chloroplast(redirected from Chloroplasts)
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an intracellular organelle (plastid) of the plant cell, in which photosynthesis occurs. Chloroplasts are green in color because of the presence of chlorophyll, the main pigment of photosynthesis. Their principal function, the capture and transformation of light energy, is also reflected in the uniqueness of their structure.
In higher plants, chloroplasts are lens-shaped bodies, measuring 3–10 micrometers in diameter and 2–5 micrometers in width. They form a system of protein-lipid membranes, embedded in the ground substance—the matrix or stroma—and separated from the cytoplasm by an outer membrane. The internal system of membranes forms a single continuous lamellar system, consisting of tiny closed flat sacs called thylakoids, which stack on top of one another to form grana. A granum may have ten to 30 thylakoids, and there can be as many as 150 grana in a chloroplast; the grana are connected to one another by large thylakoids. Such a structure substantially increases the photoactive surface of the chloroplast and ensures maximal utilization of light energy.
The initial photo stage of photosynthesis occurs in the thylakoid membrane, which consists of two layers of proteins, separated by a layer of lipids; this stage results in the formation of the two compounds necessary for the assimilation of CO2, namely, reduced nicotinamide adenine dinucleotide phosphate (NADPH) and the energy-rich adenosine triphosphate (ATP). The source of energy for the formation of ATP molecules is the difference in potentials produced on the membrane as a result of vector (directed) charge transfer. The charge is distributed on both sides of the membrane by virtue of the special arrangement of the components of the electron-transport chain that interlaces the membrane. The membranes, functioning as “partitions,” effect the spatial separation of the products of photosynthesis, such as O2 and the reducing agents, which without the membranes would interact with one another. The outer part of the thylakoid is covered with particles that measure 14–15 nanometers in diameter, which act as “coupling factors” and participate in the synthesis of ATP. The enzymes responsible for the fixation of CO2 (the dark stage of photosynthesis) are concentrated in the stroma.
Plants capable of “cooperative” photosynthesis have two types of chloroplasts, which differ in structure and functions. The first type, found in the mesophyll cells, includes small chloroplasts with grana, while the second type includes larger chloroplasts, which are present in the cells of the vascular bundles’ lining and in which the grana are rudimentary or completely absent. The second type of chloroplast is where photosystem I functions, producing ATP in the course of cyclic phosphorylation and NADPH through the decarboxylation of malic acid. The chloroplasts of the lining cells are responsible for the fixation of CO2 on rubilose diphosphate, which occurs in the course of the Calvin cycle, while the chloroplasts of the mesophyll cells are responsible for the fixation of CO2 on phosphoenolpyruvate (Hatch-Slack pathway). Thus, the interaction of both types of chloroplasts is responsible for the high efficiency of photosynthesis in plants. In addition to the enzymes of fixation of CO2, the stroma of chloroplasts also contains chains of DNA, ribosomes, starch grains, and osmiophil granules.
The presence in chloroplasts of their own genetic apparatus and of a specific protein-synthesizing system is responsible for the definite, although relative, autonomy of the chloroplasts in the cell. As the plant develops and reproduces, chloroplasts arise in new generations of cells only by division. Scientists relate their origin to symbiogenesis, speculating that the recent chloroplasts are the offspring of blue-green algae that entered into a symbiotic relationship with ancient nuclear heterotrophic cells of colorless algae or protozoans.
Chloroplasts occupy 20–30 percent of the plant cell’s volume. Some algae, such as those of the genus Chlamydomonas, contain only a single chloroplast, while a higher plant cell contains ten to 70 chloroplasts. Chloroplasts arise from proplastids, small vesicles that separate from the nucleus. At the end of the plant growing period, chloroplasts lose their green color owing to the destruction of chlorophyll and become chromoplasts.
REFERENCES“Khloroplasty i mitokhondrii.” In Voprosy membrannoi biologii. (Collection of articles.) Moscow, 1969.
Loewy, A. G., and P. Siekevitz. Struktura ifunktsiia kletki. Moscow, 1971. (Translated from English.)
Heath, O. Fotosintez. Moscow, 1972. (Translated from English.)
Baslavskaia, S. S. Fotosintez. Moscow, 1974.
Nasyrov, Iu. S. Fotosintez i genetika khloroplastov. Moscow, 1975.
Structure and Function of Chloroplasts. Edited by M. Gibbs. Boston, Mass., 1971.
R. M. BEKINA