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Reconstructing past ocean pH-depth profiles 总被引:1,自引:0,他引:1
Measurement of boron isotope compositions in species of planktonic foraminifera that calcified their tests at different depths in the water column are used to reconstruct the pH profile of the upper water column of the tropical ocean. Results for five time windows from the middle Miocene to the late Pleistocene indicate pH-depth profiles similar to that of the modern ocean in this area, which suggests that this method may greatly aid in our understanding of the global carbon cycle. 相似文献
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ZF Ren ZP Huang JW Xu JH Wang P Bush MP Siegal PN Provencio 《Science (New York, N.Y.)》1998,282(5391):1105-1107
Free-standing aligned carbon nanotubes have previously been grown above 700 degreesC on mesoporous silica embedded with iron nanoparticles. Here, carbon nanotubes aligned over areas up to several square centimeters were grown on nickel-coated glass below 666 degreesC by plasma-enhanced hot filament chemical vapor deposition. Acetylene gas was used as the carbon source and ammonia gas was used as a catalyst and dilution gas. Nanotubes with controllable diameters from 20 to 400 nanometers and lengths from 0. 1 to 50 micrometers were obtained. Using this method, large panels of aligned carbon nanotubes can be made under conditions that are suitable for device fabrication. 相似文献
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Dictyostelium discoideum: a model system for cell-cell interactions in development 总被引:27,自引:0,他引:27
P Devreotes 《Science (New York, N.Y.)》1989,245(4922):1054-1058
The cellular slime mold Dictyostelium discoideum undergoes a transition from single-celled amoebae to a multicellular organism as a natural part of its life cycle. A method of cell-cell signaling that controls chemotaxis, morphogenesis, and gene expression has developed in this organism, and a detailed understanding of this signaling system provides clues to mechanisms of intercellular communication in the development of metazoans. 相似文献
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In eukaryotic cells directional sensing is mediated by heterotrimeric guanine nucleotide-binding protein (G protein)-linked signaling pathways. In Dictyostelium discoideum amoebae and mammalian leukocytes, the receptors and G-protein subunits are uniformly distributed around the cell perimeter. Chemoattractants induce the transient appearance of binding sites for several pleckstrin homology domain-containing proteins on the inner face of the membrane. In gradients of attractant these sites are persistently present on the side of the cell facing the higher concentration, even in the absence of a functional actin cytoskeleton or cell movement. Thus, the cell senses direction by spatially regulating the activity of the signal transduction pathway. 相似文献
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Extracellular adenosine 3',5'-monophosphate (cAMP) serves multiple roles in Dictyostelium development, acting as a chemoattractant, a cell-cell signaling molecule, and an inducer of differentiation. The Dictyostelium G-protein alpha subunit G alpha 2 appears to be the major transducer linking the surface cAMP receptor to these intracellular responses. On stimulation of cells with cAMP, G alpha 2 is phosphorylated on one or more serine residues, resulting in an alteration of its electrophoretic mobility. Phosphorylation of G alpha 2 is triggered by increased occupancy of the surface cAMP receptor and is rapid and transient, coinciding with the time course of activation of physiological responses. 相似文献
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Stable integration and expression of a cry1Ia gene conferring resistance to fall armyworm and boll weevil in cotton plants 下载免费PDF全文