Eat Your Carrots! The Chemistry of Vision

 

18th-century hand-colored etching of woman pushing wheelbarrow full of carrots.

“Sandwich Carrots-Dainty Sandwich Carrots.” Hand-colored etching. Gillray, James, 1756-1815, engraver. Published by H. Humphrey, 1796 Dec 3d, London.
Image courtesy of Library of Congress.

You’ve probably heard the old adage about eating carrots for good vision. Well, there is some truth to it. Carrots contain a high concentration of β-carotene which gets broken down in the intestines to form the aldehyde (hydrocarbon) form of vitamin A, cis-retinal. Vision deteriorates in the absence of vitamin A because cis-retinal is trafficked along the protein, opsin, to produce electrochemical signals from light.

Our retinas perceive light in tiny particles called photons. As soon as these photons hit the retina, they isomerize cis-retinal to trans-retinal.  Trans-retinal then bonds to opsin to form rhodopsin. Rhodopsin is a purple pigment in the photoreceptor cells of the retina that reads blue-green light. This is the first step of the phototransduction cycle where photon energy is transferred to a series of signaling and diffusing protein complexes.

Retinal isomerism drawn with ChemDraw

Mutated forms of rhodopsin will be folded and transported differently and could lead to deteriorated vision or blindness. In more rare cases, mutations can cause rhodopsin to be constantly activated, even in the absence of light. Hypersensitivity, autoimmune disorders, and mutations can all cause rod cells in the retina to undergo apoptosis or cellular self-destruction. This sort of degradation of the retina will ultimately lead to deteriorated vision and eventually blindness.

The absorbance of cis-retinal is optimized at approximately 100 nanometers less than rhodopsin and it is a very rigid molecule because of the arrangement of its double bonds. Thanks to isomerism, we can see in color as opposed to ultraviolet! As all of the above demonstrates, our ability to see involves a series of complicated and precisely regulated bio-chemical processes, and carrots play their role.

We will be exploring more about vision loss and blindness in the upcoming GLIMPSE issue 10, Blindness. In the meantime, let us know your thoughts, research, questions, or experiences related to the topic.

If you’re interested in the chemistry of vision and why we perceive the section of the electromagnetic spectrum that we do, you might also be interested in GLIMPSE, issue 4, Color, and the article on “Human Potential for Tetrachromacy” by Kimberley A. Jameson and the online supplementary article.

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Myya McGregory is the GLIMPSE 2012 Science Writing Intern. She is a junior double-majoring in chemistry and economics at Williams College. She enjoys music, dance, and literature.

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