Chlorophylls photoreceptors

Phototactic action spectra of Phormidium autumnale and Phormidium uncinatum, measured by Nultsch86>89), show prominent maxima in the absorption range of C-phycoerythrin and smaller, but distinct, peaks in the absorption range of C-phyco-cyanin. Red light absorbed by chlorophyll a is not active, while in the blue range absorbedby the Soret band, the action spectrum shows aminimum(Fig. 6). Nultsch87) concluded that biliproteins are photoreceptors of phototaxis, but independently of the photosynthetic electron transport and phosphorylation. 

Chlorophylls occur very frequently in the plant kingdom, they are responsible for the colour of vegetables and some fruits. They also occur in algae and several bacteria. Chlorophylls in plants are photoreceptors and in photosynthesis the presence of a closed circuit of conjugated double bonds allows them to absorb light. Because of their predominant importance as photoreceptors a considerable number of analytical methods have been developed for the separation and quantitative determination. The analytical methods applied for the measurement of chlorophylls and carotenoids in food products have been reviewed previously [273],... 

The second line of evidence relates to more recent metabolic innovations such as photosynthesis. LUCA, it seems, could not photosynthesize. No form of photosynthesis based on chlorophyll is found in any archaea. A completely different form of photosynthesis, based on a pigment called bacteriorhodopsin, similar to the photoreceptor pigments in our eyes, is practised by the so-called halobacteria, archaea that live in high-salt conditions. This mode of photosynthesis is not found in any bacteria. These disparate forms of photosynthesis presumably evolved indepen-dently in bacterial and archaeal lineages some time after the age of LUCA, and subsequently remained tied to their respective domains. If a metabolic innovation as important as photosynthesis did not cross from one domain to another, there is no reason to think that other forms of respiration would have done so. We should certainly be wary of postulating that respiratory genes crossed domains unless we have evidence that they did so and the evidence from evolutionary trees suggests that they did not. 

Airborne laser fluorosensing of chlorophyll (Chi) and phycoerythrine promises to be a valuable technique for the determination of phytoplankton abundance and distribution in water [7,8]. To enhance and control photosynthesis, most free drifting algae contain a number of so-called primary absorbing pigments. The absorbed excitation energy is transferred to Chi A, whose emission (centered at 685 nm) is predominantly observed. Alternatively, the emission of the strongly fluorescent photoreceptor phycoerythrine may be measured. 

List the structural components of chlorophyll a, and explain why chlorophylls are effective photoreceptors. 

The photoreceptor molecules used by different microorganisms for light perception vary significantly and fall in different classes including BLUE proteins, cryptochromes, phototropins, phytochromes, and rhodopsins. Other prokaryotic and eukaryotic organisms use photoactive yellow proteins (PYP) which contain a 4-hydroxycinnamate chromophore (21)., chlorophylls, carotenoids, phycobilins, and pterins. Hypericins have been found to be involved in photoorientation of ciliates (22). 

In the world of photosynthetic organisms, it is a totally different story. Bilins, though cataboHcally derived from heme or chlorophyll, are of primary importance as photoreceptors and photosynthetic light harvesters. Thus, phycocyanobilin and phycoerythrobilin, suitably attached to pol) p-... 

There is considerable interest in the reduction of bacteriorhodopsin, " the photoreceptor protein of halophilic bacteria, with C/1/2 = -1 V vs. SCE (0.1 M LiC104), and in its oxidation, with U1/2 = +0.6 V. This difference of 1.6 V, which is independent of pH, coincides with its fluorescence quantum energy stored. Therefore the carotenoid chromophore may be responsible for both the reduction and oxidation processes. In this respect bacteriorhodopsin is similar to chlorophyll " and its excited state permits electron transfer from a weak donor to a weak acceptor. 

Chlorophylls are the green pigments that act as the principal photoreceptor molecules of plants. They are capable of absorbing certain wavelengths of visible light that are then converted by plants into chemical energy. Two forms of these pigments... 

For both photosensor and photocoupling light quanta must be detected by specific receptor molecules. In blue-green algae and purple bacteria the same pigments, chlorophyll and bacteriochlorophyll respectively, serve as photoreceptors for photocoupling and phototaxis. Similarly bacterio-rhodopsin, the photosynthetic pigment of Halobacterium, also mediates a step-down photophobic response. Halobacterium also possesses a second... 

Chlorophyll is the photoreceptor molecule that traps this most elusive of all powers . It is found in the chloroplasts of green plants, and is what makes green plants, green. 

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