A number of compounds share the activity of vitamin A including retinol and retinal, the alcohol and aldehyde versions of vitamin A, respectively. Retinoic acid is also sometimes referred to as vitamin A, but is actually a metabolite of retinal. Retinol and retinal have β‑ionone rigs and hydrocarbon tails for their structures. Plant compounds called carotenoids also possess vitamin A activity. Carotenoids have similar structures to retinol and retinal, but are composed of two vitamin A molecules liked by their hydrocarbon tails. Cleavage of these twin structures in the intestinal mucosa releases a molecule of vitamin A. There are over 600 known plant carotenoids, and although many have vitamin A activity, some do not. Common carotenoids in food that possess vitamin A activity include β-carotene, α-carotene and γ-carotene. Xanthophyll and lycopene are yellow and red pigmented carotenoids, respectively, that do not possess vitamin A activity. In humans, vitamin A is required for growth and differentiation of cells, and in particular is required for correct function of the retina of the eye. Carotenoids also function as lipid soluble antioxidants.