Студопедия — Morphology and Major Molecular Components of Chloroplasts Chloroplast Structure.
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Morphology and Major Molecular Components of Chloroplasts Chloroplast Structure.






The chloroplasts of algae and higher plant cells are easily seen under the light microscope as discoid or lens-shaped bodies with overall dimensions of about 5-10 µm. Usually chloro­plasts are considerably larger than the mitochondria in the same plant cell and occur in smaller numbers. The number may vary from a single chloroplast, as found in Micromonas and several other green algae, to several hundred in the cells of most higher plants.

The green pigments of chloroplasts can be seen under the light microscope to be concentrated in disclike substructures, the grana, in most eukaryotic plants. Grana are suspended in a colorless background substance in the chloroplast interior. This pigment distribution is maintained by a system of mem­branes and compartments visible in chloroplasts under the electron microscope (Figs. 8-1 and 8-2). Two continuous boundary membranes, roughly ['rʌflɪ] analogous to the inner and outer boundary membranes of mitochondria, separate the chloroplast interior from the surrounding cytoplasm. The re­gion ['riːʤ(ə)n] between these membranes, the intermembrane compart­ment, is usually difficult to trace in electron micrographs (as in Fig. 8-1) because of extensive folding of the inner boundary membrane. The two boundary membranes enclose an interior compartment, the stroma, equivalent in location to the mitochondrial matrix. Within the stroma is a complex system of membranous sacs that forms the grana and their connec­tions.

The individual unit of a granum is a flattened sac or vesi­cle, the thylakoid (from thylakos, meaning "sack" or "pouch"), consisting of a single, continuous membrane enclosing an interior thylakoid compartment. A granum is formed from a closely fused pile of these individual sacs, set one on top of another much like a stack of coins. Chloroplasts may contain from 40 to 60 grana, each formed from stacks of 2 or 3 to as many as 100 individual thylakoids.

Numerous membranous connections can be seen between the thylakoids of adjacent grana (circled in Fig. 8-1). These connections, called stromal lamellae, enclose a channel that is continuous with the thylakoid compartments of adjacent grana (diagrammed in Figs. 8-2 and 8-3). The stromal lamellae may connect the thylakoid compartments into a single, continuous internal region inside chloroplasts. Stacking and fusion of thylakoids into grana is typical of most chloroplasts of the green algae and higher plants. Elongated, single thylakoids appear along with grana in a few chloroplast types in higher plants, most notably in the bundle sheath cells of some plants with the C4 photosynthetic path­way (see pp. 188-190). Unstacked thylakoids occur exclusively in nongreen algae. In the chloroplasts of red algae, thylakoids are single and greatly elongated (Fig. 8-4). In other nongreen algae, elongated thylakoids occur in multiple layers without fusion of adjacent membranes (Fig. 8-5 shows a chloroplast of this type from a brown alga). Despite these variations, the basic structure of thylakoids is uniform. Each thylakoid con­sists of a single, continuous membrane enclosing a flattened inner compartment that is completely separated from the sur­rounding stroma by the thylakoid membrane.

The chloroplast stroma may contain a variety of inclusions suspended in the regions surrounding the thylakoid and stromal lamellae membranes. Most conspicuous of these are starch granules (Fig. 8-6), found in chloroplasts after a period of active photosynthesis in the light. The stroma also frequently contains dense, spherical particles called osmiophilic granules or droplets (visible in Fig. 8-1), probably representing aggregates of lipids suspended in the stromal solution. In algae, large bodies called pyrenoids appear in the stroma (see Fig. 8-6b). Because these bodies are frequently surrounded by a layer of starch granules, they have traditionally been regarded as func­tioning in carbohydrate synthesis. This conclusion is sup­ported by the work of R. H. Holdsworth (1971), who analyzed isolated pyrenoids isolated from a green alga. Holdsworth found an enzyme active in the initial step in carbohydrate synthesis (ribulose diphosphate carboxylase) to make up a large part of the mass of the pyrenoids.

The chloroplast stroma, like the mitochondrial matrix, contains DNA, ribosomes, and all of the biochemical factors required for DNA replication, RNA transcription, and protein synthesis. Of the visible elements of this system, the DNA ap­pears at scattered locations in the stroma as faintly visible threads (see Fig. 12-43). The ribosomes, conspicuously smaller than the ribosomes of the surrounding cytoplasm, are distributed throughout the stroma (see Fig. 13-34; the func­tions of the chloroplast DNA and ribosomes in replication, transcription, and protein synthesis are discussed at length in Supplements 12-2 and 13-1).Glycolipids constitute about 45% of the lipid fraction. These lipids are built up from a glycerol backbone, combined with two fatty acid chains and a mono- or disaccharide carbo­hydrate group (Fig. 8-7). Some of the glycolipids, about 4% of the total lipid fraction, contain sulfur atoms as a part of their carbohydrate groups. One of these unusual glycolipids, termed sulfolipids, is shown in Fig. 8-7. Phospholipids, at about 10% of the total, form a much smaller fraction of the lipid content than they do in mitochondria. Chloroplast quinones ['kwɪniːn], including plastoquinones (Fig. 8—7c) and vitamin K, make up about 6% of the total chloroplast lipids; some of these molecules function in photosynthetic electron transport. Sterols are present in chloroplast membranes at concentrations less than 1%. The remaining lipids of chloroplasts consist primarily of the photosynthetic pigments, the chlorophylls and carotenoids, which make up 28 to 30% of the total.

The predominant lipid pigments of chloroplasts are the chlorophylls, which absorb red and blue wavelengths['weɪvleŋθ] most strongly and transmit green light[laɪt]. The various chlorophylls have in common a porphyrin['pôrfərin] ring to which a long phytol side chain (an alcohol) is attached (Fig. 8-8). The porphyrin ring of chlorophyll is similar to the prosthetic groups of the cyto­chromes and hemoglobin except for the presence of an extra subunit (shaded in Fig. 8-8) linked at one side. A magnesium atom is bound at the center of the porphyrin ring in the chlorophylls. At least three major types, chlorophyll a, b, and c, which differ only in the side groups bound to the porphyrin ring, occur in different eukaryotic organisms.

All photosynthetic eukaryotes utilize chlorophyll a as their primary photosynthetic pigment. In the green algae and higher plants, chlorophyll b occurs as a second major photo­synthetic pigment. Chlorophyll c occurs with chlorophyll a in the brown algae, diatoms, and dinoflagellates. Chlorophyll a is also found as the sole chlorophyll pigment in the blue-green algae. These chlorophyll pigments occur in combination with specific proteins, held in place in the amino acid chain of the protein by hydrophobic, noncovalent bonds (see, for example, Markwell, Thomber, and Boggs, 1979).

The wavelengths of light absorbed by the different chlorophylls vary depending on their state of binding within chloroplast membranes. Purified chlorophyll a, for example, absorbs light most strongly at wavelengths of 420 and 675 nm in acetone solutions. Within membranes, chlorophyll a may show other absorption maxima, at 660, 670, 678, 685, or 700 nm, along with the 420- and 675-nm maxima. These changes in absorption are due to interactions between the chlorophylls and between the chlorophylls and their binding proteins, and not to chemical changes in the chlorophyll molecules. The dif­ferently absorbing forms of chlorophyll a, for example, are chemically identical.

The carotenoids (Fig. 8-9) are also found in all photo­synthetic organisms. These lipid-soluble pigments absorb light at blue wavelengths, from 400 to 550 nm, and transmit yellow and orange. All of the carotenoids are based on a long carbon chain containing 40 carbon atoms, linked together by double and single bonds. Various substitutions in the side groups at­tached to the 40-carbon backbone give rise to the different carotenoid pigments. Two types of carotenoids occur in both eukaryotes and prokaryotes. The carotenes are pure hydrocar­bons; most abundant among these in higher plants is fi-carotene (Fig. 8-9a). The second type of carotenoid, the carotenols, con­tains hydroxyl (—OH) groups at both ends of the carbon chain. The yellow pigment xanthophyll (Fig. 8-9b) is the predominant carotenol in higher plants.

A final group of pigments is found in addition to chloro­phyll a in the blue-green and red algae. These pigments, called phycobilins, contain a light-absorbing molecular structure re­sembling the porphyrin ring of hemoglobin except that the complex occurs in an extended, linear form (Fig. 8-10). Two kinds of phycobilins, the phycocyanins andphycoerythrins, occur in both the blue-green and red algae. In both groups, the phy-cobilin pigments occur in combination with proteins in com­plexes known as phycobiliproteins.

The carotenoids and phycobilins function as accessory pig­ments that take up light at wavelengths not directly absorbed by the chlorophylls. The light energy absorbed by these pig­ments and by chlorophylls b or c is eventually transferred to chlorophyll a, which is the photosynthetic pigment immedi­ately involved in transforming light into chemical energy. The net effect of the entire complement of photosynthetic pigments is to greatly broaden the spectrum of wavelengths usable as energy sources for photosynthesis.







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