The retinol form functions as a storage form of the vitamin, and can be converted to and from its visually active aldehyde form, retinal. The associated
acid (retinoic
acid),
a metabolite that can be irreversibly synthesized from
vitamin A, has only partial vitamin A activity, and does not function in the retina or some essential parts of the reproductive system.
All forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a retinyl group.