Chlorophyll

Animation depicting nearly four years worth of SeaWiFS ocean chlorophyll concentration.

These chlorophyll maps show milligrams of chlorophyll per cubic meter of seawater each month. Places where chlorophyll amounts were very low, indicating very low numbers of phytoplankton, are blue. Places where chlorophyll concentrations were high, meaning many phytoplankton were growing, are yellow. The observations come from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite. Land is dark gray, and places where MODIS could not collect data because of sea ice, polar darkness, or clouds are light gray.The highest chlorophyll concentrations, where tiny surface-dwelling ocean plants are thriving, are in cold polar waters or in places where ocean currents bring cold water to the surface, such as around the equator and along the shores of continents. It is not the cold water itself that stimulates the phytoplankton. Instead, the cool temperatures are often a sign that the water has welled up to the surface from deeper in the ocean, carrying nutrients that have built up over time. In polar waters, nutrients accumulate in surface waters during the dark winter months when plants can’t grow. When sunlight returns in the spring and summer, the plants flourish in high concentrations.^[1]

Chlorophyll (also chlorophyl) is a term used for several closely related green pigments found in cyanobacteria and the chloroplasts of algae and plants.^[2] Its name is derived from the Greek words χλωρός, chloros ("green") and φύλλον, phyllon ("leaf").^[3] Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to absorb energy from light. Chlorophyll absorbs light most strongly in the blue portion of the electromagnetic spectrum, followed by the red portion. Conversely, it is a poor absorber of green and near-green portions of the spectrum, hence the green color of chlorophyll-containing tissues.^[4] Chlorophyll was first isolated and named by Joseph Bienaimé Caventou and Pierre Joseph Pelletier in 1817.^[5]

Gallery

• 

  Chlorophyll gives leaves their green color and absorbs light that is used in photosynthesis.

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  Chlorophyll is found in high concentrations in chloroplasts of plant cells.

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  Absorption maxima of chlorophylls against the spectrum of white light.

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  SeaWiFS-derived average sea surface chlorophyll for the period 1998 to 2006.

Chlorophyll and photosynthesis

Chlorophyll is vital for photosynthesis, which allows plants to absorb energy from light.^[6]

Chlorophyll molecules are specifically arranged in and around photosystems that are embedded in the thylakoid membranes of chloroplasts.^[7] 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 centres, named P680 and P700, respectively. These centres 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),^[8][9][10] 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.

Chemical structure

Space-filling model of the chlorophyll a molecule

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. This was discovered in 1906,^[11] and was the first time that magnesium had been detected in living tissue.^[12] 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. After initial work done by German chemist Richard Willstätter spanning from 1905 to 1915, the general structure of chlorophyll a was elucidated by Hans Fischer in 1940. By 1960, when most of the stereochemistry of chlorophyll a was known, Robert Burns Woodward published a total synthesis of the molecule.^[12][13] In 1967, the last remaining stereochemical elucidation was completed by Ian Fleming,^[14] and in 1990 Woodward and co-authors published an updated synthesis.^[15] Chlorophyll f was announced to be present in cyanobacteria and other oxygenic microorganisms that form stromatolites in 2010;^[16][17] a molecular formula of C₅₅H₇₀O₆N₄Mg and a structure of (2-formyl)-chlorophyll a were deduced based on NMR, optical and mass spectra.^[18] The different structures of chlorophyll are summarized below:

Structure of chlorophyll a	Structure of chlorophyll b	Structure of chlorophyll d
Structure of chlorophyll c1	Structure of chlorophyll c2

When leaves degreen in the process of plant senescence, chlorophyll is converted to a group of colourless tetrapyrroles known as nonfluorescent chlorophyll catabolites (NCC's) with the general structure:

These compounds have also been identified in several ripening fruits.^[19]

Spectrophotometry

Absorbance spectra of free chlorophyll a (blue) and b (red) in a solvent. The spectra of chlorophyll molecules are slightly modified in vivo depending on specific pigment-protein interactions.

Measurement of the absorption of light is complicated by the solvent used to extract it from plant material, which affects the values obtained,

• In diethyl ether, chlorophyll a has approximate absorbance maxima of 430 nm and 662 nm, while chlorophyll b has approximate maxima of 453 nm and 642 nm.^[20]
• The absorption peaks of chlorophyll a are at 665 nm and 465 nm. Chlorophyll a fluoresces at 673 nm (maximum) and 726 nm. The peak molar absorption coefficient of chlorophyll a exceeds 10⁵ M⁻¹ cm⁻¹, which is among the highest for small-molecule organic compounds.
• In 90% acetone-water, the peak absorption wavelengths of chlorophyll a are 430 nm and 664 nm; peaks for chlorophyll b are 460 nm and 647 nm; peaks for chlorophyll c1 are 442 nm and 630 nm; peaks for chlorophyll c2 are 444 nm and 630 nm; peaks for chlorophyll d are 401 nm, 455 nm and 696 nm.^[21]

By measuring the absorption of light in the red and far red regions it is possible to estimate the concentration of chlorophyll within a leaf.^[22]
In his scientific paper Gitelson (1999) states, "The ratio between chlorophyll fluorescence, at 735 nm and the wavelength range 700nm to 710 nm, F735/F700 was found to be linearly proportional to the chlorophyll content (with determination coefficient, r2, more than 0.95) and thus this ratio can be used as a precise indicator of chlorophyll content in plant leaves."^[23] The fluorescent ratio chlorophyll content meters use this technique.

Biosynthesis

In plants, chlorophyll may be synthesized from succinyl-CoA and glycine, although the immediate precursor to chlorophyll a and b is protochlorophyllide. In Angiosperm plants, the last step, conversion of protochlorophyllide to chlorophyll, is light-dependent and such plants are pale (etiolated) if grown in the darkness. Non-vascular plants and green algae have an additional light-independent enzyme and grow green in the darkness instead.

Chlorophyll itself is bound to proteins and can transfer the absorbed energy in the required direction. Protochlorophyllide occurs mostly in the free form and, under light conditions, acts as a photosensitizer, forming highly toxic free radicals. Hence, plants need an efficient mechanism of regulating the amount of chlorophyll precursor. In angiosperms, this is done at the step of aminolevulinic acid (ALA), one of the intermediate compounds in the biosynthesis pathway. Plants that are fed by ALA accumulate high and toxic levels of protochlorophyllide; so do the mutants with the damaged regulatory system.^[24]

Chlorosis is a condition in which leaves produce insufficient chlorophyll, turning them yellow. Chlorosis can be caused by a nutrient deficiency of iron—called iron chlorosis—or by a shortage of magnesium or nitrogen. Soil pH sometimes plays a role in nutrient-caused chlorosis; many plants are adapted to grow in soils with specific pH levels and their ability to absorb nutrients from the soil can be dependent on this.^[25] Chlorosis can also be caused by pathogens including viruses, bacteria and fungal infections, or sap-sucking insects.

Complementary light absorbance of anthocyanins with chlorophylls

Superposition of spectra of chlorophyll a and b with oenin (malvidin 3O glucoside), a typical anthocyanidin, showing that, while chlorophylls absorb in the blue and yellow/red parts of the visible spectrum, oenin absorbs mainly in the green part of the spectrum, where chlorophylls don't absorb at all.

Anthocyanins are other plant pigments. The absorbance pattern responsible for the red color of anthocyanins may be complementary to that of green chlorophyll in photosynthetically active tissues such as young Quercus coccifera leaves. It may protect the leaves from attacks by plant eaters that may be attracted by green color.^[26]

Culinary use

Chlorophyll is registered as a food additive (colorant), and its E number is E140. Chefs use chlorophyll to color a variety of foods and beverages green, such as pasta and absinthe.^[27] Chlorophyll is not soluble in water, and it is first mixed with a small quantity of vegetable oil to obtain the desired solution. Extracted liquid chlorophyll was considered to be unstable and always denatured until 1997, when Frank S. & Lisa Sagliano used freeze-drying of liquid chlorophyll at the University of Florida and stabilized it as a powder, preserving it for future use.^[28]

Alternative medicine

Many claims are made about the healing properties of chlorophyll, but most have been disproved or are exaggerated by the companies that are marketing them. Quackwatch, a website dedicated to debunking false medical claims, has a quote from Toledo Blade (1952) which claims ^[29] "Chlorophyll Held Useless As Body Deodorant",^[30] but later has John C. Kephart pointing out "No deodorant effect can possibly occur from the quantities of chlorophyll put in products such as gum, foot powder, cough drops, etc. To be effective, large doses must be given internally".^[31]

See also

Wikimedia Commons has media related to Chlorophyll.

• Bacteriochlorophyll, related compounds in phototrophic bacteria
• Chlorophyll a, an essential chlorophyll pigment
• Chlorophyll b, also an essential chlorophyll pigment
• Chlorophyllin, a semi-synthetic derivative of chlorophyll
• Deep chlorophyll maximum
• Grow light, a lamp that promotes photosynthesis
• Chlorophyll fluorescence, to measure plant stress

References

External links

• Oregon University of Health & Sciences
• PDF review-Chlorophyll d: the puzzle resolved
• Light Absorption by Chlorophyll – NIH books

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