Retinal is also known as retinaldehyde. Retinal is one of the many forms of vitamin A. Retinal is bound to proteins called opsins and is the chemical basis of animal vision. Retinal allows certain microorganisms to convert light into metabolic energy. Opsins are a group of light-sensitive proteins found in photoreceptor cells of the retina. Vision begins with the photoisomerization of retinal. When the 11-cis-retinal chromophore absorbs a photon it isomerizes from the 11-cis state to the all-trans state. The absorbance spectrum of the chromophore depends on its interactions with the opsin protein to which it is bound.