From wikipedia,
[edit] Chlorophyll and photosynthesis
Chlorophyll is vital for
photosynthesis, which allows plants to obtain energy from 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 molecules 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. Because of chlorophyll’s selectivity regarding the wavelength of light it absorbs, areas of a leaf containing the molecule will appear green.
The two currently accepted photosystem units are Photosystem II and Photosystem I, which have their own distinct reaction center chlorophylls, named P680 and P700, respectively.
[2] These pigments are named after the wavelength (in
nanometers) of their red-peak absorption maximum. The identity, function and spectral properties of the types of chlorophyll 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),
[3][4][5] these chlorophyll pigments can be separated in a simple paper chromatography experiment, and, based on the number of polar groups between chlorophyll a and chlorophyll b, will chemically separate out on the paper.
The function of the reaction center chlorophyll is to use the energy absorbed by and transferred to it from the other chlorophyll pigments 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 chlorophyll pigments 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 NADP+ 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 chlorophyll pigments, 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.
