全文获取类型
收费全文 | 19350篇 |
免费 | 68篇 |
国内免费 | 123篇 |
专业分类
系统科学 | 118篇 |
丛书文集 | 135篇 |
教育与普及 | 48篇 |
理论与方法论 | 86篇 |
现状及发展 | 8182篇 |
研究方法 | 989篇 |
综合类 | 9749篇 |
自然研究 | 234篇 |
出版年
2013年 | 175篇 |
2012年 | 325篇 |
2011年 | 526篇 |
2010年 | 130篇 |
2008年 | 384篇 |
2007年 | 442篇 |
2006年 | 446篇 |
2005年 | 421篇 |
2004年 | 390篇 |
2003年 | 438篇 |
2002年 | 434篇 |
2001年 | 819篇 |
2000年 | 747篇 |
1999年 | 461篇 |
1994年 | 315篇 |
1992年 | 405篇 |
1991年 | 278篇 |
1990年 | 347篇 |
1989年 | 300篇 |
1988年 | 273篇 |
1987年 | 335篇 |
1986年 | 330篇 |
1985年 | 403篇 |
1984年 | 295篇 |
1983年 | 277篇 |
1982年 | 255篇 |
1981年 | 238篇 |
1980年 | 237篇 |
1979年 | 627篇 |
1978年 | 461篇 |
1977年 | 432篇 |
1976年 | 371篇 |
1975年 | 390篇 |
1974年 | 461篇 |
1973年 | 418篇 |
1972年 | 390篇 |
1971年 | 454篇 |
1970年 | 598篇 |
1969年 | 460篇 |
1968年 | 470篇 |
1967年 | 437篇 |
1966年 | 415篇 |
1965年 | 307篇 |
1959年 | 147篇 |
1958年 | 250篇 |
1957年 | 161篇 |
1956年 | 157篇 |
1955年 | 140篇 |
1954年 | 147篇 |
1948年 | 128篇 |
排序方式: 共有10000条查询结果,搜索用时 13 毫秒
101.
S. F. Wang J. C. Braekman D. Daloze J. Pasteels P. Soetens N. V. Handjieva P. Kalushkov 《Cellular and molecular life sciences : CMLS》1996,52(6):628-630
The isolation of N-quinaldyl-L-arginine·HCl (1) from the CoccinellidaeSubcoccinella-24-punctata is reported. The structure, first established on the basis of the analysis of the spectral properties of1, has been confirmed by synthesis. The alkaloid is of endogenous origin and markedly deterrent to ants. 相似文献
102.
Molecular evidence for increased hematopoietic proliferation in the spleen of the b/b laboratory rat
S. Savković S. Pavlović T. Mitrović M. Joksimović J. Marjanović V. Glišin Z. Popović 《Cellular and molecular life sciences : CMLS》1996,52(8):807-811
The splenomegaly and the appearance of a significant number of CFU-E (erythroid colony-forming units) and BFU-E1 (erythroid burst-forming units) in the Belgrade laboratory rat (b/b) spleen prompted us to analyse further the molecular evidence for increased hematopoietic proliferation in the b/b spleen. Messenger RNAs (mRNAs) specific for globins, proteins for iron transport and deposition and the band 3 protein were used in rat erythropoietic tissues as markers for proliferation and erythroid differentiation. In the b/b spleen, all mRNAs analysed display an erythroid-specific pattern of expression. This analysis also revealed an enhanced level of mRNA for ferritin in the +/b spleen, whereas erythrocyte-specific mRNA production was normal. 相似文献
103.
Proteins enter the secretory pathway by two general routes. In one, the complete polypeptide is made in the cytoplasm and held in an incompletely folded state by chaperoning adenosine triphosphatases (ATPases) such as hsp70. InSaccharomyces cerevisiae, fully synthesized secretory precursors engage the endoplasmic reticulum (ER) membrane by interaction with a set of Sec proteins comprising the polypeptide translocation apparatus (Sec61p, Sec62p, Sec63p, Sec71p, Sec72p). Productive interaction requires displacement of hsp70 from the precursor, a reaction that is facilitated by Ydj1p, a homologue of theEscherichia coli DnaJ protein. Both DnaJ and Ydj1p regulate chaperone activity by stimulating the ATPase activity of their respective hsp70 partners (E. coli DnaK andS. cerevisiae Ssa1p, resepectively). In the ER lumen, another hsp70 chaperone, BiP, binds ATP and interacts with the ER membrane via its contact with a peptide loop of Sec63p. This loop represents yet another DnaJ homologue in that it contains a region of 70 residue similarity to the J box, the most conserved region of the DnaJ family of proteins. In the presence of ATP, under conditions in which BiP can bind to Sec63p, the secretory precursor passes from the cytosol into the lumen through a membrane channel formed by Sec61 p. A second route to the membrane pore that is used by many other secretory precursors, particularly in mammalian cells, requires that the polypeptide engage the ER membrane as the nascent chain emerges from the ribosome. Such cotranslational translocation bypasses the need for certain Sec proteins, instead utilizing an alternate set of cytosolic and membrane factors that allows the nascent chain to be inserted directly into the Sec61p channel. 相似文献
104.
105.
The rotation rates of asteroids, which are deduced from periodic fluctuations in their brightnesses, are controlled by mutual collisions. The link between asteroid spin and collision history is usually made with reference to impact experiments on centimetre-scale targets, where material strength governs the impact response. Recent work, however, indicates that for objects of the size of most observed asteroids (> or = 1 km in diameter), gravity rather than intrinsic strength controls the dynamic response to collisions. Here we explore this idea by modelling the effect of impacts on large gravitating bodies. We find that the fraction of a projectile's angular momentum that is retained by a target asteroid is both lower and more variable than expected from laboratory experiments, with spin evolution being dominated by 'catastrophic' collisions that eject approximately 50 per cent of the target's mass. The remnant of an initially non-rotating silicate asteroid that suffers such a collision rotates at a rate of approximately 2.9 per day, which is close to the observed mean asteroid rotation rate of approximately 2.5 d-1. Moreover, our calculations suggest that the observed trend in the mean spin frequency for different classes of asteroids (2.2 d-1 for C-type asteroids, 2.5 d-1 for S-type, and 4.0 d-1 for M-type) is due to increasing mean density, rather than increasing material strength. 相似文献
106.
107.
S. Fujino K. Satoh T. Nakai K. Togashi T. Kado M. Fujino T. Arima M. Fujino 《Cellular and molecular life sciences : CMLS》1993,49(2):138-144
The excitation-contraction (E-C) coupling process in single twitch fibres from frog toe muscle was inhibited selectively by phenylglyoxal (PGO), a specific guanidyl modifying reagent. A new protein (31.5 kDa), which has PGO-binding ability and seems to play a key role in the E-C coupling process, was solubilized from transverse tubule membrane-junctional sarcoplasmic reticulum complexes (TTM-JSR) of frog skeletal muscles, using14C-PGO. The monoclonal antibody against this protein applied extracellularly inhibited the E-C coupling process of the single fibres. This protein appears to constitute the very first step of input for E-C coupling. It is considered to behave as an indispensable part of an electrometer to measure membrane potentials. Therefore, the name electrometrin is suggested for the new protein. 相似文献
108.
109.
The Black Sea is the world's largest anoxic basin; it is also a contemporary analogue of the environment in which carbonaceous shales and petroleum source beds formed. Recently, Repeta et al. reported that anoxygenic photosynthesis may be an important component of carbon cycling in the present Black Sea, owing to a shoaling of the chemocline and consequent penetration of the photic zone by anaerobic waters in the past few decades. It has been suggested that this was due to an anthropogenic decrease in freshwater input to the Black Sea, although natural causes were not ruled out. Here we report the distributions of sequestered photosynthetic pigments in eight core samples of sediments from the Black Sea ranging in age from zero to 6,200 years before the present. Our results show that photosynthetic green sulphur bacteria (Chlorobiaceae [correction of Clorobiaceae]) have been active in the Black Sea for substantial periods of time in the past. This finding indicates that the penetration of the photic zone by anaerobic waters is not a recent phenomenon, and suggests that natural causes for shoaling of the chemocline are more likely than anthropogenic ones. 相似文献
110.
Summary Replicative DNA synthesis in permeable mouse sarcoma cells was more sensitive to ethidium bromide (EtBr) than bleomycin-induced unscheduled DNA synthesis (UDS). A similar difference in sensitivity to EtBr was observed between DNA polymerases and . The difference in sensitivity to EtBr of replicative DNA synthesis and UDS in the present system seems to reflect mainly the sensitivity difference between DNA polymerases and .Acknowledgments. The authors wish to thank Nippon Kayaku Co. (Tokyo, Japan) for providing copper-free bleomycin A2. This research was supported in part by a grant from the Japan Ministry of Education, Science and Culture. 相似文献