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Chlorophyll (also chlorophyl) is any of several related green
pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...
s found in
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...
and in the
chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it i ...
s of algae and
plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...
s. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to absorb
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...
from light. Chlorophylls absorb light most strongly in the blue portion of the
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging fro ...
as well as the red portion. Conversely, it is a poor absorber of green and near-green portions of the spectrum. Hence chlorophyll-containing tissues appear green because green light, diffusively reflected by structures like cell walls, is less absorbed. Two types of chlorophyll exist in the photosystems of green plants: chlorophyll ''a'' and ''b''.


History

Chlorophyll was first isolated and named by Joseph Bienaimé Caventou and
Pierre Joseph Pelletier Pierre-Joseph Pelletier (, , ; 22 March 1788 – 19 July 1842) was a French chemist and pharmacist who did notable research on vegetable alkaloids, and was the co-discoverer with Joseph Bienaimé Caventou of quinine, caffeine, and strychnine. ...
in 1817. The presence of
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...
in chlorophyll was discovered in 1906, and was that element's first detection in living tissue. 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 Ian Lancaster Fleming (28 May 1908 – 12 August 1964) was a British writer who is best known for his postwar ''James Bond'' series of spy novels. Fleming came from a wealthy family connected to the merchant bank Robert Fleming & Co., an ...
, and in 1990 Woodward and co-authors published an updated synthesis. Chlorophyll ''f'' was announced to be present in
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...
and other oxygenic microorganisms that form stromatolites in 2010; a molecular formula of C55H70O6N4Mg and a structure of (2- formyl)-chlorophyll ''a'' were deduced based on NMR, optical and mass spectra.


Photosynthesis

Chlorophyll is vital for
photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
, which allows plants to absorb energy from
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...
. Chlorophyll molecules are arranged in and around
photosystem Photosystems are functional and structural units of protein complexes involved in photosynthesis. Together they carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. Photosy ...
s that are embedded in the
thylakoid Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thyl ...
membranes of
chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it i ...
s. In these complexes, chlorophyll serves three functions: # The function of the vast majority of chlorophyll (up to several hundred molecules per photosystem) is to absorb light. # Having done so, these same centers execute their second function: The transfer of that energy by
resonance energy transfer Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscilla ...
to a specific chlorophyll pair in the reaction center of the photosystems. # This specific pair performs the final function of chlorophylls: Charge separation, which produces the unbound protons (H) and electrons (e) that separately propel biosynthesis. The two currently accepted photosystem units are and which have their own distinct reaction centres, named P700 and
P680 P680, or photosystem II primary donor, is the reaction-center chlorophyll ''a'' molecular dimer associated with photosystem II in plants, algae, and cyanobacteria, and central to oxygenic photosynthesis. Etymology Its name is derived from th ...
, 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. The function of the reaction center of chlorophyll is to absorb light energy and transfer it to other parts of the photosystem. The absorbed energy of the photon is transferred to an electron in a process called charge separation. The removal of the electron from the chlorophyll is an oxidation reaction. The chlorophyll donates the high energy electron to a series of molecular intermediates called an
electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples ...
. The charged reaction center of chlorophyll (P680+) is then reduced back to its ground state by accepting an electron stripped from water. 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 as it accepts the electron, via many intermediates in the thylakoid membrane, by electrons coming, 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 pump H+ ions across the thylakoid membrane, setting up a proton-motive force a chemiosmotic potential used mainly in the production of ATP (stored chemical energy) or to reduce NADP+ to NADPH. NADPH is a universal
agent Agent may refer to: Espionage, investigation, and law *, spies or intelligence officers * Law of agency, laws involving a person authorized to act on behalf of another ** Agent of record, a person with a contractual agreement with an insuranc ...
used to reduce CO2 into sugars as well as other biosynthetic reactions. Reaction center chlorophyll–protein complexes are capable of directly absorbing light and performing charge separation events without the assistance of other chlorophyll pigments, but the probability of that happening under a given light intensity is small. Thus, the other chlorophylls in the photosystem and antenna pigment proteins 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

Several chlorophylls are known. All are defined as derivatives of the parent chlorin by the presence of a fifth, ketone-containing ring beyond the four pyrrole-like rings. Most chlorophylls are classified as chlorins, which are reduced relatives of porphyrins (found in
hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythroc ...
). They share a common biosynthetic pathway with porphyrins, including the precursor uroporphyrinogen III. Unlike hemes, which contain iron bound to the N4 center, most chlorophylls bind
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...
. The axial
ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elect ...
s attached to the Mg2+ center are often omitted for clarity. Appended to the chlorin ring are various side chains, usually including a long phytyl chain (). The most widely distributed form in terrestrial plants is chlorophyll ''a''. The only difference between chlorophyll ''a'' and chlorophyll ''b'' is that the former has a
methyl In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in ...
group where the latter has a formyl group. This difference causes a considerable difference in the absorption spectrum, allowing plants to absorb a greater portion of visible light. The structures of chlorophylls are summarized below: chlorophyll a.svg, chlorophyll ''a'' chlorophyll b.svg, chlorophyll ''b'' chlorophyll c1.svg, chlorophyll ''c1'' chlorophyll c2.svg, chlorophyll ''c2'' Chlorophyll d.svg, chlorophyll ''d'' Chlorophyll f_vert.svg, chlorophyll ''f''


Measurement of chlorophyll content

Chlorophylls can be extracted from the protein into organic solvents. In this way, the concentration of chlorophyll within a leaf can be estimated. Methods also exist to separate chlorophyll ''a'' and chlorophyll ''b''. In
diethyl ether Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable li ...
, chlorophyll ''a'' has approximate absorbance maxima of 430 nm and 662 nm, while chlorophyll ''b'' has approximate maxima of 453 nm and 642 nm. The absorption peaks of chlorophyll ''a'' are at 465 nm and 665 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. Ratio fluorescence emission can be used to measure chlorophyll content. By exciting chlorophyll ''a'' fluorescence at a lower wavelength, the ratio of chlorophyll fluorescence emission at and can provide a linear relationship of chlorophyll content when compared with chemical testing. The ratio ''F''735/''F''700 provided a correlation value of ''r''2 0.96 compared with chemical testing in the range from 41 mg m−2 up to 675 mg m−2. Gitelson also developed a formula for direct readout of chlorophyll content in mg m−2. The formula provided a reliable method of measuring chlorophyll content from 41 mg m−2 up to 675 mg m−2 with a correlation ''r''2 value of 0.95.


Biosynthesis

In some plants, chlorophyll is derived from glutamate and is synthesised along a branched biosynthetic pathway that is shared with heme and
siroheme Siroheme (or sirohaem) is a heme-like prosthetic group at the active sites of some enzymes to accomplish the six-electron reduction of sulfur and nitrogen. It is a cofactor at the active site of sulfite reductase, which plays a major role in su ...
.
Chlorophyll synthase In enzymology, chlorophyll synthase () is an enzyme that catalyzes the chemical reaction :chlorophyllide a + phytyl diphosphate \rightleftharpoons chlorophyll a + diphosphate The two substrates of this enzyme are chlorophyllide ''a'' and phyt ...
is the enzyme that completes the biosynthesis of chlorophyll ''a'': :chlorophyllide ''a'' + phytyl diphosphate \rightleftharpoons chlorophyll ''a'' + diphosphate This converion forms an ester of the carboxylic acid group in chlorophyllide ''a'' with the 20-carbon diterpene alcohol phytol. Chlorophyll ''b'' is made by the same enzyme acting on chlorophyllide ''b''. In
Angiosperm Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants ...
plants, the later steps in the biosynthetic pathway are light-dependent. Such plants are pale ( etiolated) if grown in darkness. Non-vascular plants and green algae have an additional light-independent
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
and grow green even in darkness. Chlorophyll is bound to
proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
.
Protochlorophyllide Protochlorophyllide,KEGG compound database entr/ref> or monovinyl protochlorophyllide, is an intermediate in the biosynthesis of chlorophyll ''a''. It lacks the phytol side-chain of chlorophyll and the reduced pyrrole in ring D. Protochlorophy ...
, one of the biosynthetic intermediates, occurs mostly in the free form and, under light conditions, acts as a photosensitizer, forming
free radicals In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spon ...
, which can be toxic to the plant. Hence, plants regulate the amount of this chlorophyll precursor. In angiosperms, this regulation is achieved at the step of
aminolevulinic acid δ-Aminolevulinic acid (also dALA, δ-ALA, 5ALA or 5-aminolevulinic acid), an endogenous non-proteinogenic amino acid, is the first compound in the porphyrin synthesis pathway, the pathway that leads to heme in mammals, as well as chlorophyll in p ...
(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 a damaged regulatory system.


Senescence and the chlorophyll cycle

The process of plant senescence involves the degradation of chlorophyll: for example the enzyme
chlorophyllase Chlorophyllase is an essential enzyme in chlorophyll metabolism. It is a membrane proteins commonly known as chlase (EC 3.1.1.14, CLH) with systematic name chlorophyll chlorophyllidohydrolase. It catalyzes the reaction :chlorophyll + H2O = phy ...
() hydrolyses the phytyl sidechain to reverse the reaction in which chlorophylls are biosynthesised from chlorophyllide ''a'' or ''b''. Since chlorophyllide ''a'' can be converted to chlorophyllide ''b'' and the latter can be re-esterified to chlorophyll ''b'', these processes allow cycling between chlorophylls ''a'' and ''b''. Moreover, chlorophyll ''b'' can be directly reduced (via ) back to chlorophyll ''a'', completing the cycle. In later stages of senescence, chlorophyllides are 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 ripening fruits and they give characteristic autumn colours to
deciduous In the fields of horticulture and Botany, the term ''deciduous'' () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, ...
plants.


Distribution

The 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 Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. ...
, 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 Thriving is a condition beyond mere survival, implying growth and positive development. Youth development The synthesis of existing lines of research has given a lens through which to view research, theory, and practice in the field of yo ...
, 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 cannot grow. When sunlight returns in the spring and summer, the plants flourish in high concentrations.Chlorophyll : Global Maps
Earthobservatory.nasa.gov. Retrieved on 2014-02-02.


Culinary use

Synthetic chlorophyll is registered as a food additive colorant, and its
E number E numbers ("E" stands for "Europe") are codes for substances used as food additives, including those found naturally in many foods such as vitamin C, for use within the European Union (EU) and European Free Trade Association (EFTA). Commonly ...
is E140. Chefs use chlorophyll to color a variety of foods and beverages green, such as pasta and spirits. Absinthe gains its green color naturally from the chlorophyll introduced through the large variety of herbs used in its production. Chlorophyll is not soluble in water, and it is first mixed with a small quantity of vegetable oil to obtain the desired
solution Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approximate solutions within specified error bounds * Solutio ...
.


Biological use

A 2002 study found that "leaves exposed to strong light contained degraded major antenna proteins, unlike those kept in the dark, which is consistent with studies on the illumination of
isolated proteins Protein purification is a series of processes intended to isolate one or a few proteins from a complex mixture, usually Cell biology, cells, Tissue (biology), tissues or whole organisms. Protein purification is vital for the specification of the fu ...
". This appeared to the authors as support for the
hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obse ...
that " active oxygen species play a role
in vivo Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and p ...
" in the short-term behaviour of plants.


See also

* Bacteriochlorophyll, related compounds in phototrophic bacteria *
Chlorophyllin Chlorophyllin refers to any one of a group of closely related water-soluble salts that are semi-synthetic derivatives of chlorophyll, differing in the identity of the cations associated with the anion. Its most common form is a sodium/copper deriv ...
, a semi-synthetic derivative of chlorophyll * Deep chlorophyll maximum * Chlorophyll fluorescence, to measure plant stress


References

{{Authority control Tetrapyrroles Photosynthetic pigments Articles containing video clips E-number additives