunlight (280–315 nm) can penetrate the upper epidermis and reach the basal layer.Visible light and UVA can't — this is why sunlight through glass doesn't make D: glass blocks UVB.
2 · B-ring opens
7-DHC catches a UVB photon, a C–C bond in its B ring breaks, and the molecule opens into previtamin D3.7-dehydrocholesterol (7-DHC) in the skin catches a UVB photon. The energy breaks a specific C–C bond in its B ring — the molecule goes from closed to open, becoming previtamin D3. No enzyme needed; pure photochemistry.
3 · → cholecalciferol
Unstable previtamin D3 spontaneously rearranges at skin temperature into stable over a few hours.Previtamin D3 is unstable and spontaneously rearranges at skin temperature into stable (cholecalciferol) — what supplement labels call Vitamin D3. Again no enzyme needed; temperature-driven isomerisation taking a few hours.
4 · Into the blood
enters the blood through dermal capillaries, binds vitamin D binding protein and is carried to the liver. enters circulation via dermal capillaries below the skin, binds to vitamin D binding protein (), and is transported to the liver — the story's next stop. A single full sun exposure provides enough D3 to maintain plasma for several days.