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chlorophyll (klôrˈəfĭlˌ), green pigment that gives most plants their color and enables them to carry on the process of photosynthesis. Chemically, chlorophyll has several similar forms, each containing a complex ring structure and a long hydrocarbon tail. The molecular structure of the chlorophylls is similar to that of the heme portion of hemoglobin, except that the latter contains iron in place of magnesium. Within the photosynthetic cells of plants the chlorophyll is in the chloroplasts—small, roundish, dense protoplasmic bodies that contain the grana, or disks, where the chlorophyll molecules are located. Most forms of chlorophyll absorb light in the red and blue-violet portions of the visible spectrum; the green portion is not absorbed and, reflected, gives chlorophyll its characteristic color. Chlorophyll f absorbs near infrared wavelengths that are slightly beyond the red portion of the visible spectrum. Chlorophyll tends to mask the presence of colors in plants from other substances, such as the carotenoids. When the amount of chlorophyll decreases, the other colors become apparent. This effect can be seen most dramatically every autumn when the leaves of trees “turn color.”
Chlorophyta (klōrŏfˈətə), phylum (division) of the kingdom Protista consisting of the photosynthetic organisms commonly known as green algae. The organisms are largely aquatic or marine. The various species can be unicellular, multicellular, coenocytic (having more than one nucleus in a cell), or colonial. Those that are motile have two apical or subapical flagella. A few types are terrestrial, occurring on moist soil, on the trunks of trees, on moist rocks, and even in snowbanks. Various species are highly specialized, some living exclusively on turtles, sloths, or within the gill mantles of marine mollusks.
It is generally accepted that early chlorophytes gave rise to the plants. Cells of the Chlorophyta contain organelles called chloroplasts in which photosynthesis occurs; the photosynthetic pigments chlorophyll a and chlorophyll b, and various carotenoids, are the same as those found in plants and are found in similar proportions. Chlorophytes store their food in the form of starch in plastids and, in many, the cell walls consist of cellulose. Unlike in plants, there is no differentiation into specialized tissues among members of the division, even though the body, or thallus, may consist of several different kinds of cells. There are four evolutionary lineages of green algae. Most living species are grouped in classes that are coextensive with three of these lineages.
This group contains the largest number of species of the division. They can have two or more flagella, near the apex of the cell. Mitosis in this class involves phycoplasts, microtubules that develop between and separate the daughter nuclei. This characteristic is not seen in any other organism, implying that no organisms have descended from this class. There are a variety of asexual and sexual reproductive techniques. Sexual reproduction is characterized by the formation of a zygospore (a dormant diploid zygote protected by a thick wall) that later undergoes meiosis.
The class includes unicellular organisms such as those in the genus Chlamydomonas with their two apical flagella and nonmotile organisms such as Chlorella, which is being cultivated for use as a dietary supplement. Colonial genera of Chlorophyceae include Hydrodictyon (the “water net”) and the so-called volvocine line of flagellated specimens that range from simple colonies of Gonium to the intricate spinning spheres of Volvox, which can consist of up to 60,000 cells and exhibit some cellular specialization. The most complex of the class are the filamentous members, some of which exhibit features that are seen primarily in plants. Despite this similarity the class is not believed to have been the evolutionary source of plants.
Charophyceae are of great fossil age; the stoneworts date as far back as the late Silurian period. Cells of this class are asymetrical. Those that are motile have two flagella, at right angles near the apex of the cell. Sexual reproduction in this class, as in Chlorophyceae, is characterized by the formation of a zygospore and zygotic meiosis. Unlike in the other two common classes of green algae, but as with plants, the nuclear envelope disintegrates when mitosis begins. During cell division the mitotic spindle is present; in some a phragmoplast similar to those seen in plants aids in the formation of a cell plate. Plants are thought to have evolved from early species of Charophyceae.
The class includes Spirogyra, familiar filamentous algae that float on ponds and lakes in slimy masses. The desmids are single cells noted for their extraordinary symmetry and geometrical beauty. They are found only in fresh (usually still) water and often take an important place in the food chains of small nutrient-poor ponds and peat bogs. The stoneworts consist of a complex branched thallus with an erect stemlike structure and many whorls of short branches. They occur in shallow fresh or brackish water and especially in water rich in calcium, where they become stiff and lime-encrusted, a characteristic that has made them plentiful in the fossil record.
Ulvophyceae contains marine organisms that take a variety of shapes that may consist of a few cells, long filaments, thin sheets of cells, or coenocytic cells. Most approach being radially symmetrical. They have an alternation of generations and unlike in the other classes, meiosis occurs in the spores rather than the zygotes. When present, there can be two or more apical flagella. During mitosis, the nuclear envelope and the mitotic spindle persist, as they do in the Charophyceae.
The class Ulvophyceae includes sea lettuce, or Ulva, bright green, leaflike algae that grows in shallow waters on rocks and piers. Ventricaria is an egg-shaped, coenocytic alga, familiar in warm seas. Some organisms of Ulvophyceae produce toxins that discourage predation. The chloroplasts of some others become symbionts after they are retained in the bodies of sea slugs that eat the algae. They continue to perform photosynthesis, providing the slug with needed oxygen.
See also seaweed.
See H. C. Bold and M. J. Wynne, Introduction to the Algae: Structure and Reproduction (1985); C. A. Lembi and J. R. Waaland, Algae and Human Affairs (1988); C. van den Hoek, Algae: an Introduction to Phycology (1994).
The generic name for the intensely colored green pigments which are the photoreceptors of light energy in photosynthesis. These pigments belong to the tetrapyrrole family of organic compounds.
Five closely related chlorophylls, designated a through e, occur in higher plants and algae. The principal chlorophyll (Chl) is Chl a, found in all oxygen-evolving organisms; photosynthetic bacteria, which do not evolve O2, contain instead bacteriochlorophyll (Bchl). Higher plants and green algae contain Chl b, the ratio of Chl b to Chl a being 1:3. Chlorophyll c (of two or more types) is present in diatoms and brown algae. Chlorophyll d, isolated from marine red algae, has not been shown to be present in the living cell in large enough quantities to be observed in the absorption spectrum of these algae. Chlorophyll e has been isolated from cultures of two algae, Tribonema bombycinum and Vaucheria hamata. In higher plants the chlorophylls and the above-mentioned pigments are contained in lipoprotein bodies, the plastids. See Carotenoid, Cell plastids, Photosynthesis
Chlorophyll molecules have three functions: They serve as antennae to absorb light quanta; they transmit this energy from one chlorophyll to another by a process of “resonance transfer;” and finally, this chlorophyll molecule, in close association with enzymes, undergoes a chemical oxidation (that is, an electron of high potential is ejected from the molecule and can then be used to reduce another compound). In this way the energy of light quanta is converted into chemical energy.
The chlorophylls are cyclic tetrapyrroles in which four 5-membered pyrrole rings join to form a giant macrocycle. Chlorophylls are members of the porphyrin family, which plays important roles in respiratory pigments, electron transport carriers, and oxidative enzymes. See Porphyrin
It now appears that the chlorophyll a group may be made up of several chemically distinct Chl a species. The structure of monovinyl cholorophyll a, the most abundant of the Chl a species, is shown in the illustration.
The two major pigments of protoplasm, green chlorophyll and red heme, are synthesized from ALA (δ-aminolevulinic acid) along the same biosynthetic pathway to protoporphyrin. ALA is converted in a series of enzymic steps, identical in plants and animals, to protoporphyrin. Here the pathway branches to form (1) a series of porphyrins chelated with iron, as heme and related cytochrome pigments; and (2) a series of porphyrins chelated with magnesium which are precursors of chlorophyll. See Hemoglobin
Chlorophylls reemit a fraction of the light energy they absorb as fluorescence. Irrespective of the wavelength of the absorbed light, the emitted fluorescence is always on the long-wavelength side of the lowest energy absorption band, in the red or infrared region of the spectrum.
The fluorescent properties of a particular chlorophyll are functions of the structure of the molecule and its immediate environment. Thus, the fluorescence spectrum of chlorophyll in the living plant is always shifted to longer wavelengths relative to the fluorescence spectrum of a solution of the same pigment. This red shift is characteristic of aggregated chlorophyll.
the green pigment of plants, by means of which plants capture the energy of sunlight and effect photosynthesis (seePHOTOSYNTHESIS). It is localized in special cell structures—chloroplasts and chromatophores—and is associated with the proteins and lipids of the chloroplast membranes. The basic structural unit of the chlorophyll molecule is a magnesium complex of the porphyrin cycle; the high-molecular-weight alcohol phytol, which is attached to the propionic-acid radical in the IV pyrrole ring, gives chlorophyll the capacity to become fixed in the lipid layer of the chloroplast membranes.
Higher plants and green algae contain chlorophylls a and b, brown algae and diatoms contain chlorophylls a and c, and red algae contain chlorophylls a and d. Photosynthesizing bacteria contain close analogs of chlorophyll known as bacteriochlorophylls. Structurally, chlorophyll is closely related to other natural porphyrin complexes with iron, namely, cytochromes, which are the respiratory pigments, and heme, the red pigment that gives blood its characteristic color. It is also related to prosthetic groups of certain enzymes, namely, peroxidase and catalase.
The name “chlorophyll” was given in 1817 by the French chemists P. Pelletier and J. Caventou to a green alcohol solution of a mixture of plant pigments. In the early 20th century, the Russian scientist M. S. Tsvet (Tswett) was the first to distinguish chlorophylls a and b, using the chromatographic method, which he developed. The chemical structure of chlorophyll was elucidated by the German scientists R. Willstátter, A. Stoll (1913), and H. Fischer (1930–40). The complete synthesis of chlorophyll was effected by the American chemist R. Woodward. Chlorophyll’s role in photosynthesis was proved by the classic works of K. A. Timiriazev. The biosynthetic pathways of chlorophyll were elucidated by the American scientists D. Shemin and S. Granick, among others. The Soviet scientists T. N. Godnev and A. A. Shlyk made important contributions to the study of chlorophyll.
The principal biosynthetic pathway of chlorophyll is determined by the condensation of two molecules of δ-aminolevulinic acid with the formation of porphobilinogen—a derivative of pyrrole, which as a result of a series of enzymatic steps yields protoporphyrin IX, a compound containing a porphyrin nucleus. From protoporphyrin is formed the immediate precursor of chlorophyll—protochlorophyllide, which already contains a magnesium atom. Chlorophyll is formed from protochlorophyllide by means of successive reactions of reduction and the addition of phytol. The reduction stage of protochlorophyllide is effected in the light in higher plants and in the dark in lower plants.
In the chloroplasts and chromatophores a large part of the chlorophyll, whose content generally constitutes 0.5 to 1.5 percent of the dry weight, is found in the form of light-gathering “antennae,” while a smaller amount is found in the reaction centers, which participate directly in the photosynthetic electron-transfer chain. Upon absorbing a quantum of light, the chlorophyll molecule enters an excited state (a singlet state of excitation lasts about 10–9 sec), which may then pass to the triplet state, a long-lived excited state lasting as long as 10–3 sec. Chlorophyll molecules excited by light are capable of transferring an electron from a donor molecule to an acceptor molecule.
The mechanisms of these reactions in model systems have been elucidated by the Soviet scientists A. A. Krasnovskii and V. B. Evstigneev, among others. The ability of excited chlorophyll to transfer electrons ensures the functioning of the reaction centers of the photosystems in the photosynthetic electron-transfer chain. The use of techniques involving spectral analysis and low temperatures has shown that in the initial photo stage, the bacteriochlorophyll and possibly the chlorophyll of the active center give up their electron to an acceptor molecule (ubiquinone, ferrodoxin). This initial process is bound to the chain of enzymatic reactions that lead to the formation of reduced pyridine nucleotides and adenosine triphosphate, which ensure the functioning of the carbon cycle. Thus, the light absorbed by chlorophyll is converted to the potential chemical energy of the organic photosynthetic products and molecular oxygen. The light absorbed by chlorophyll also produces other photobiological phenomena in cells; for example, it induces the generation of an electric potential on the membranes of the chloroplasts and influences the movement of unicellular organisms (phototaxis).
Much attention is being focused on the properties of chlorophyll at various levels of molecular organization, inasmuch as these properties are closely related to the fundamental phenomenon of the conversion of light energy to chemical energy in the process of photosynthesis.
REFERENCESTimiriazev, K. A. “Solntse, zhizn’ i khlorofill.” Izbr. soch., vol. 1. Moscow, 1948.
Godnev, T. N. Stroenie khlorofilla i metody ego kolichestvennogo opredeleniia. Minsk, 1952.
Khlorofill: Sb. st. Minsk, 1974.
Krasnovskii, A. A. Preobrazovanie energii sveta pri fotosinteze: Molekuliarnye mekhanizmy. Moscow, 1974. (Bakhovskie chteniia, 29.)
Vernon, L. P., and G. R. Sealy. The Chlorophylls. New York–London, 1966.
A. A. KRASNOVSKII