Chlorophyll

Chlorophyll, also chlorophyl is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros ("green") and φύλλον, phyllon ("leaf"). Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light. Chlorophyll absorbs light most strongly in the blue portion of the electromagnetic spectrum, followed by the red portion. However, it is a poor absorber of green and near-green portions of the spectrum; hence the green color of chlorophyll-containing tissues. Chlorophyll was first isolated by Joseph Bienaimé Caventou and Pierre Joseph Pelletier in 1817.

Chlorophyll and photosynthesis

Chlorophyll molecules are specifically arranged in and around photosystems that 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.

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. 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), 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 that reduces P680+ ultimately comes from the oxidation of water into O2 and H+ through several intermediates. This reaction is how photosynthetic organisms such as 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 used mainly 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.

A green sea slug, Elysia chlorotica, has been found to use the chlorophyll it has eaten to perform photosynthesis for itself. This process is known as kleptoplasty, and no other animal has been found to have this ability.

Why green and not black

It is as of yet unclear exactly why plants (or rather, their light absorbing molecule, chlorophyll) have mostly evolved to be green. Green plants reflect mostly green and near-green light to viewers rather than absorbing it. Other parts of the system of photosynthesis still allow green plants to use the green light spectrum (e.g. through a light-trapping leaf structure, carotenoids, etc). Green plants do not use a large part of the visible spectrum as efficiently possible. A black plant can absorb more radiation, and this could be very useful - issues of managing this extra heat and radiation notwithstanding (e.g. some plants must close their openings, called stoma, on hot days to avoid losing too much water). More precisely, the question becomes why the only light absorbing molecule used for power in plants is green and not simply black.

Biologist John Berman offers that evolution is not an engineer, and so it is often subject to various limitations that an engineer is not. Even if black were better, evolution's limitations can prevent species from climbing to the absolute highest peak on the fitness landscape. Berman says that evolving pigments that work nearly as well as chlorophyll, or that successfully modify it, could be very difficult. In fact, all higher plants (Embryophytes) are believed to have evolved from a common ancestor that is a sort of green algae - with the idea being that chlorophyll evolved only once. 

Shil DasSarma, a microbial geneticist at the University of Maryland, points out that species of archae do use another light absorbing molecule, Retinal, to extract energy from sunlight's green spectrum. He describes the view of some scientists that such green-light-absorbing archae once dominated the earth environment. This could have left open a "niche" for green organisms which would absorb the other wavelengths of sunlight. This is just a possibility, however, and Berman says scientists are still not convinced of any one explanation.

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 chlorin ring is a magnesium ion. For the structures depicted in this article, some of the ligands attached to the Mg2+ center are omitted for clarity. The chlorin ring can have several different side chains, usually including a long phytol chain. There are a few different forms that occur naturally, but the most widely distributed form in terrestrial plants is chlorophyll a. The general structure of chlorophyll a was elucidated by Hans Fischer in 1940, and by 1960, when most of the stereochemistry of chlorophyll a was known, Robert Burns Woodward published a total synthesis of the molecule as then known. In 1967, the last remaining stereochemical elucidation was completed by Ian Fleming, and in 1990 Woodward and co-authors published an updated synthesis. In 2010, a near-infrared light photosynthetic pigment called Chlorophyll f may have been discovered in cyanobacteria and other oxygenic microorganisms that form stromatolites. Based on NMR data, optical and mass spectra, it is thought to have a structure of C55H70O6N4Mg or 2-formyl-chlorophyll a.