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Carotene and Vitamin A
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Phototropism is the tendency to be attracted to light and in the context of this site refers to this tendency in plants. In plants, the wavelengths of light that plants employ for phototropism is different than that used in photosynthesis so it seems logical that a different photoactive pigment is responsible. The red wavelengths of sunlight are used in photosynthesis whereas phototropism appears to use the wavelengths found in the blue, violet and green ends of the spectrum. Thus, the pigments used for phototropism must be a yellow color as this pigment would be absorbing that end of the spectrum from sunlight. All plants that exhibit phototropism have these pigments, known as carotenes.
It is also known that some invertebrates exhibit phototropism. Some sessile hydroids, for example, preferentialy grow toward the sun. These animals appear to be sensitive to the same wavelengths of light as plants and use carotenes as well to provide the organism with the ability to detect light and grow differentially in response. This is an important clue in understanding the basis of vision. |
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| This background has all led us up to being able to understand the basis of vision. First it is worth noting that of the all the major phyla in the world, only 3 have developed image-resolving eyes. It is also believed that these phyla developed their vision systems independantly of one another yet all three utilize the same basic chemical principles as the basis for vision.
George Wald was the pioneer in this discovery of the chemical basis of vision. He found that Vitamin A plays a key role in vision. Vitamin A, in turn is, exactly one half of a carotene molecule with an hydrogen and a hydroxyl (OH) attached to the broken end. It's no accident that we emply this same chemical in vision that plants use for phototropism. In fact, we still rely on plants for this chemical. Animals, humans included, cannot make their own vitamin A. The adage about eating carrots for enhanced vision has it's basis on our dependance on carotene to make vitamin A and one of the effects of vitamin A deficiency is, in fact, loss of vision. But what exacctly does vitamin A do? |
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The short term answer is that it turns into retinene.
George Wald discovered that it was a derivative of vitamin A, called retinene which is bound to a protein belonging to a group called opsins that is the key to vision. This opsin/retinene complex holds the key to how light can be passed from the physical realm outside the call to become some sort of signal within the body. This point is worth repeating. Think of what vision is. Photons of light are all around you as you read this. You see the screen and you read this text. When you close your eyes, of course, all goes dark. The photons, billions of them, are still out there, crashing into your eyelids. But the cells the compose your eyelids have no means to "record" the event of a photon striking them. The phone either bounces off (is reflected), passes through (in which case you would see light) or is wholly or partially absorbed as heat. There is no mechanism to pass this event on to your brain other than eventually feeling warmth if the light intensity is sufficient.
Vision is quite different. It requires a mechanism to turn the physical energy contained in a quanta of light into an electrochemical signal that can reach your brain. In order to understand this, we need to take one more small detour to understand a chemical concept known as isomerization. |
References used in the Vision sections of this site. Hartline., H.K., Wagner, H.G., Ratcliff, Floyd, Inhibition in the Eye of Limulus, Journal of General Physiology, 1956, 39:5 pp.651-673 Westerman, L.A., Barlow, R.B., Ultraviolet responses of the Limulus mediann ocellus, Biological Bulletin, 1981 161 352-353 Barlow, R.B., Ireland, C.I., Kass, L., Vision in Limulus mating behavior, Biological Bulletin, 1981 161 339-340 Powers, M.K., Barlow, R.B., Circadian changes in visual sensitivity of Limulus: behavioral evidence, Biological Bulletin, 1981 161 350-351 Hubbard, Ruth. Retinene Isomerase, Journal of General Physiology, Vol 39, No. 6 pp.935-962 Wald, G., Human Vision and the Spectrum, Science, 1945, 101, 653 Wald, G., Life and Light, Scientific American, Oct. 1959, pp 92-108 Invertebrate Photoreceptors, A Comparative Analysis, Jerome J. Wolken, Academic Press, NY, 1971 Kimbel, R.L., Poincelot, R.P., Abrahamson, E.W., Chromophore Transfer from Lipid to Protein in Bovine Rhodopsin, Biochemistry 1970 9:8 1817 Westerman, L.A, Barlow, R.B, Ultraviolet responses of the Limulus median ocellus, Biological Bulletin General Scientific Meetings. 161:3 352-353 Barlow, R.B, Ireland, L.C., Cass, L., Viision in Limulus mating behavior, Biological Bulletin General Scientific Meetings. 161:3 339-340 Sargent, William., The Year of the Crab., W.W. Norton & Company 1987