Chlorophyll

Chlorophyll is a green photosynthetic pigment found in most plants, algae, and cyanobacteria. Its name is derived from ancient Greek: chloros = green and phyllon = leaf. Chlorophyll absorbs most strongly in the blue and red but poorly in the green portions of the electromagnetic spectrum, hence the green color of chlorophyll-containing tissues like plant leaves.

Chlorophyll and photosynthesis

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 hundered 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, however, contain phycobilin pigments of cyanobacteria and red algae can absorb green light relatively well and they can exploit the little remaining green light in these habitats.

Chemical structure

Chlorophyll is a chlorin pigment, which is structurally similar to and produced through the same metabolic pathway as other porphyrin pigments such as heme. At the center of the porphyrin ring is a magnesium ion. The chlorin ring can have several different side chains, usually including a long phytyl chain. There are a few different forms that occur naturally:

	Chlorophyll a	Chlorophyll b	Chlorophyll c1	Chlorophyll c2	Chlorophyll d
Molecular formula	C₅₅H₇₂O₅N₄Mg	C₅₅H₇₀O₆N₄Mg	C₃₅H₃₀O₅N₄Mg	C₃₅H₂₈O₅N₄Mg	C₅₄H₇₀O₆N₄Mg
C3 group	-CH=CH₂	-CH=CH₂	-CH=CH₂	-CH=CH₂	-CHO
C7 group	-CH₃	-CHO	-CH₃	-CH₃	-CH₃
C8 group	-CH₂CH₃	-CH₂CH₃	-CH₂CH₃	-CH=CH₂	-CH₂CH₃
C17 group	-CH₂CH₂COO-Phytyl	-CH₂CH₂COO-Phytyl	-CH=CHCOOH	-CH=CHCOOH	-CH₂CH₂COO-Phytyl
C17-C18 bond	Single	Single	Double	Double	Single
Occurrence	Universal	Mostly plants	Various algae	Various algae	cyanobacteria

Historical evidence for the importance of chlorophyll in photosynthesis

A simple experiment can show how chlorophyll is associated with photosynthesis. After destarching a leaf from a variegated plant and exposing it to light for several hours, starch can be seen to rapidly accumulate again by staining with iodine solution. Variegated leaves have green areas that contain chlorophyll and white areas that do not. The iodine-stained starch only accumulates in regions of the leaf that were green and therefore contained chlorophyll. This shows that photosynthesis does not occur in areas where chlorophyll is absent, and constitutes evidence that the presence of chlorophyll is a requirement for photosynthesis.

References

Speer, Brian R. (1997). "Photosynthetic Pigments" in UCMP Glossary (online). University of California, Berkeley Museum of Paleontology. Verified availability March 20, 2006.
PDF review-Chlorophyll d: the puzzle resolved

Photosynthetic pigments

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Chlorophyll and photosynthesis

Chemical structure

Historical evidence for the importance of chlorophyll in photosynthesis

References