全文获取类型
收费全文 | 43340篇 |
免费 | 88篇 |
国内免费 | 146篇 |
专业分类
系统科学 | 208篇 |
丛书文集 | 824篇 |
教育与普及 | 115篇 |
理论与方法论 | 231篇 |
现状及发展 | 19298篇 |
研究方法 | 1681篇 |
综合类 | 20690篇 |
自然研究 | 527篇 |
出版年
2013年 | 300篇 |
2012年 | 576篇 |
2011年 | 1117篇 |
2010年 | 258篇 |
2008年 | 754篇 |
2007年 | 782篇 |
2006年 | 821篇 |
2005年 | 830篇 |
2004年 | 734篇 |
2003年 | 792篇 |
2002年 | 768篇 |
2001年 | 1334篇 |
2000年 | 1217篇 |
1999年 | 805篇 |
1994年 | 373篇 |
1992年 | 786篇 |
1991年 | 619篇 |
1990年 | 681篇 |
1989年 | 673篇 |
1988年 | 642篇 |
1987年 | 685篇 |
1986年 | 682篇 |
1985年 | 857篇 |
1984年 | 670篇 |
1983年 | 570篇 |
1982年 | 523篇 |
1981年 | 499篇 |
1980年 | 603篇 |
1979年 | 1398篇 |
1978年 | 1180篇 |
1977年 | 1114篇 |
1976年 | 835篇 |
1975年 | 909篇 |
1974年 | 1243篇 |
1973年 | 1074篇 |
1972年 | 1072篇 |
1971年 | 1332篇 |
1970年 | 1754篇 |
1969年 | 1329篇 |
1968年 | 1284篇 |
1967年 | 1275篇 |
1966年 | 1126篇 |
1965年 | 820篇 |
1959年 | 483篇 |
1958年 | 729篇 |
1957年 | 565篇 |
1956年 | 496篇 |
1955年 | 433篇 |
1954年 | 470篇 |
1948年 | 298篇 |
排序方式: 共有10000条查询结果,搜索用时 239 毫秒
1.
R.S.K. Barnes 《Journal of Natural History》2018,52(1-2):87-113
Areas of the Knysna estuarine bay in the Western Cape are dominated by three endemic South African truncatelloid microgastropods, temporarily known as ‘Hydrobia’ knysnaensis (Krauss), ‘Assiminea’ capensis (Sowerby) and ‘Assiminea’ globulus Connolly. Although first described 80–170 years ago and present in abundance (up to 100,000 m?2), they remain surrounded by confusion and still await taxonomic assignment, largely because they appear most atypical members of their groups by virtue of anatomy and/or biogeography and/or habitat. This study contributes in-life perspectives to morphological and phylogenetic analyses known to be on-going. At Knysna, they are syntopic: at least two occurring in >85% and all three in >40% of individual 0.0026 m2 samples from their region of dominance. Nevertheless, they tend to greater abundance in divergent microhabitats; ‘A.’ globulus dominating higher tidal levels, and ‘A.’ capensis and ‘Hydrobia’ lower ones; the former especially unvegetated sediment, the latter, if anything, seagrass. Interspecific feeding interactions appear unlikely to be responsible for these patterns, other evidence suggesting that all are maintained below carrying capacity. Field biology of ‘H.’ knysnaensis generally appears equivalent to that of northern-hemisphere intertidal hydrobiids and that of ‘A.’ globulus is typically assimineid, albeit at atypically low shore height. Unlike assimineids, however, ‘A.’ capensis is truly aquatic. The success of these truncatelloids in unusual circumstances may be consequent on the absence from South Africa of other microgastropod groups that fill their niches elsewhere in the southern hemisphere. 相似文献
2.
Patricio Atanes Inmaculada Ruz-Maldonado Ross Hawkes Bo Liu Min Zhao Guo Cai Huang Israa Mohammed Al-Amily Albert Salehi Stefan Amisten Shanta J. Persaud 《Cellular and molecular life sciences : CMLS》2018,75(16):3039-3050
Introduction
Islets synthesise and secrete numerous peptides, some of which are known to be important regulators of islet function and glucose homeostasis. In this study, we quantified mRNAs encoding all peptide ligands of islet G protein-coupled receptors (GPCRs) in isolated human and mouse islets and carried out in vitro islet hormone secretion studies to provide functional confirmation for the species-specific role of peptide YY (PYY) in mouse islets.Materials and methods
GPCR peptide ligand mRNAs in human and mouse islets were quantified by quantitative real-time PCR relative to the reference genes ACTB, GAPDH, PPIA, TBP and TFRC. The pathways connecting GPCR peptide ligands with their receptors were identified by manual searches in the PubMed, IUPHAR and Ingenuity databases. Distribution of PYY protein in mouse and human islets was determined by immunohistochemistry. Insulin, glucagon and somatostatin secretion from islets was measured by radioimmunoassay.Results
We have quantified GPCR peptide ligand mRNA expression in human and mouse islets and created specific signalomes mapping the pathways by which islet peptide ligands regulate human and mouse GPCR signalling. We also identified species-specific islet expression of several GPCR ligands. In particular, PYY mRNA levels were ~ 40,000-fold higher in mouse than human islets, suggesting a more important role of locally secreted Pyy in mouse islets. This was confirmed by IHC and functional experiments measuring insulin, glucagon and somatostatin secretion.Discussion
The detailed human and mouse islet GPCR peptide ligand atlases will allow accurate translation of mouse islet functional studies for the identification of GPCR/peptide signalling pathways relevant for human physiology, which may lead to novel treatment modalities of diabetes and metabolic disease.3.
4.
5.
6.
Archive for History of Exact Sciences - We show that Dedekind, in his proof of the principle of definition by mathematical recursion, used implicitly both the concept of an inductive cone from an... 相似文献
7.
8.
Danielle Kamato Muhamad Ashraf Rostam Rebekah Bernard Terrence J. Piva Nitin Mantri Daniel Guidone Wenhua Zheng Narin Osman Peter J. Little 《Cellular and molecular life sciences : CMLS》2015,72(4):799-808
G protein-coupled receptor (GPCR) signalling is mediated through transactivation-independent signalling pathways or the transactivation of protein tyrosine kinase receptors and the recently reported activation of the serine/threonine kinase receptors, most notably the transforming growth factor-β receptor family. Since the original observation of GPCR transactivation of protein tyrosine kinase receptors, there has been considerable work on the mechanism of transactivation and several pathways are prominent. These pathways include the “triple membrane bypass” pathway and the generation of reactive oxygen species. The recent recognition of GPCR transactivation of serine/threonine kinase receptors enormously broadens the GPCR signalling paradigm. It may be predicted that the transactivation of serine/threonine kinase receptors would have mechanistic similarities with transactivation of tyrosine kinase pathways; however, initial studies suggest that these two transactivation pathways are mechanistically distinct. Important questions are the relative importance of tyrosine and serine/threonine transactivation pathways, the contribution of transactivation to overall GPCR signalling, mechanisms of transactivation and the range of cell types in which this phenomenon occurs. The ultimate significance of transactivation-dependent signalling remains to be defined but it appears to be prominent and if so will represent a new cell signalling frontier. 相似文献
9.
10.
Ten-day treatment with nicotine (5, 25 or 50 micrograms/ml drinking water) dose-dependently intensified gastric ulceration induced by cold-restraint, and emptying rate. Stomach contractions produced by graded doses of bethanechol i.v. were elevated further by nicotine treatment. It is suggested that chronic nicotine administration produces hypersensitivity of the gastric muscarinic receptors; stomach hypermotility contributes to the ulcer-worsening action of the alkaloid. 相似文献