Chloroplasts starch

Swimming cells have a single nucleus and two flagella inserted into a minute papilla at the anterior end of the cell the cell wall is thin. Most of the cell volume is occupied by one or more grass-green chloroplasts. In the most frequently used species, C. reinhardtii, only one cup-shaped chloro-plast is present one or more pyrenoids are present within the chloroplast starch grains surround the pyrenoid. 

A further ramification of the translocator-mediated exchange of exported triose phosphate and imported Pj pertains to starch synthesis. When cytosolic metabolism and Pj availability are limited, leading to a high 3-PGA/P ratio in the chloroplast, starch synthesis will be stimulated. This occurs because ADP-glucose py-rophosphorylase, the major regulatory enzyme in starch synthesis, is strongly activated by 3-PGA and inhibited by Pj [29]. As mentioned above, starch synthesis from triose phosphate will release Pj, relieving to some extent the Pj limitation of CO2 fixation. 

During very active periods of photosynthesis, triose phosphates are converted to starch. Under normal conditions, approximately 30% of the CO, fixed by leaves is incorporated into starch, which is stored as water-insoluble granules. During a subsequent dark period, most chloroplast starch is degraded and converted to sucrose. Sucrose is then exported to storage organs and rapidly growing tissues. In these tissues (e.g, tubers and seeds), most sucrose molecules are used to synthesize starch, which is stored primarily within a specialized plastid called an amyloplast. 

Where C5 represents RuBP and C3 is a three-carbon carbohydrate, either glyceraldehyde or dihydroxy acetone. The C3 can either be exported from the chloroplast directly, used for the synthesis of lipids and proteins within the chloroplast, or used to produce starch (a polymer of glucose, a Ce carbohydrate) that can be retained in the chloroplast (in cells without chloroplasts, starch can occur in the cytosol). 

Fig. 3. Primary carbon metabolism in a photosynthetic C3 leaf. An abbreviated depiction of foliar C02 uptake, chloroplastic light-reactions, chloroplastic carbon fixation (Calvin cycle), chloroplastic starch synthesis, cytosolic sucrose synthesis, cytosolic glycolysis, mitochondrial citric acid cycle, and mitochondrial electron transport. The photorespiration cycle spans reactions localized in the chloroplast, the peroxisome, and the mitochondria. Stacked green ovals (chloroplast) represent thylakoid membranes. Dashed arrows near figure top represent the C02 diffusion path from the atmosphere (Ca), into the leaf intercellular airspace (Ci), and into the stroma of the chloroplast (Cc).SoHd black arrows represent biochemical reactions. Enzyme names and some substrates and biochemical steps have been omitted for simplicity. The dotted line in the mitochondria represents the electron transport pathway. Energy equivalent intermediates (e.g., ADP, UTP, inorganic phosphate Pi) and reducing equivalents (e.g., NADPH, FADH2, NADH) are labeled in red. Membrane transporters Aqp (CO2 conducting aquaporins) and TPT (triose phosphate transporter) are labeled in italics. Mitochondrial irmer-membrane electron transport and proton transport proteins are labeled in small case italics.

The ultrastructure of Stichococcus diplosphaera in squamules of Endo-carport pusillum is very similar to that of Trebouxia. Stichococcus has a chloroplast which almost fills the cell and contains a central pyrenoid. Ten parallel chloroplast lamellae pass through the pyrenoid. Pyrenoglobuli were found at the periphery of the pyrenoid and in the stroma of the chloroplast. Starch was present also (Ahmadjian and Jacobs, 1970). 

Starch is stored in plant cells in the form of granules in the stroma of plas-tids (plant cell organelles) of two types chloroplasts, in which photosynthesis takes place, and amyloplasts, plastids that are specialized starch accumulation bodies. When starch is to be mobilized and used by the plant that stored it, it must be broken down into its component monosaccharides. Starch is split into its monosaccharide elements by stepwise phosphorolytic cleavage of glucose units, a reaction catalyzed by starch phosphorylase (Figure 7.23). This is formally an a(1 4)-glucan phosphorylase reaction, and at each step, the prod-... 

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. 

Heldt H W, Chon C J, Maronde D, Harold A, Stankovic Z S, Walker D A, Kraminer A, Kirk M R and Heber U (1977), Role of orthophosphate and other factors in the regulation of starch formation in leaves and isolated chloroplasts , Plant Physiol, 59, 1146-1155. 

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. 

Starch, a reserve polysaccharide widely distributed in plants, is the most important carbohydrate in the human diet. In plants, starch is present in the chloroplasts in leaves, as well as in fruits, seeds, and tubers. The starch content is especially high in cereal grains (up to 75% of the dry weight), potato tubers (approximately 65%), and in other plant storage organs. 

Cushman, K. E., Tibbitts, T. W., Sharkey, T. D., Wise, R. R. (1995). Constant-light injury of potato Temporal and spatial patterns of carbon dioxide assimilation, starch content, chloroplast integrity, and necrotic lesions. J. Amer. Soc. Hort. Sci., 120,1032-1040. 

Topical eukaryotic cells (Fig. 1-7) are much larger than prokaryotic cells—commonly 5 to 100 pm in diameter, with cell volumes a thousand to a million times larger than those of bacteria. The distinguishing characteristics of eukaryotes are the nucleus and a variety of membrane-bounded organelles with specific functions mitochondria, endoplasmic reticulum, Golgi complexes, and lysosomes. Plant cells also contain vacuoles and chloroplasts (Fig. 1-7). Also present in the cytoplasm of many cells are granules or droplets containing stored nutrients such as starch and fat. 

The chloroplast stroma contains all the enzymes necessary to convert the triose phosphates produced by C02 assimilation (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate) to starch, which is temporarily stored in the chloroplast as insoluble granules. Aldolase condenses the trioses to fructose 1,6-bisphos-phate fructose 1,6-bisphosphatase produces fructose 6-phosphate phosphohexose isomerase yields glucose 6-phosphate and phosphoglucomutase produces glucose 1-phosphate, the starting material for starch synthesis (see Section 20.3). 

Sucrose synthesis in the cytosol and starch synthesis in the chloroplast are the major pathways by which the excess triose phosphate from photosynthesis is harvested. Sucrose synthesis (described below) releases four Pi molecules from the four triose phosphates required to make sucrose. For every molecule of triose phosphate removed from the chloroplast, one Pj is transported into the chloroplast, providing the ninth Pj mentioned above, to be used in regenerating ATP. If this exchange were blocked, triose phosphate synthesis would quickly deplete the available Pj in the chloroplast, slowing ATP synthesis and suppressing assimilation of C02 into starch. 

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