全文获取类型
收费全文 | 15679篇 |
免费 | 63篇 |
国内免费 | 97篇 |
专业分类
系统科学 | 102篇 |
丛书文集 | 120篇 |
教育与普及 | 95篇 |
理论与方法论 | 82篇 |
现状及发展 | 6210篇 |
研究方法 | 678篇 |
综合类 | 8271篇 |
自然研究 | 281篇 |
出版年
2013年 | 112篇 |
2012年 | 244篇 |
2011年 | 574篇 |
2010年 | 118篇 |
2009年 | 171篇 |
2008年 | 269篇 |
2007年 | 323篇 |
2006年 | 305篇 |
2005年 | 342篇 |
2004年 | 388篇 |
2003年 | 312篇 |
2002年 | 306篇 |
2001年 | 498篇 |
2000年 | 490篇 |
1999年 | 351篇 |
1992年 | 302篇 |
1991年 | 237篇 |
1990年 | 259篇 |
1989年 | 244篇 |
1988年 | 253篇 |
1987年 | 264篇 |
1986年 | 228篇 |
1985年 | 338篇 |
1984年 | 251篇 |
1983年 | 199篇 |
1982年 | 174篇 |
1981年 | 170篇 |
1980年 | 213篇 |
1979年 | 467篇 |
1978年 | 384篇 |
1977年 | 339篇 |
1976年 | 303篇 |
1975年 | 309篇 |
1974年 | 400篇 |
1973年 | 377篇 |
1972年 | 324篇 |
1971年 | 443篇 |
1970年 | 567篇 |
1969年 | 423篇 |
1968年 | 398篇 |
1967年 | 400篇 |
1966年 | 426篇 |
1965年 | 277篇 |
1964年 | 104篇 |
1959年 | 132篇 |
1958年 | 250篇 |
1957年 | 146篇 |
1956年 | 155篇 |
1955年 | 118篇 |
1948年 | 135篇 |
排序方式: 共有10000条查询结果,搜索用时 356 毫秒
31.
It is well known that transmitter secretion requires specialized secretory organelles, the synaptic vesicles, for the packaging, storage and exocytotic release of the transmitter. Here we report that when acetylcholine (ACh) is loaded into an isolated Xenopus myocyte, there is spontaneous quantal release of ACh from the myocyte which results in activation of its own surface ACh channels and the appearance of membrane currents resembling miniature endplate currents. This myocyte secretion probably reflects Ca(2+)-regulated exocytosis of ACh-filled cytoplasmic compartments. Furthermore, step depolarization of the myocyte membrane triggers evoked ACh release from the myocyte with a weak excitation-secretion coupling. These findings suggest that quantal transmitter secretion does not require secretory pathways unique to neurons and that the essence of presynaptic differentiation may reside in the provision of transmitter supply and modification of the preexisting secretion pathway. 相似文献
32.
A. F. McDonagh Y. -M. Pu D. A. Lightner 《Cellular and molecular life sciences : CMLS》1992,48(3):246-248
The characteristic circular dichroism of bilirubin bound to human serum albumin undergoes a remarkable sign inversion on addition of halothane, chloroform and other volatile anesthetics. This sign inversion, which is completely reversed by removal of the anesthetic, reflects a pronounced conformational change of the bound ligand; probably a complete inversion of chirality. The observation suggests that association of volatile anesthetics with proteins can markedly alter the internal topography of receptor sites and potentially influence the stereoselectivity of ligand binding. 相似文献
33.
Short alanine peptides, containing 16 or 17 residues, appear to form alpha-helices in aqueous solution. But the main spectroscopic analyses used on helical peptides (circular dichroism and nuclear magnetic resonance) cannot distinguish between an alpha-helix (in which the ith residue is hydrogen-bonded to residue i + 4; ref. 9) and the next most common peptide helix, the 3(10)-helix10 (i-->i + 3 hydrogen-bonding). To address this problem we have designed single and doubly spin-labelled analogues of alanine-based peptides in which the nitroxide spin label forms an unbranched side chain extending from the sulphur atom of a cysteine residue. Here we report the circular dichroism, Fourier-transform infrared and electron-spin resonance spectra of these peptides under helix-forming conditions. The infrared absorbance gives an amide I' band with a frequency that is substantially different from that observed for alpha-helices. The electron-spin resonance spectra of doubly labelled helices show that the ranking of distances between side chains, around a single turn (residues 4-8), is inconsistent with an alpha-helical structure. Our experiments suggest that the more likely peptide geometry is a 3(10)-helix. 相似文献
34.
K K Leong K M Yu W B Lee 《厦门大学学报(自然科学版)》2002,(Z1)
Product data management (PDM) has been accepted as an important tool for the manufacturing industries. In recent years, more and mor e researches have been conducted in the development of PDM. Their research area s include system design, integration of object-oriented technology, data distri bution, collaborative and distributed manufacturing working environment, secur ity, and web-based integration. However, there are limitations on their rese arches. In particular, they cannot cater for PDM in dis... 相似文献
35.
Summary Host cell reactivation capacity for ozone T3 phage was determined for differentE. coli strains deficient in one or more of the DNA repair mechanisms. The results indicate that DNA polymerase I appears to play a key role in the repair of damage produced on the DNA by ozone while thelexA gene product seems to play a minor one.This research was sponsored by the National Sciences and Engineering Council of Canada. One of us (PLH) acknowledges a scholarship from the NSERCof Canada. 相似文献
36.
Chromosomal effects of adeno-associated virus vector integration. 总被引:17,自引:0,他引:17
Adeno-associated virus (AAV) vectors are currently being used in several clinical gene-therapy trials (see the NIH OBA Human Gene Transfer Clinical Trials Database); however, little is known about the chromosomal effects of vector integration. Here we report that integrated vector proviruses are associated with chromosomal deletions and other rearrangements and are frequently located on chromosome 19 (although not at the wildtype AAV integration site). 相似文献
37.
Jacob W. Ulvila 《Journal of forecasting》1985,4(4):377-385
Recent years have seen an increasing cross-fertilization between the fields of decision analysis and forecasting. Decision-analytic models often require forecasts as inputs, and aspects of the Bayesian decision-theoretic framework underlying decision analysis have proved useful to forecasting, particularly in contexts where subjective judgemental inputs are required. This paper describes the use of decision tree analysis for forecasting and illustrates its use for corporate divisional forecasting and planning. A specialized decision-analytic technique, acts as events, is also described and illustrated to forecast a new product's earnings. Conclusions are drawn about the applicability of decision analysis for forecasting. 相似文献
38.
J. W. Faigle H. Stierlin H. Mory T. Winkler H. -P. Kriemler 《Cellular and molecular life sciences : CMLS》1985,41(4):476-478
Summary Indoxyl derivatives were detected as minor products among the urinary metabolites of two trial drugs, a benzodiazepine (GP 55 129) and a benzophenone (CGP 11 952). Their structures were elucidated by NMR and mass spectroscopy. Presumably, metabolites containing potential aldehyde functions react spontaneously with endogenous indoxyl. Such derivatives have not hitherto been encountered in drug metabolism. 相似文献
39.
A topoisomerase II inhibitor, novobiocin, and a deacetylase inhibitor, butyrate, synergistically transformed human liver cells into fibroblast-like cells. This morphological change was associated with an increased production of procollagen type III peptide and a simultaneous assembly of actin, tubulin, vimentin and cytokeratin. Novobiocin and butyrate had no marked effect on the phosphorylation state of cytokeratin proteins, but synergistically enhanced [3H]acetate uptake. From these results, it can be speculated that protein acetylation plays an important role in inducing the assembly of cytoskeletal proteins and the morphological transformation of human liver cells. 相似文献
40.