- Chlorophyll a
Chlorophyll a Identifiers PubChem ChemSpider Properties Molecular formula C55H72MgN4O5 Molar mass 893.49 g mol−1 Melting point
117-120 °C, 390-393 K, 243-248 °F
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Chlorophyll a is a specific form of chlorophyll used in oxygenic photosynthesis. It absorbs most energy from wavelengths of violet-blue and orange-red light. This photosynthetic pigment is essential for photosynthesis in eukaryotes, cyanobacteria and prochlorophytes because of its role as primary electron donor in the electron transport chain. Chlorophyll a also transfers resonance energy in the antenna complex, ending in the reaction center where specific chlorophyll’s P680 and P700 are located.
Distribution of Chlorophyll a
Chlorophyll a is essential for most photosynthetic organisms to release chemical energy but is not the only pigment that can be used for photosynthesis. All oxygenic photosynthetic organisms use chlorophyll a, but differ in accessory pigments like chlorophylls b and c. Chlorophyll a can also be found in very small quantities in the green sulfur bacteria, an anaerobic photoautotroph. These organisms use bacteriochlorophyll and some chlorophyll a but do not produce oxygen. Anoxygenic photosynthesis is the term applied to this process, unlike oxygenic photosynthesis where oxygen is produced during the light reactions of photosynthesis.
The molecular structure of chlorophyll a consists of a N-ring with a Mg center, side chains, and a hydrocarbon tail.
Chlorophyll a contains a central magnesium ion encased in a 4-ion nitrogen ring known as a chlorin ring. The chlorin ring is a heterocyclic compound derived from a pyrrole that encases a metal. The Mg within the center uniquely defines the structure as a chlorophyll molecule.
Side chains are attached to the porphyrin ring of chlorophyll a. Different side chains characterize each type of chlorophyll molecules, and alters the absorption spectrum of light. Chlorophyll a contains only methyl groups (CH3) as side chains. Chlorophyll b replaces a methyl group at the C-3 position on the ring (Green Box in Figure) with an aldehyde group. The porphyrin ring of bacteriochlorophyll is saturated, and lacking alternation of double and single bonds causing variation in absorption of light.
A tail attached to the porphyrin ring (of the chlorophyll a molecule) is a long hydrocarbon tail. This long hydrophobic extension anchors the chlorophyll a molecule to other hydrophobic proteins in the thylakoid membrane of the chloroplast.
Chlorophyll a biosynthetic pathway utilizes a variety of enzymes. Genes code for the enzymes on the Mg-tetrapyrroles of both bacteriochlorophyll a and chlorophyll a. It begins with glutamic acid, which is transformed into a 5-aminolevulinic acid (ALA). Two molecules of ALA are then reduced to porphobilinogen (PBG), and four molecules of PBG are then coupled, forming protoporphyrin IX. When forming protoporphyrin, Mg-chelatase acts as a catalyst for the insertion of Mg into the chlorophyll a structure. The pathway then uses either a light-dependent process, driven by the enzyme protochlorophyllide, which is a precursor to the production of a chlorophyll a molecule, or a light-independent process driven by other enzymes, to form a cyclic ring, and the reduction of another ring in the structure. Attachment of the phytol tail completes the process of chlorophyll biosynthesis.
Reactions of Photosynthesis
Absorbance of Light
Chlorophyll a absorbs light within the violet, blue and red wavelengths while mainly reflecting green. This reflectance gives chlorophyll its green appearance. Accessory photosynthetic pigments broaden the spectrum of light absorbed, increasing the range of wavelengths that can be used in photosynthesis. The addition of chlorophyll b next to chlorophyll a extends the absorption spectrum. In low light conditions, plants produce a greater ratio of chlorophyll b to chlorophyll a molecules, increasing photosynthetic yield.
Absorption of light by photosynthetic pigments converts photons into chemical energy. Light energy radiating onto the chloroplast strikes the pigments in the thylakoid membrane and excites their electrons. Since the chlorophyll a molecules only capture certain wavelengths, organisms may use accessory pigments to capture a wider range of light energy shown as the yellow circles. It then transfers captured light from one pigment to the next as resonance energy, passing energy one pigment to the other until reaching the special chlorophyll a molecules in the reaction center. These special chlorophyll a molecules are located in both photosystem II and photosystem I. They are known as P680 for Photosystem II and P700 for Photosystem I. P680 and P700 are the primary electron donors to the electron transport chain.
Primary Electron Donation
Chlorophyll a is very important in the energy phase of photosynthesis. Two electrons need to be passed to an electron acceptor for the process of photosynthesis to proceed. Within the reaction centers of both photosystems there are a pair of chlorophyll a molecules that pass electrons on to the transport chain through redox reactions.
- Photosystem II light harvesting protein
- Chlorophyll, green pigment that contains sub essential and accessory pigments
- Chlorophyll b, an accessory pigment of chlorophyll, found in many land plants
- Chlorophyll c, an accessory pigment of chlorophyll
- Methyl group, hydrophobic functional group attached to a CH3
- Photosystem I
- Photosystem II
- Zeiger & Taiz 2006, Topic 7.11: Chlorophyll Biosynthesis
- ^ PHOTOSYNTHESIS
- ^ a b c d e f Raven, Peter H.; Evert, Ray F.; Eichhorn, Susan E. (2005). "Photosynthesis, Light, and Life". Biology of Plants (7th ed.). W.H. Freeman. pp. 119–127. ISBN 0716798115.
- ^ a b Papageorgiou,G, and Govindjee (2004). Chlorophyll a Fluorescence, A Signature of Photosynthesis. 19. Springer. p. 14,48,86
- ^ a b c Eisen JA, Nelson KE, Paulsen IT, et al. (July 2002). "The complete genome sequence of Chlorobium tepidum TLS, a photosynthetic, anaerobic, green-sulfur bacterium". Proc. Natl. Acad. Sci. U.S.A. 99 (14): 9509–14. doi:10.1073/pnas.132181499. PMC 123171. PMID 12093901. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=12093901. See pages 9514,48,86.
- ^ a b c Zeiger, Eduardo; Taiz, Lincoln (2006). "Ch. 7: Topic 7.11: Chlorophyll Biosynthesis". Plant physiology (4th ed.). Sunderland, Mass: Sinauer Associates. ISBN 0-87893-856-7. http://4e.plantphys.net/article.php?ch=0&id=76.
- ^ a b c Lange, L.; Nobel, P.; Osmond, C.; Ziegler, H. (1981). Physiological Plant Ecology I – Responses to the Physical Environment. 12A. Springer-Verlag. pp. 67, 259.
- ^ Campbell, Mary K.; Farrell, Shawn O. (20 November 2007). Biochemistry (6th ed.). Cengage Learning. pp. 647. ISBN 9780495390411. http://books.google.com/books?id=NYa45_BxgukC&pg=PA647.
- ^ a b c Suzuki JY, Bollivar DW, Bauer CE (1997). "Genetic Analysis of Chlorophyll biosynthesis." (PDF). Annu. Rev. Genet 31 (1): 61–89. doi:10.1146/annurev.genet.31.1.61. ftp://18.104.22.168/pub/3.%C9%FA%CE%EF%CE%C4%CF%D7/Annual%20Review/Annual%20Review%20of%20Genetics-Palo%20Alto/1997/Genetic%20analysis%20of%20chlorophyll%20biosynthesis.pdf.
- ^ Zeiger & Taiz 2006, Figure 7.11.A: The biosynthetic pathway of chlorophyll
- ^ a b Ishikita H, Saenger W, Biesiadka J, Loll B, Knapp EW (June 2006). "How photosynthetic reaction centers control oxidation power in chlorophyll pairs P680, P700, and P870". Proc. Natl. Acad. Sci. U.S.A. 103 (26): 9855–60. doi:10.1073/pnas.0601446103. PMC 1502543. PMID 16788069. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=16788069.
Types of Plant pigments Flavonoids BetalainsBetacyanins • Betaxanthins Carotenoids Other Types of tetrapyrroles Bilanes
porphyrinsPorphyrinogensIsobacteriochlorinsSiroheme · SirohydrochlorinCorphins
biochemical families: prot · nucl · carb (glpr, alco, glys) · lipd (fata/i, phld, strd, gllp, eico) · amac/i · ncbs/i · ttpy/i
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