Chloroplasts Are the Site of Photosynthesis

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It is well known that photosynthetic organisms, such as green plants, convert carbon dioxide (GOg) and water to carbohydrates such as glucose (written here as GgHjgOg) and molecular oxygen (O2). 

Lush rain forest vegetation. Photosynthesis linked to oxygen plays an essential role in all life, plant and animal. 

4 Evolutionary Implications of Photosynthesis with and witheut Oxygen 

Outer boundary, Inner boundary membrane / membrane 

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Chloroplasts are the site of photosynthesis, the reactions by which light energy is converted to metabolically useful chemical energy in the form of ATP. These reactions occur on the thylakoid membranes. The formation of carbohydrate from CO9 takes place in the stroma. Oxygen is evolved during photosynthesis. Chloroplasts are the primary source of energy in the light. 

In green plants, chloroplasts are the site of photosynthesis, and we begin by examining their presence, location and stmcture in leaf tissue at several different organizational levels, starting with the leaf itself, as illustrated in Pig. 13. Pigs. 13 (A) and (B) are sketches of a (spinach) leaf and a magnified cross-... 

Chloroplasts (29-36) are the sites of photosynthesis and their ribosomes can carry out protein synthesis. Chloroplasts that contain chlorophylls and carotenoids, are disc shaped and 4-6 pm in diameter. These plastids are comprised of a ground substance (stroma) and are traversed by thylakoids (flattened membranous sacs). The thylakoids are stacked as grana. In addition, the chloroplasts of green algae and plants contain starch grains, small lipid oil droplets, and DNA. 

Chloroplasts in plant cells are surrounded by a double membrane and have an internal membrane system of thylakoid vesicles that are stacked up to form grana. The thylakoid vesicles contain chlorophyll and are the site of photosynthesis. Carbon dioxide (C02) fixation takes place in the stroma, the soluble matter around the thylakoid vesicles. 

Chloroplasts, which are found in green plants and green algae, are the sites of photosynthesis. Both mitochondria and chloroplasts contain DNA that differs from that found in the nucleus, and both carry out transcription and protein synthesis distinct from that directed by the nucleus. 

Plastids are any of a number of interrelated organelles occurring in the cytoplasm of plant cells in which starch, oil, protein, pigments, etc., are stored. The chlorophyll-containing chloroplasts, the site of photosynthesis, are referred to as green plastids. 

In addition to their plasma membrane eukaryotic cells also contain internal membranes that define a variety of organelles (fig. 17.2). Each of these organelles is specialized for particular functions The nucleus synthesizes nucleic acids, mitochondria oxidize carbohydrates and lipids and make ATP, chloroplasts carry out photosynthesis, the endoplasmic reticulum and the Golgi apparatus synthesize and secrete proteins, and lysosomes digest proteins. Additional membranes divide mitochondria and chloroplasts into even finer, more specialized subcompartments. Like the plasma membrane, organellar membranes act as barriers to the leakage of proteins, metabolites, and ions they contain transport systems for import and export of materials, and they are the sites of enzymatic activities as diverse as cholesterol biosynthesis and oxidative phosphorylation. 

The thylakoids and stroma are the sites of the so-called light and dark reactions of photosynthesis, respectively. This compartmentalization of photosynthetic functions was recognized by Park and Pon when they broke open the chloroplasts, separated the contents into thylakoid and stroma fractions and examined their properties. The specific activities of the thylakoids include photochemical reactions, electron transport, oxygen evolution, ATP synthesis and NADP reduction, while the stroma contains enzymes for CO2 fixation driven by ATP and NADPH and other biochemical reactions in the dark. Our understanding and appreciation of the detailed structure and organization of the thylakoid membranes has increased tremendously in recent years. Further discussion of thylakoid structure will be continued in section VII on page 26. 

Chloioplasts are lens-shaped organelles in higher photosynthetic algae and plants. They, like mitochondria, are enclosed within their own intracellular manbranes, reproduce themselves, and contain their own DNA that governs replication of chloroplastic proteins. Chloioplasts are the sites for photosynthesis and contain pigments, including chlorophyll. 

Lupine alkaloids are formed in the green, aerial parts of Lupinus polyphyllus that incorporate labeled cadaverine into the lupanine skeleton, consistent with the fact that the enzymes of alkaloid biosynthesis, in this case, are located in the chloroplast stroma (Hartmann, 1985). Roots of the intact plants or in vitro cultured roots do not. A similar situation obtains for coniine in Conium maculatum, where the en-Z5nnes occur in both the chloroplasts and mitochondria. However, alkaloids are rarely formed in plastids (Hartmann, 1985), but are usually formed in the cytoplasm. Chloroplasts are not only the site of photosynthesis, but also of lipid, amino acid, and terpenoid biosynthesis (Schultz et al., 1985 Wink, 1987). 

Those succulents which unify the sites of malic acid synthesis, storage (large vacuoles), and conversion (chloroplasts) all within the same cells, can be expected to have CAM (see Fig. 2.3). In contrast, those succulents where the potential malic acid stores (water cells) are spatially separated from the sites of photosynthesis will not or only very weakly perform CAM, probably because of the complexity of transport of malic acid. 

The isolation of chloroplasts capable of performing all of the reactions normally regarded as photosynthetic including the fixation of carbon dioxide, the evolution of oxygen and the synthesis of sugars and polysaccharides unequivocally demonstrates that the site of photosynthesis is the chloroplast. Experiments involving the fractionation of chloroplasts have further shown that the dark reactions associated with carbon dioxide fixation are located in the stroma of the chloroplast and the light reactions , electron transport and photophosphorylation, takes place in the lamellar systems. 

Plastids are organelles of plant cells that serve a variety of purposes.45 Most important are the chloroplasts, the chlorophyll-containing sites of photosynthesis. Like mitochondria they contain folded internal membranes (see Fig. 23-19) and several small molecules of DNA. 

Photosynthesis in plants can be inhibited by many conditions, such as invasion by viral pathogens. The effects of TMV on photosynthesis are quite variable (4-6), which may reflect the different effects on photosynthesis induced by the different TMV strains. One of the key issues still to be resolved with the pathology of viral infection is what viral effect causes the expression of disease symptoms. Since TMV infection can affect leaf development and pigmentation and often plant growth, it is quite likely that a primal site of TMV infection is in the chloroplast. However, it is not known if the expression of chlorotic symptoms from TMV infection is the result of the inhibition of photosynthesis (5,7,8) or if TMV inhibits some other host process which induces symptoms and this then causes the inhibition of photosynthesis. 

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