Chlorophyll

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Biography

Chlorophyll (also chlorophyl ) is a green pigment found in almost all plant s, alga e, and cyanobacteria . Its name is derived from the Greek language|Greek words ??????, chloros ("green") and f?????, phyllon ("leaf"). 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. 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.

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Chlorophyll and photosynthesis

Chlorophyll is vital for photosynthesis , which allows plants to absorb energy from light.

Chlorophyll molecules are specifically arranged in and around photosystem s that are embedded in the thylakoid membranes of chloroplast s. 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.Green, 1984 These pigments are named after the wavelength (in nanometer s) 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 chemiosmosis|chemiosmotic potential used mainly to produce Adenosine triphosphate|ATP chemical energy; and those electrons ultimately reduce NADP⁺ to NADPH , a universal redox|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 pigment s, 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 still is unclear exactly why plants 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 as possible. A black plant can absorb more radiation, and this could be very useful, if extra heat produced is effectively disposed of (e.g., some plants must close their openings, called stoma ta, on hot days to avoid losing too much water, which leaves only conduction, convection, and radiative heat-loss as solutions). The question becomes why the only light-absorbing molecule used for power in plants is green and not simply black.

The biologist John Berman has offered the opinion that evolution is argument from poor design|not an engineering process , and so it is often subject to various limitations that an engineer or other designer is not. Even if black leaves were better, evolution's limitations can prevent species from climbing to the absolute highest peak on the fitness landscape . Berman wrote that achieving pigments that work better than chlorophyll could be very difficult. In fact, all higher plants ( embryophyte s) are thought to have evolved from a common ancestor that is a sort of green algae – with the idea being that chlorophyll has evolved only once.
. Askabiologist.org.uk. Retrieved on 2012-04-15.

Shil DasSarma, a microbial geneticist at the University of Maryland , has pointed out that species of archaea do use another light-absorbing molecule, retinal , to extract power from the green spectrum. He described 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 that would absorb the other wavelengths of sunlight. This is just a possibility, and Berman wrote that scientists are still not convinced of any one explanation.. Livescience.com (2007-04-10). Retrieved on 2012-04-15.

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 . At the time of its discovery in the early 1900s, this was the first time that this element had been detected in living tissue. For the structures depicted in this article, some of the ligand s 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.
In 1967, the last remaining stereochemical elucidation was completed by Ian Fleming (chemist)|Ian Fleming , and in 1990 Woodward and co-authors published an updated synthesis. Chlorophyll f was announced to be present in cyanobacteria and other oxygenic microorganisms that form stromatolites in 2010;Jabr, Ferris . Scientific American. August 19, 2010. Retrieved on 2012-04-15.. New Scientist. August 19, 2010. Retrieved on 2012-04-15. a molecular formula of C₅₅H₇₀O₆N₄Mg and a structure of (2- formyl )-chlorophyll a were deduced based on NMR, optical and mass spectra. The different structures of chlorophyll are summarized below:


Molecular formula	C_{55H_{72O_{5N_4Mg}}}	C_{55H_{70O_{6N_4Mg}}}	C_{35H_{30O_{5N_4Mg}}}	C_{35H_{28O_{5N_4Mg}}}	C_{54H_{70O_{6N_4Mg}}}	C_{55H_{70O_{6N_4Mg}}}
C2 group	-CH₃	-CH₃	-CH₃	-CH₃	-CH₃	-CHO
C3 group	-CH=CH₂	-CH=CH₂	-CH=CH₂	-CH=CH₂	-CHO	-CH=CH₂
C7 group	-CH₃	-CHO	-CH₃	-CH₃	-CH₃	-CH₃
C8 group	-CH_2CH₃	-CH_2CH₃	-CH_2CH₃	-CH=CH₂	-CH_2CH₃	-CH_2CH₃
C17 group	-CH_{2CH_2COO-Phytyl}	-CH_{2CH_2COO-Phytyl}	-CH=CHCOOH	-CH=CHCOOH	-CH_{2CH_2COO-Phytyl}	-CH_{2CH_2COO-Phytyl}
C17-C18 bond	Single (chlorin)	Single (chlorin)	Double (porphyrin)	Double (porphyrin)	Single (chlorin)	Single (chlorin)
Occurrence	Universal	Mostly plants	Various algae	Various algae	Cyanobacteria	Cyanobacteria

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

These compounds have also been identified in several ripening fruits.

Spectrophotometry

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& nbsp;nm and 662& nbsp;nm, while chlorophyll b has approximate maxima of 453& nbsp;nm and 642& nbsp;nm.Gross, 1991

The absorption peaks of chlorophyll a are at 665& nbsp;nm and 465& nbsp;nm. Chlorophyll a fluorescence|fluoresces at 673& nbsp;nm (maximum) and 726& nbsp;nm. The peak Molar absorptivity|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& nbsp;nm and 664& nbsp;nm; peaks for chlorophyll b are 460& nbsp;nm and 647& nbsp;nm; peaks for chlorophyll c1 are 442& nbsp;nm and 630& nbsp;nm; peaks for chlorophyll c2 are 444& nbsp;nm and 630& nbsp;nm; peaks for chlorophyll d are 401& nbsp;nm, 455& nbsp;nm and 696& nbsp;nm.

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.

While absorption instruments have been proven over time to work well as a non-destructive test on many types of samples, they do have limitations. Absorption technique limitations:

The sample to be measured must completely cover the instrument’s aperture, with no holes.

Samples must be thin enough to allow transmission of the measuring wavelengths of light.

Surfaces must be relatively flat and uniform.

Variable fluorescence (the Kautsky effect|Kautsky induction effect) caused by the red wavelength of absorption instruments, limits the repeatability of measurements at the same location.

Selection of the measuring area, on smaller leaves, can cause measurement variations due to internal leaf structure variation. Veins and midribs can cause significant variation on samples.

Wavelengths used in absorption methods, typically limit linear correlation with chemical chlorophyll content methods to concentration levels below 300& nbsp;mg/m².

As a result, absorption instruments do not work with conifer needles, turf grasses, Arabidopsis leaves, moss, most CAM plants such as prickly pear cactus, and Agave, fruit, stems, petioles, lichens, and algae on rocks. Furthermore, it is difficult to get reliable readings on very small leaf plants such as immature rice and wheat.

A superior method is to measure the chlorophyll content using chlorophyll fluorescence .
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." Non-destructive, hand-held meters use this effect to measure the chlorophyll content of samples where traditional absorbance techniques cannot be used.
This new method has many advantages over older, absorption techniques. These include:-
#Samples do not need to fill the measuring aperture, so small samples can be measured.
# Samples do not need to allow transmission of the measuring beam, so thick samples can be measured.
# Surfaces do not need to be flat and uniform, so difficult plant morphologies can be measured.
# No Kautsky effect|Kautsky induction effect, so measurements can be repeated rapidly at the same sight.
# Small sample area, so naturally occurring structures like mid ribs and veins can be avoided.
# Higher measuring range (675& nbsp;mg/m², as opposed to 300& nbsp;mg/m² with absorption techniques), so many more samples can be measured.

By measuring chlorophyll fluorescence , plant ecophysiology can be investigated. Chlorophyll fluorescence#Chlorophyll fluorometers|Chlorophyll fluorometers are used by plant researchers to assess plant stress.

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 ( etiolation|etiolated ) if grown in the darkness. Non-vascular plant s 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.

Chlorosis is a condition in which leaf|leaves produce insufficient chlorophyll, turning them yellow. Chlorosis can be caused by a nutrient deficiency of Iron deficiency (plant disorder)|iron --called iron chlorosis—or by a shortage of Magnesium deficiency|magnesium or Nitrogen deficiency|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.
Chlorosis can also be caused by pathogens including viruses, bacteria and fungal infections, or sap-sucking insects.

Complementary light absorbance of anthocyanins with chlorophylls

Anthocyanin s are other plant pigment s. 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.The importance of being red when young: anthocyanins and the protection of young leaves of Quercus coccifera from insect herbivory and excess light. Panagiota Karageorgou and Yiannis Manetas, Tree Physiol, 2006, 26 (5), pages 613-621,

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 .
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.

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