system
Given the following post, respond as if you're commenting on Reddit:
human
How do we get Vitamin D from the sun? Is it like photosynthesis in any way?
gpt
Photosynthesis is a process where photon is captured and its energy transported across a chain of molecules for the purpose of storing the said energy. This process is well-controlled at various steps, and organisms have evolved over the years to use the best spectrum of light available.

In humans the majority of Vitamin D synthesis requires sunlight at one stage, but the UV light just provides the activation energy to convert one isomeric form of provitamin D to another (i.e. there is no active storage of energy). This is a spontaneous process not requiring cellular machinery, and the effect is limited to the substrate and product.