Chlorophyll

"Leaf green" redirects here. For the video game, see Pokémon FireRed and LeafGreen.

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

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

Absorption maxima of chlorophylls against the spectrum of white light.^{[citation needed]}

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

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.^[1] Chlorophyll was first isolated by Joseph Bienaimé Caventou and Pierre Joseph Pelletier in 1817.^[2]

Chlorophyll and photosynthesis

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

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.^[3] 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),^[4][5][6] 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?

Black plants can absorb more radiation, and yet most plants are green

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, notwitstandanding the problems of disposing of this extra heat (e.g., some plants must close their openings, called stomata, on hot days to avoid losing too much water). 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 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 (embryophytes) are believed to have evolved from a common ancestor that is a sort of green algae - with the idea being that chlorophyll has evolved only once. ^[7]

Shil DasSarma, a microbial geneticist at the University of Maryland, has pointed out that species of archae 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.^[8]

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. 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.^[9] In 1967, the last remaining stereochemical elucidation was completed by Ian Fleming,^[10] and in 1990 Woodward and co-authors published an updated synthesis.^[11] In 2010, a near-infrared-light photosynthetic pigment called chlorophyll f might have been discovered in cyanobacteria and other oxygenic microorganisms that form stromatolites.^[12][13] Based on NMR data, optical and mass spectra, it is thought to have a structure of C₅₅H₇₀O₆N₄Mg or [2-formyl]-chlorophyll a.^[14]

The different structures of chlorophyll are summarized below:

	Chlorophyll a	Chlorophyll b	Chlorophyll c1	Chlorophyll c2	Chlorophyll d	Chlorophyll f
Molecular formula	C55H72O5N4Mg	C55H70O6N4Mg	C35H30O5N4Mg	C35H28O5N4Mg	C54H70O6N4Mg	C55H70O6N4Mg
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	-CH2CH3	-CH2CH3	-CH2CH3	-CH=CH2	-CH2CH3	-CH2CH3
C17 group	-CH2CH2COO-Phytyl	-CH2CH2COO-Phytyl	-CH=CHCOOH	-CH=CHCOOH	-CH2CH2COO-Phytyl	-CH2CH2COO-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

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.^[15]

Spectrophotometry

Absorbance spectra of free chlorophyll a (green) 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.^{[16][specify]}
• 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.^{[citation needed]}
• 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.^[17]

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

Biosynthesis

Further information: Protochlorophyllide

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.^[18]

Further information: Chlorosis

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.^[19] Chlorosis can also be caused by pathogens including viruses, bacteria and fungal infections, or sap-sucking insects.

Measuring chlorophyll

The chlorophyll content of leaves can be non-destructively measured using hand-held, battery-powered meters.

The absorption spectrum of chlorophyll, showing the transmittance band measured by a CCM200 Chlorophyll Meter to calculate the relative chlorophyll content

Chlorophyll Content meters measure the optical absorption of a leaf to estimate its chlorophyll content. Chlorophyll molecules absorb in the blue and red bands, but not the green and infra-red bands. Chlorophyll content meters measure the amount of absorption at the red band to estimate the amount of chlorophyll present in the leaf. To compensate for varying leaf thickness, Chlorophyll Meters also measure absorption at the infrared band, which is not significantly affected by chlorophyll.

The measurements made by these devices are simple, quick and relatively inexpensive. They now, typically, have large data storage capacity, averaging and graphical displays.^[20]

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.^[21] 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.^[22]

See also

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

References

1. ^ Speer, Brian R. (1997). "Photosynthetic Pigments". UCMP Glossary (online). University of California Museum of Paleontology. http://www.ucmp.berkeley.edu/glossary/gloss3/pigments.html. Retrieved 2010-07-17. 
2. ^ Delépine, Marcel (September 1951). "Joseph Pelletier and Joseph Caventou". Journal of Chemical Education 28 (9): 454. doi:10.1021/ed028p454. ISSN 0021-9584. http://pubs.acs.org/doi/abs/10.1021/ed028p454. 
3. ^ Green, 1984
4. ^ Marker, A. F. H. (1972). "The use of acetone and methanol in the estimation of chlorophyll in the presence of phaeophytin". Freshwater Biology 2 (4): 361. doi:10.1111/j.1365-2427.1972.tb00377.x 
5. ^ Jeffrey, S. W.; Shibata, Kazuo (February 1969). "Some Spectral Characteristics of Chlorophyll c from Tridacna crocea Zooxanthellae". Biological Bulletin (Marine Biological Laboratory) 136 (1): 54–62. doi:10.2307/1539668. JSTOR 1539668 
6. ^ Gilpin, Linda (21 March 2001). "Methods for analysis of benthic photosynthetic pigment". School of Life Sciences, Napier University. http://www.lifesciences.napier.ac.uk/teaching/MB/benchl01.html. Retrieved 2010-07-17. 
7. ^ askabiologist.org/uk, Jonathan Max Berman, "Why did plants evolve green, not black?"
8. ^ Livescience.com Early Earth Was Purple, Study Suggests
9. ^ Woodward RB, Ayer WA, Beaton JM (July 1960). "The total synthesis of chlorophyll". Journal of the American Chemical Society 82 (14): 3800–3802. doi:10.1021/ja01499a093. http://pubs.acs.org/doi/abs/10.1021/ja01499a093. 
10. ^ Fleming, Ian (14 October 1967). "Absolute Configuration and the Structure of Chlorophyll". Nature 216 (5111): 151–152. doi:10.1038/216151a0. http://www.nature.com/nature/journal/v216/n5111/abs/216151a0.html. 
11. ^ Robert Burns Woodward, William A. Ayer, John M. Beaton, Friedrich Bickelhaupt, Raymond Bonnett, Paul Buchschacher, Gerhard L. Closs, Hans Dutler, John Hannah, Fred P. Hauck, et al. (1990). "The total synthesis of chlorophyll a" (PDF, 0.5 MB). Tetrahedron 46 (22): 7599–7659. doi:10.1016/0040-4020(90)80003-Z. http://media.iupac.org/publications/pac/1961/pdf/0203x0383.pdf. 
12. ^ http://www.scientificamerican.com/article.cfm?id=new-form-chlorophyll
13. ^ http://www.newscientist.com/article/dn19338-infrared-chlorophyll-could-boost-solar-cells.html
14. ^ Chen, M. .; Schliep, M. .; Willows, R. D.; Cai, Z. -L.; Neilan, B. A.; Scheer, H. . (2010). "A Red-Shifted Chlorophyll". Science 329 (5997): 1318–1319. Bibcode 2010Sci...329.1318C. doi:10.1126/science.1191127. PMID 20724585.  edit
15. ^ Müller, Thomas; Ulrich, Markus; Ongania, Karl-Hans; Kräutler, Bernhard. (2007). "Colorless Tetrapyrrolic Chlorophyll Catabolites Found in Ripening Fruit Are Effective Antioxidants". Angewandte Chemie International Edition 46 (45): 8699–8702. doi:10.1002/anie.200703587. ISSN 1433-7851. PMC 2912502. PMID 17943948. http://www3.interscience.wiley.com/journal/116331819/abstract. 
16. ^ Gross, 1991
17. ^ Larkum, edited by Anthony W. D. Larkum, Susan E. Douglas & John A. Raven (2003). Photosynthesis in algae. London: Kluwer. ISBN 0792363337. 
18. ^ Meskauskiene R, Nater M, Goslings D, Kessler F, op den Camp R, Apel K. (23 October 2001). "FLU: A negative regulator of chlorophyll biosynthesis in Arabidopsis thaliana". Proceedings of the National Academy of Sciences 98 (22): 12826–12831. doi:10.1073/pnas.98.22.12826. ISSN 0027-8424. PMC 60138. PMID 11606728. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=60138. 
19. ^ Duble, Richard L.. "Iron Chlorosis in Turfgrass". Texas A&M University. http://plantanswers.tamu.edu/turf/iron.html. Retrieved 2010-07-17. 
20. ^ http://www.adc.co.uk/Products/CCM-200_plus_Chlorophyll_Content_Meter
21. ^ Adams, Jad (2004). Hideous absinthe : a history of the devil in a bottle. Madison, Wisconsin: University of Wisconsin Press. p. 22. ISBN 9780299200008. http://books.google.com/?id=N7rKrszRxFM. 
22. ^ US patent 5820916, Sagliano, Frank S. & Sagliano, Elizabeth A., "Method for growing and preserving wheatgrass nutrients and products thereof", issued 1998-10-13 

External links

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