Plastid

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plastid

[′plas·təd]
(cell and molecular biology)
One of the specialized cell organelles containing pigments or protein materials, often serving as centers of special metabolic activities; examples are chloroplasts and leukoplasts.

Plastid

 

any of various intracellular organelles that contain pigments and synthesize organic matter in the cytoplasm of autotrophic plants. In higher plants there are three types of plastids: green chloroplasts, colorless leucoplasts, and variously colored chromoplasts. The entire plastid complex of an organism is called a plastidome.

Chloroplasts are lenticular or rounded corpuscles that measure 4–6 μ in diameter (less commonly, up to 9 μ; in rare exceptions, up to 24 μ). They contain about 50 percent protein, 35 percent lipids, and 7 percent pigments, as well as a small quantity of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Plastids closely interact with other cell components; however, they have some genetic autonomy, since they contain DNA and RNA. Chloroplast pigments in higher plants are represented by green chlorophylls a and b and by carotenoids—red-orange carotene and yellow xanthophyll. The DNA in the chloroplast differs somewhat from that of the nucleus; however, it does resemble the DNA of blue-green algae and bacteria.

Under an ordinary microscope, the structure of the chloroplast appears to be granular. Studies with the electron microscope have established that the chloroplast is separated from the cytoplasm by a two-layer lipid-protein membrane.

The colorless stroma, or matrix, of the chloroplast has a lamellar system consisting of two types of flat saccules, or thylakoids, which form from the lipid-protein membrane. The smaller saccules, which are located in the granular structures, are gathered into packets resembling stacks of coins. The larger saccules are distributed between the smaller ones and in the intergranular areas of the stroma (thylakoids of the stroma). On the outer surface on the thylakoids the protein component of the membranes consists of globular protein-enzymes (polyenzyme complexes). The membrane is also made up of chlorophylls and carotenoids, forming a lipid-protein-pigment complex, in which photosynthesis is effected in the presence of light. This type of structure increases by many times the chloroplast’s active synthesizing surface. Such plastids are capable of reproducing by division into two approximately equal parts. A chloroplast also reproduces by budding: a small part bubbles, enlarges, and develops into a new chloroplast.

Leucoplasts are small, rounded or elongate, colorless corpuscles that are present in all living plant cells. Simple organic compounds are synthesized in the leucoplasts into more complex substances—starch and possibly fats and proteins—which are stored in the tissues of tubers, roots, rhizomes, and seed endosperm. Leucoplasts are divided, according to the type of matter accumulated, into amyloplasts, elaioplasts, and proteinoplasts. Like chloroplasts, they have a membrane consisting of two lipoid-protein layers, but their stroma contains only one or several processes of the inner layer of the membrane (similar to those in mitochondria).

Chromoplasts may be rounded, irregularly angled, or even needle-shaped. They contain carotenoids and impart yellow and orange colors to autumn leaves, to the bracts of the perianth, and to ripening and mature fruits of such plants as the plastid is capable of changing into any of the other forms. For example, leucoplasts may turn into chloroplasts (for example, the greening of potato tubers in the light). By autumn, the chloroplasts lose chlorophyll and are changed into chromoplasts, which, in turn, are capable of changing into chloroplasts (for example, the greening of the tops of carrot roots in light). All plastids have a common origin. They develop from initial particles—small, bubble-like formations that separate from the membrane of the cell nucleus. Many Soviet and foreign biologists regard plastids as modified blue-green algae that entered at the dawn of life into symbiotic relationships with the cells of heterotrophic organisms.

In most algae the plastids are represented by one or several chromatophores that differ in shape and size: an unbroken (in Mougeotia) or perforated (in Cladophora) lamella, two stellate lamellae (in Zygonema), one or two spirally wound strips (in Spirogyra), and a cup-shaped membrane (in Chlamydomonas). Chromatophores usually have specialized areas, or pyrenoids, in which reserve matter—the products of synthesizing activity—is accumulated. In a number of algae the chromatophores contain, in addition to chlorophylls and carotenoids, other pigments that mask the green coloring of the chlorophyll (for example, infusorial earth diatoms, phycoxanthin in brown algae, blue phycocyanin in blue-green algae, and red fucoerythrin in red algae). Chromatophores have casings of two lipid-protein membranes that have the same structure as those in plastids of higher plants. In the stroma there is a multilayer structure of several lamellae that are similar to the thylakoids of chloroplat stroma.

REFERENCES

Guliaev, V. A. “Osobennosti stroeniia rastitel’nykh kletok.” In Rukovodstvo po tsitologii, vol. 1. Moscow-Leningrad, 1965.
Frey-Wyssling, A., and K. Miuletaler. Ul’trastruktura rastitel’noi kletki. Moscow, 1968. (Translated from English.)
Sager, R. “Struktura khloroplasta i ee sviaz’ s fotosinteticheskoi aktivnost’iu.” In the collection Struktura i funktsiia fotosinteticheskogo apparata. Moscow, 1962. (Translated from English.)
Wettstein, D. “Formirovanie plastidnykh struktur.” In the collection Struktura i funktsiia fotosinteticheskogo apparata. Moscow, 1962. (Translated from English.)

D. A. TRANKOVSKII