VV LoRetinalOur body cannot make retinal from scratch; it’s made from closely
related molecules that we eat: vitamin A and carotenoids. Vitamin A,
also called retinol, differs from retinal by the addition of hydrogen to
the oxygen atom at the end of the chain (converting the aldehyde to an
alcohol). Carotenoids, such as beta-carotene, are widespread in plants
and are the most abundant chemical precursors to retinal and retinol
in nature. Beta-carotene is the molecule that gives carrots their orange
color.
SOR RRR RA RBSBeta-caroteneIt is found in many of the plants we eat: lettuce, kale, chard, pumpkin
and other squashes, spinach, sweet potatoes, and tomatoes, as well
as carrots. Our body knows how to take a molecule of beta-carotene,
cleave it in half, and chemically modify it a wee bit, yielding two mole-
cules of retinal.
When the retinal molecule is bound to the protein in rhodopsin
or one of the cone opsins, it occurs in a form (called the 11-cis isomer
of retinal) where the carbon-chain portion of the molecule is bent or
kinked. The absorption of a photon of light by the retinal molecule
triggers a change in the shape of the chain so that it rotates around the