Green photoreceptors

Post-ingestion from a-, (3- y-carotene other carotenes from plant leaves a wide variety of fruit, root seed sources e.g. Daucus carota (carrot) (Apiaceae) [root] Retinal covalently linked to opsins (— light receptor Rhodopsins in vision) colour blind John Dalton (atomic theory, 1766-1844) bequeathed his eyes to science 2 centuries on molecular biology confirmed the absence of the gene for the green photoreceptor opsin... 

The molecular basis of most of these processes is not yet clear. Most of what we know concerns the absorption of quanta by the photoreceptor pigments, while stimulus transformation and signal transmission have as yet been investigated only in part and in a very few organisms. In blue-green algae, not even the movement mechanism is clear, so that any model of the motor response is highly speculative. 

Cahill We did an action spectrum for suppression of melatonin in IKenopus retina. We tried to deal with this variability issue. We got some estimates from our all-fit program of 95% confidence intervals which we did plot on the action spectrum. When we did this we were fairly disappointed with what we were able to exclude as possible photoreceptors. We were able to eliminate the 650 nm cones, but anything from 500 nm down we couldn t really throw out. When we plotted the actual points they fell very nicely on the green opsin. The peak could have been over a range of 40 nm. 

Figure 5.5.10-11 Wald figure 4 with overlay. Wald used the curve labeled green adapted to justify his claim that the long wavelength photoreceptor had a peak absorption at 575 nm. The short dashed curve through the same points contains k, km k 1000 110 275. The peak near 575-580 nm is actually due to the Bezold Effect. The alternate (long dashes) adaptation curve correctly isolates the peak absorption of the L-channel at 625 nm. See text.

Our retina has red, green, and blue cones which include rhodopsins as photoreceptors [6-8], Phytochromes are photo-sensors of green plants [9], Biological luminescences from fireflies [10] and some jellyfishes [11] are also beautiful activities of living organism. Recently, fluorescent proteins are routinely applied as molecular markers for gene expression in the field of molecular biology [12]. 

Fig. 3. Examples of natural photoantenna chromophores (2) 5,10-methenyltetrahydrofolate (MTHF), a blue light photoreceptor pigment present in photolyase and some cryptochromes (3) Pheophytin a, the primary electron acceptor in cyanobacterial oxygenic photosynthesis. (4) 11-cis-retinal, which is involved as sensory photoreceptor component in the opsin-based visual process of animals and (5) the p-hydroxy-benzylidene-imidazolinone chromophore (HBDI) of the green fluorescent protein from bioluminescent marine species.

Cone cells, like rod cells, contain visual pigments. Like rhodopsin, these photoreceptor proteins are members of the 7TM receptor family and utilize 11-cA-retinal as their chromophore. In human cone cells, there are three distinct photoreceptor proteins with absorption maxima at 426, 530, and 560 nm (Figure 32.26). These absorbances correspond to (in fact, define) the blue, green, and red regions of the spectrum. Recall that the absorption maximum for rhodopsin is 500 nm. 

The amino acid sequences of the cone photoreceptors have been compared with each other and with rhodopsin. The result is striking. Each of the cone photoreceptors is approximately 40% identical in sequence with rhodopsin. Similarly, the blue photoreceptor is 40% identical with each of the green and red photoreceptors. The green and red photoreceptors, however, are > 95% identical with each other, differing in only 15 of 364 positions (Figure 32.27). 

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