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.