全文获取类型
收费全文 | 2595篇 |
免费 | 213篇 |
专业分类
2808篇 |
出版年
2022年 | 19篇 |
2021年 | 39篇 |
2020年 | 18篇 |
2019年 | 43篇 |
2018年 | 42篇 |
2017年 | 43篇 |
2016年 | 45篇 |
2015年 | 86篇 |
2014年 | 94篇 |
2013年 | 158篇 |
2012年 | 189篇 |
2011年 | 192篇 |
2010年 | 95篇 |
2009年 | 75篇 |
2008年 | 142篇 |
2007年 | 142篇 |
2006年 | 136篇 |
2005年 | 111篇 |
2004年 | 126篇 |
2003年 | 120篇 |
2002年 | 109篇 |
2001年 | 97篇 |
2000年 | 88篇 |
1999年 | 67篇 |
1998年 | 31篇 |
1997年 | 18篇 |
1996年 | 27篇 |
1995年 | 20篇 |
1994年 | 36篇 |
1993年 | 13篇 |
1992年 | 46篇 |
1991年 | 36篇 |
1990年 | 29篇 |
1989年 | 28篇 |
1988年 | 21篇 |
1987年 | 19篇 |
1986年 | 20篇 |
1985年 | 17篇 |
1984年 | 20篇 |
1983年 | 12篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1980年 | 8篇 |
1979年 | 11篇 |
1977年 | 7篇 |
1976年 | 11篇 |
1975年 | 11篇 |
1973年 | 9篇 |
1972年 | 8篇 |
1970年 | 6篇 |
排序方式: 共有2808条查询结果,搜索用时 15 毫秒
51.
R. M. Alvarez B. Rodríguez J. M. Romano A. O. Díaz E. Gómez D. Miró L. Navarro G. Saura J. L. García 《World journal of microbiology & biotechnology》1992,8(2):214-215
Microbial lipids produced byRhodotorula glutinis grown in continuous culture with molasses under nitrogen-limiting conditions were evaluated and the effects of growth rate on fatty acid composition were studied. As the growth rate decreased, cell biomass, lipid content and lipid yield gradually increased. The maximum lipid content recorded was 39% (w/w) of dry cell biomass at a dilution rate of 0.04 h–1. The growth rate also affected fatty acid composition: oleic acid decreased with decreasing growth rate while stearic acid increased. 相似文献
52.
53.
The crystal structures of chenodeoxycholic acid (CDCA), ursodeoxycholic acid (7beta isomer of CDCA) and their other two epimers (3beta,7alpha- and 3beta,7beta-isomers) have been resolved. The four isomers were recrystallized from p-xylene. CDCA crystal is hexagonal P6(5) while the crystals of the other three isomers are orthorhombic (P2(1)2(1)2(1) space group). Only the 3beta,7beta isomer forms an inclusion complex with the solvent with a 1:1 stoichiometry. In all cases, the three hydrogen bond sites (the two hydroxy groups, O3-H and O7-H, and the carboxylic acid group of the side chain, O24bO24a-H) simultaneously act as hydrogen bond donors and acceptors. By considering that O24a is always donor and O24b is always acceptor, the hydrogen bond sequences can be understood on the basis of the interaction between the two hydroxy groups. However the comparison between the four compounds is complicated by the existence of two molecules in the asymmetric unit in the UDCA crystal resulting in that the same hydrogen bond site (for instance O3) can be donor towards two different acceptors (either O7 or O24b). As in the case of the four isomers of deoxycholic acid (Steroids 2004, 69, 379), the other three isomers present a donor-->acceptor sequence, which is O7-->O3 when O3-H is beta and O3-->O7 when O3-H is alpha. The spatial orientation of the carboxylic acid of the side chain is referred to two almost perpendicular planes (defined by (1) the carbon atoms C1/C6-C17/C20 and by (2) the methyl groups C18-C19 and the two carbon atoms to which they are linked, C10 and C13, respectively). Only the side chain of CDCA evidences a positive deviation towards the hydrophobic beta side of the molecule. 相似文献
54.
Alvarez S Goodger JQ Marsh EL Chen S Asirvatham VS Schachtman DP 《Journal of proteome research》2006,5(4):963-972
The xylem in plants has mainly been described as a conduit for water and minerals, but emerging evidence also indicates that the xylem contains protein. To study the proteins in xylem sap, we characterized the identity and composition of the maize xylem sap proteome. The composition of the xylem sap proteome in maize revealed proteins related to different phases of xylem differentiation including cell wall metabolism, secondary cell wall synthesis, and programmed cell death. Many proteins were found to be present as multiple isoforms and some of these isoforms are glycosylated. Proteins involved in defense mechanisms were also present in xylem sap and the sap proteins were shown to have antifungal activity in bioassays. 相似文献
55.
Cross talk between gibberellin and cytokinin: the Arabidopsis GA response inhibitor SPINDLY plays a positive role in cytokinin signaling 总被引:21,自引:0,他引:21 下载免费PDF全文
Greenboim-Wainberg Y Maymon I Borochov R Alvarez J Olszewski N Ori N Eshed Y Weiss D 《The Plant cell》2005,17(1):92-102
SPINDLY (SPY) is a negative regulator of gibberellin (GA) responses; however, spy mutants exhibit various phenotypic alterations not found in GA-treated plants. Assaying for additional roles for SPY revealed that spy mutants are resistant to exogenously applied cytokinin. GA also repressed the effects of cytokinin, suggesting that there is cross talk between the two hormone-response pathways, which may involve SPY function. Two spy alleles showing severe (spy-4) and mild (spy-3) GA-associated phenotypes exhibited similar resistance to cytokinin, suggesting that SPY enhances cytokinin responses and inhibits GA signaling through distinct mechanisms. GA and spy repressed numerous cytokinin responses, from seedling development to senescence, indicating that cross talk occurs early in the cytokinin-signaling pathway. Because GA3 and spy-4 inhibited induction of the cytokinin primary-response gene, type-A Arabidopsis response regulator 5, SPY may interact with and modify elements from the phosphorelay cascade of the cytokinin signal transduction pathway. Cytokinin, on the other hand, had no effect on GA biosynthesis or responses. Our results demonstrate that SPY acts as both a repressor of GA responses and a positive regulator of cytokinin signaling. Hence, SPY may play a central role in the regulation of GA/cytokinin cross talk during plant development. 相似文献
56.
Yadvinder Malhi Timothy R. Baker Oliver L. Phillips Samuel Almeida Esteban Alvarez Luzmilla Arroyo Jerome Chave Claudia I. Czimczik Anthony Di Fiore Niro Higuchi Timothy J. Killeen Susan G. Laurance William F. Laurance Simon L. Lewis Lina María Mercado Montoya Abel Monteagudo David A. Neill Percy Núez Vargas Sandra Patio Nigel C.A. Pitman Carlos Alberto Quesada Rafael Salomo Jos Natalino Macedo Silva Armando Torres Lezama Rodolfo Vsquez Martínez John Terborgh Barbara Vinceti Jon Lloyd 《Global Change Biology》2004,10(5):563-591
The net primary production of tropical forests and its partitioning between long‐lived carbon pools (wood) and shorter‐lived pools (leaves, fine roots) are of considerable importance in the global carbon cycle. However, these terms have only been studied at a handful of field sites, and with no consistent calculation methodology. Here we calculate above‐ground coarse wood carbon productivity for 104 forest plots in lowland New World humid tropical forests, using a consistent calculation methodology that incorporates corrections for spatial variations in tree‐size distributions and wood density, and for census interval length. Mean wood density is found to be lower in more productive forests. We estimate that above‐ground coarse wood productivity varies by more than a factor of three (between 1.5 and 5.5 Mg C ha?1 a?1) across the Neotropical plots, with a mean value of 3.1 Mg C ha?1 a?1. There appear to be no obvious relationships between wood productivity and rainfall, dry season length or sunshine, but there is some hint of increased productivity at lower temperatures. There is, however, also strong evidence for a positive relationship between wood productivity and soil fertility. Fertile soils tend to become more common towards the Andes and at slightly higher than average elevations, so the apparent temperature/productivity relationship is probably not a direct one. Coarse wood productivity accounts for only a fraction of overall tropical forest net primary productivity, but the available data indicate that it is approximately proportional to total above‐ground productivity. We speculate that the large variation in wood productivity is unlikely to directly imply an equivalent variation in gross primary production. Instead a shifting balance in carbon allocation between respiration, wood carbon and fine root production seems the more likely explanation. 相似文献
57.
Direct action of endocrine disrupting chemicals on human sperm 总被引:1,自引:0,他引:1
Dorte L Egeberg Luis Alvarez Christoph Brenker Anders Rehfeld Hanne Frederiksen Benjamin Wäschle U Benjamin Kaupp Melanie Balbach Dagmar Wachten Niels E Skakkebaek Kristian Almstrup Timo Strünker 《EMBO reports》2014,15(7):758-765
Synthetic endocrine disrupting chemicals (EDCs), omnipresent in food, household, and personal care products, have been implicated in adverse trends in human reproduction, including infertility and increasing demand for assisted reproduction. Here, we study the action of 96 ubiquitous EDCs on human sperm. We show that structurally diverse EDCs activate the sperm‐specific CatSper channel and, thereby, evoke an intracellular Ca2+ increase, a motility response, and acrosomal exocytosis. Moreover, EDCs desensitize sperm for physiological CatSper ligands and cooperate in low‐dose mixtures to elevate Ca2+ levels in sperm. We conclude that EDCs interfere with various sperm functions and, thereby, might impair human fertilization. 相似文献
58.
The leader protease (Lpro) from foot-and-mouth disease virus (FMDV) has the ability to cleave eIF4G, leading to a blockade of cellular protein synthesis. In contrast to previous reports, our present findings demonstrate that FMDV Lpro is able to increase translation driven by FMDV IRES. Additionally, inactivation of eIF2 subsequent to phosphorylation induced by arsenite or thapsigargin in BHK cells blocks protein synthesis directed by FMDV IRES, whereas in the presence of Lpro, significant translation is found under these conditions. This phenomenon was also observed in cell-free systems after induction of eIF2 phosphorylation by addition of poly(I:C). 相似文献
59.
60.
Functional significance of isoform diversification in the protocadherin gamma gene cluster 总被引:1,自引:0,他引:1
WV Chen FJ Alvarez JL Lefebvre B Friedman C Nwakeze E Geiman C Smith CA Thu JC Tapia B Tasic JR Sanes T Maniatis 《Neuron》2012,75(3):402-409
The mammalian Protocadherin (Pcdh) alpha, beta, and gamma gene clusters encode a large family of cadherin-like transmembrane proteins that are differentially expressed in individual neurons. The 22 isoforms of the Pcdhg gene cluster are diversified into A-, B-, and C-types, and the C-type isoforms differ from all other clustered Pcdhs in sequence and expression. Here, we show that mice lacking the three C-type isoforms are phenotypically indistinguishable from the Pcdhg null mutants, displaying virtually identical cellular and synaptic alterations resulting from neuronal apoptosis. By contrast, mice lacking three A-type isoforms exhibit no detectable phenotypes. Remarkably, however, genetically blocking apoptosis rescues the neonatal lethality of the C-type isoform knockouts, but not that of the Pcdhg null mutants. We conclude that the role of the Pcdhg gene cluster in neuronal survival is primarily, if not specifically, mediated by its C-type isoforms, whereas a separate role essential for postnatal development, likely in neuronal wiring, requires isoform diversity. 相似文献