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Post by dipjyoti » Sun Sep 24, 2006 7:48 am

Chlorophyll is an essential component of photosynthesis, which helps plants get energy from the light. Chlorophyll molecules are specifically arranged in and around pigment protein complexes called photosystems, which are embedded in the thylakoid membranes of chloroplasts. In these complexes, chlorophyll serves two primary functions. The function of the vast majority of chlorophyll (up to several hundred per photosystem) is to absorb light and transfer that light energy by resonance energy transfer to a specific chlorophyll pair in the reaction center of the photosystems. Photosystem II and Photosystem I have their own distinct reaction center chlorophylls, named P680 and P700, respectively. These pigments are named after the wavelength (in nanometers) of their red-peak absorption maximum. The identity, function and spectral properties of the chlorophylls in each photosystem are distinct and determined by each other and the protein structure surrounding them. Once extracted from the protein into a solvent (such as acetone or methanol), these chlorophylls lose those distinctions and become a homogenous mixture of identical molecules.

The function of the reaction center chlorophyll is to use the light energy absorbed by and transferred to it from the other chlorophylls in the photosystems to undergo a charge separation, a specific redox reaction in which the chlorophyll donates an electron into a series of molecular intermediates called an electron transport chain. The charged reaction center chlorophyll (P680+) is then reduced back to its ground state by accepting an electron. In Photosystem II, the electron which reduces P680+ ultimately comes from the oxidation of water into O2 and H+ through several intermediates. This reaction is how photosynthetic organisms like plants produce O2 gas, and is the source for practically all the O2 in Earth's atmosphere. Photosystem I typically works in series with Photosystem II, thus the P700+ of Photosystem I is usually reduced, via many intermediates in the thylakoid membrane, by electrons ultimately from Photosystem II. Electron transfer reactions in the thylakoid membranes are complex, however, and the source of electrons used to reduce P700+ can vary. The electron flow produced by the reaction center chlorophylls is used to shuttle H+ ions across the thylakoid membrane, setting up a chemiosmotic potential mainly used to produce ATP chemical energy, and those electrons ultimately reduce NAD+ to NADPH a universal reductant used to reduce CO2 into sugars as well as for other biosynthetic reductions.
Reaction center chlorophyll-protein complexes are capable of directly absorbing light and performing charge separation events without other chlorophylls, but the absorption cross section (the likelihood of absorbing a photon under a given light intensity) is small. Thus, the remaining chlorophylls in the photosystem and antenna pigment protein complexes associated with the photosystems all cooperatively absorb and funnel light energy to the reaction center. Besides chlorophyll a, there are other pigments, called accessory pigments, which occur in these pigment-protein antenna complexes. They include other forms of chlorophyll, such as chlorophyll b in green algal and higher plant antennae, while other algae may contain chlorophyll c or d. In addition, there are many non-chlorophyll accessory pigments, such as carotenoids or phycobilins which also absorb light and transfer that light energy to the photosystem chlorophylls. Some of these accessory pigments, particularly the carotenoids, also serve to absorb and dissipate excess light energy, or work as antioxidants. The large, physically associated group of chlorophylls and other accessory pigments is sometimes referred to as a pigment bed, though this term is losing prominence with the advent of detailed knowledge of the structural organization of the photosystem and antenna complexes.

The different chlorophyll and non-chlorophyll pigments associated with the photosystems all have different spectra, either because the spectra of the different chlorophylls are modified by their local protein environment, or because the accessory pigments have intrinsically different absorption spectra from chlorophyll. The net result is that, in vivo the total absorption spectrum is broadened and flattened such that a wider range of red, orange, yellow and blue light can be absorbed by plants and algae. Most photosynthetic organisms do not have pigments which absorb green light well, thus most remaining light under leaf canopies in forests or under water with abundant plankton is green, a spectral effect called the "green window". Some organisms, such as cyanobacteria and red algae, contain accessory phycobilin pigments that can absorb green light relatively well and thus they can exploit the little remaining green light in these habitats.


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