全文获取类型
收费全文 | 1965篇 |
免费 | 11篇 |
国内免费 | 33篇 |
专业分类
系统科学 | 46篇 |
丛书文集 | 14篇 |
教育与普及 | 4篇 |
理论与方法论 | 45篇 |
现状及发展 | 860篇 |
研究方法 | 177篇 |
综合类 | 817篇 |
自然研究 | 46篇 |
出版年
2019年 | 13篇 |
2018年 | 19篇 |
2017年 | 21篇 |
2016年 | 21篇 |
2015年 | 17篇 |
2014年 | 17篇 |
2013年 | 42篇 |
2012年 | 70篇 |
2011年 | 93篇 |
2010年 | 25篇 |
2009年 | 11篇 |
2008年 | 50篇 |
2007年 | 50篇 |
2006年 | 71篇 |
2005年 | 55篇 |
2004年 | 59篇 |
2003年 | 91篇 |
2002年 | 126篇 |
2001年 | 144篇 |
2000年 | 95篇 |
1999年 | 66篇 |
1997年 | 11篇 |
1992年 | 25篇 |
1990年 | 18篇 |
1989年 | 15篇 |
1988年 | 20篇 |
1987年 | 24篇 |
1986年 | 21篇 |
1985年 | 21篇 |
1984年 | 21篇 |
1983年 | 16篇 |
1982年 | 12篇 |
1981年 | 15篇 |
1980年 | 24篇 |
1979年 | 38篇 |
1978年 | 35篇 |
1977年 | 34篇 |
1976年 | 30篇 |
1975年 | 39篇 |
1974年 | 29篇 |
1973年 | 30篇 |
1972年 | 43篇 |
1971年 | 43篇 |
1970年 | 31篇 |
1969年 | 32篇 |
1968年 | 25篇 |
1967年 | 29篇 |
1966年 | 33篇 |
1965年 | 21篇 |
1958年 | 9篇 |
排序方式: 共有2009条查询结果,搜索用时 15 毫秒
821.
J. C. de Freitas L. A. Blankemeier R. S. Jacobs 《Cellular and molecular life sciences : CMLS》1984,40(8):864-865
Summary The irreversible neurotoxic action of -bungarotoxin (-BuTx) can be prevented by preincubation of the toxin with manoalide, a non-steroidal anti-inflammatory agent. Manoalide was also found to inactivate purified phospholipase A2 and thus prevent hydrolysis of phosphatidylcholine. PLA2 is a component found in several neurotoxic venoms and is also a rate limiting enzyme important in phospholipid metabolism and prostaglandin synthesis in man.This work is a result of research sponsored in part by NOAA-Department of Commerce under Sea Grant No. NA80AA-D-00120 and the California State Resources Agency R/MP-21. The US Government is authorized to produce and distribute reprints for governmental purposes. A preliminary report of aspects of this work appeared in Fedn Proc.42 (1983) 374. We also acknowledge the technical assistance of Dan Rohrer. Dr George Taborski's advice and assistance during the course of this work is gratefully acknowledged.J. C. de Freitas is a post-doctoral fellow supported by Fundacao de Amparo à Pesquisa do Estado de São Paulo (81/0263-9) and Conselho Nacional de Desenvolvimento Cientifico e Tecnológico (201.171/82), Brazil. 相似文献
822.
W H Peters H G Swarts J J de Pont F M Schuurmans Stekhoven S L Bonting 《Nature》1981,290(5804):338-339
(Na+ + K+)ATPase contains two different subunits, a catalytic subunit (alpha) and a subunit with uncertain function (beta). The enzyme binds ATP, ouabain and vanadate, and can be phosphorylated by ATP as well as by inorganic phosphate. From the previously reported maximal binding and phosphorylation capacities of 3.5--4.3 nmol P per mg protein (based on Lowry protein determination) and the earlier molecular weight value of approximately 250,000, a molar binding and phosphorylation capacity of 0.87--1.07 mol per mol enzyme was derived. As it is generally agreed that the enzyme molecule contains two alpha subunits or even a multiple of two, it has been suggested that the enzyme operates by means of a so-called "half-of-the-sites mechanism" whereby only of the two alpha subunits can be phosphorylated at any one time. We now present evidence that every alpha subunit can be phosphorylated simultaneously, which rules out the operation of such a mechanism. 相似文献
823.
Frequencies of amylase variants in Drosophila melanogaster 总被引:9,自引:0,他引:9
824.
18 rats were treated with L-ASA before heart transplantation and daily thereafter until death or rejection. 22 animals acted as controls. A significantly higher post-operative mortality rate, without any significant modification of the transplant survival time, was found in L-ASA-treated group. 相似文献
825.
L. Botte S. Scippa M. de Vincentiis 《Cellular and molecular life sciences : CMLS》1979,35(9):1228-1230
Summary X-ray histospectrographic analysis at the scanning and transmission electron microscope (STEM) are made on the blood cells ofPhallusia mamillata Cuvier andCiona intestinalis, to study the direct intracellular sites of accumulation of vanadium. The results show a clear accumulation of vanadium on the membrane and in the granules of vacuoles of amebocytes, signet ring cell, compartment cell and traces of metal in the vanadophores of vanadocytes.We thank Dr C. Weichan and Mr G. Hubert for kind hospitality and for technical assistance in the Department of Electron Microscopy of Siemens Laboratory in Karlsruhe (BRD). We thank also Dr V. Andria of Siemens Elettra in Rome (Italy). 相似文献
826.
Summary Shortening of nucleologenesis time in a synchronous cell population, labelled as binucleate, by a caffeine pulse and fall in frequency of prophases are related to narciclasine inhibition of protein synthesis inAllium cepa L. meristems.Acknowledgments. We gratefully acknowledge Dr D. Vázquez and Dr A. Jiménez, from the Instituto de Bioquímica de Macromoléculas, for calling our attention to this protein synthesis inhibitor and providing us samples both of narciclasine andNarcissus bulbs. We also greatly appreciate Dr B. Rodriguez and Dr F. M. Panizo of the Instituto de Química Orgánica (Departamento de Productos Naturales) for their valuable help in isolation and characterization of narciclasine, and for laboratory facilities. The work has been partially supported by the Comisión Asesora para la Investigación Científica (Spain). A.S. has a fellowship awarded by the Instituto de Cultura Hispánica, Spain. 相似文献
827.
de la Cruz C Huang Q Lynn JW Li J Ratcliff W Zarestky JL Mook HA Chen GF Luo JL Wang NL Dai P 《Nature》2008,453(7197):899-902
Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-T(c)) copper oxides, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron or hole doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at approximately 137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-T(c) copper oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state. 相似文献
828.
Sánchez-Lavega A Orton GS Hueso R García-Melendo E Pérez-Hoyos S Simon-Miller A Rojas JF Gómez JM Yanamandra-Fisher P Fletcher L Joels J Kemerer J Hora J Karkoschka E de Pater I Wong MH Marcus PS Pinilla-Alonso N Carvalho F Go C Parker D Salway M Valimberti M Wesley A Pujic Z 《Nature》2008,451(7177):437-440
The atmospheres of the gas giant planets (Jupiter and Saturn) contain jets that dominate the circulation at visible levels. The power source for these jets (solar radiation, internal heat, or both) and their vertical structure below the upper cloud are major open questions in the atmospheric circulation and meteorology of giant planets. Several observations and in situ measurements found intense winds at a depth of 24 bar, and have been interpreted as supporting an internal heat source. This issue remains controversial, in part because of effects from the local meteorology. Here we report observations and modelling of two plumes in Jupiter's atmosphere that erupted at the same latitude as the strongest jet (23 degrees N). The plumes reached a height of 30 km above the surrounding clouds, moved faster than any other feature (169 m s(-1)), and left in their wake a turbulent planetary-scale disturbance containing red aerosols. On the basis of dynamical modelling, we conclude that the data are consistent only with a wind that extends well below the level where solar radiation is deposited. 相似文献
829.
Cyclic dermal BMP signalling regulates stem cell activation during hair regeneration 总被引:3,自引:0,他引:3
In the age of stem cell engineering it is critical to understand how stem cell activity is regulated during regeneration. Hairs are mini-organs that undergo cyclic regeneration throughout adult life, and are an important model for organ regeneration. Hair stem cells located in the follicle bulge are regulated by the surrounding microenvironment, or niche. The activation of such stem cells is cyclic, involving periodic beta-catenin activity. In the adult mouse, regeneration occurs in waves in a follicle population, implying coordination among adjacent follicles and the extrafollicular environment. Here we show that unexpected periodic expression of bone morphogenetic protein 2 (Bmp2) and Bmp4 in the dermis regulates this process. This BMP cycle is out of phase with the WNT/beta-catenin cycle, thus dividing the conventional telogen into new functional phases: one refractory and the other competent for hair regeneration, characterized by high and low BMP signalling, respectively. Overexpression of noggin, a BMP antagonist, in mouse skin resulted in a markedly shortened refractory phase and faster propagation of the regenerative wave. Transplantation of skin from this mutant onto a wild-type host showed that follicles in donor and host can affect their cycling behaviours mutually, with the outcome depending on the equilibrium of BMP activity in the dermis. Administration of BMP4 protein caused the competent region to become refractory. These results show that BMPs may be the long-sought 'chalone' inhibitors of hair growth postulated by classical experiments. Taken together, results presented in this study provide an example of hierarchical regulation of local organ stem cell homeostasis by the inter-organ macroenvironment. The expression of Bmp2 in subcutaneous adipocytes indicates physiological integration between these two thermo-regulatory organs. Our findings have practical importance for studies using mouse skin as a model for carcinogenesis, intra-cutaneous drug delivery and stem cell engineering studies, because they highlight the acute need to differentiate supportive versus inhibitory regions in the host skin. 相似文献
830.
Ants travelling to and fro between their nest and a foraging area may follow stereotyped foodward and homeward routes that are guided by different visual and directional memory sequences. Honeybees are known to fly a feeder-to-hive or hive-to-feeder vector according to whether or not they have recently fed--their feeding state controls which compass direction they select. We show here that the feeding state of the wood ant Formica rufa also determines the choice between an outward or inward journey, but by priming the selective retrieval of visual landmark memories. 相似文献