全文获取类型
收费全文 | 18464篇 |
免费 | 1620篇 |
国内免费 | 8篇 |
出版年
2023年 | 84篇 |
2022年 | 116篇 |
2021年 | 414篇 |
2020年 | 235篇 |
2019年 | 290篇 |
2018年 | 366篇 |
2017年 | 320篇 |
2016年 | 502篇 |
2015年 | 861篇 |
2014年 | 941篇 |
2013年 | 1104篇 |
2012年 | 1479篇 |
2011年 | 1362篇 |
2010年 | 905篇 |
2009年 | 851篇 |
2008年 | 1128篇 |
2007年 | 1148篇 |
2006年 | 1022篇 |
2005年 | 1052篇 |
2004年 | 956篇 |
2003年 | 887篇 |
2002年 | 842篇 |
2001年 | 176篇 |
2000年 | 154篇 |
1999年 | 222篇 |
1998年 | 246篇 |
1997年 | 172篇 |
1996年 | 152篇 |
1995年 | 121篇 |
1994年 | 132篇 |
1993年 | 122篇 |
1992年 | 133篇 |
1991年 | 104篇 |
1990年 | 135篇 |
1989年 | 117篇 |
1988年 | 98篇 |
1987年 | 98篇 |
1986年 | 65篇 |
1985年 | 102篇 |
1984年 | 103篇 |
1983年 | 64篇 |
1982年 | 91篇 |
1981年 | 74篇 |
1980年 | 52篇 |
1979年 | 54篇 |
1978年 | 41篇 |
1977年 | 41篇 |
1976年 | 35篇 |
1975年 | 25篇 |
1974年 | 34篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Patrick Dreyfus Dina Zevin-Sonkin Shlomo Seidman Catherine Prody Rivka Zisling Haim Zakut Hermona Soreq 《Journal of neurochemistry》1988,51(6):1858-1867
To study the polymorphism of human cholinesterases (ChEs) at the levels of primary sequence and three-dimensional structure, a fragment of human butyrylcholinesterase (BuChE) cDNA was subcloned into the pEX bacterial expression vector and its polypeptide product analyzed. Immunoblot analysis revealed that the clone-produced BuChE peptides interact specifically with antibodies against human and Torpedo acetylcholinesterase (AChE). Rabbit polyclonal antibodies prepared against the purified clone-produced BuChE polypeptides interacted in immunoblots with denatured serum BuChE as well as with purified and denatured erythrocyte AChE. In contrast, native BuChE tetramers from human serum, but not AChE dimers from erythrocytes, interacted with these antibodies in solution to produce antibody-enzyme complexes that could be precipitated by second antibodies and that sedimented faster than the native enzyme in sucrose gradient centrifugation. Furthermore, both AChE and BuChE dimers from muscle extracts, but not BuChE tetramers from muscle, interacted with these antibodies. To reveal further whether the anti-cloned BuChE antibodies would interact in situ with ChEs in the neuromuscular junction, bundles of muscle fibers were microscopically dissected from the region in fetal human diaphragm that is innervated by the phrenic nerve. Muscle fibers incubated with the antibodies and with 125I-Protein A were subjected to emulsion autoradiography, followed by cytochemical ChE staining. The anti-cloned BuChE antibodies, as well as anti-Torpedo AChE antibodies, created patches of silver grains in the muscle endplate region stained for ChE, under conditions where control sera did not. These findings demonstrate that the various forms of human AChE and BuChE in blood and in neuromuscular junctions share sequence homologies, but also display structural differences between distinct molecular forms within particular tissues, as well as between similarly sedimenting molecular forms from different tissues. 相似文献
72.
Patrick Sullivan 《CMAJ》1988,138(2):160-161
73.
74.
Michael L. Mavrovouniotis Patrick Bayol Tu-Kien Michael Lam George Stephanopoulos Gregory Stephanopoulos 《Biotechnology Techniques》1988,2(1):23-28
Using Gibbs Energies of compounds, as well as Gibbs Energy changes and equilibrium constants of biochemical reactions, the contributions of functional groups to the Gibbs Energy (in aqueous solution, temperature 25°C, and pH=7) have been estimated. These contributions allow the estimation of the Gibbs Free Energy and the equilibrium constant of a biochemical reaction, given the structure of the reactants and products. 相似文献
75.
76.
Alterations in lipid content and composition in the N-nitrosodiethylamine-induced hepatocarcinoma were investigated. Rats were administrated with N-nitrosodiethylamine in the drinking water for 12 weeks followed by normal tap water for another 6 weeks. The cholesterol content in the liver was increased shortly after the administration of N-nitrosodiethylamine and remained elevated after the removal of the nitrosoamine from the water. The phosphatidylethanolamine level was elevated during N-nitrosodiethylamine administration with a concomitant reduction in phosphatidylcholine level. Lysophosphatidylcholine and sphingomyelin levels were increased during the last four weeks of the study. The level of phosphatidylinositol was substantially reduced after eight weeks of N-nitrosodiethylamine treatment, and remained low during the post-treatment period. We postulate that changes in lysophosphatidylcholine and sphingomyelin may be a compensatory mechanism for maintaining the asymmetrical distribution of choline-containing lipids in the outer leaflet of the membrane. The elevated level of cholesterol may be a useful indicator for the early detection of N-nitrosodiethylamine-induced hepatocarcinoma. 相似文献
77.
Synopsis We investigated the ability of two congeneric species of sunfish to learn to forage on a novel prey item in feeding arenas
containing structured habitats. Eight bluegill sunfish and eight pumpkinseed sunfish were given the opportunity to forage
on whiteworms daily for 10 days. Each day, several behavioural measures were recorded for each fish. Both species of sunfish
learned to feed over the 10-day period but the bluegill sunfish learned to feed more quickly than the pumpkinseed sunfish.
Pumpkinseeds, however, attained a higher level of foraging efficiency. The differences in learning and foraging efficiency
were related to body morphology. 相似文献
78.
Somatic embryogenesis and plant regeneration from immature embryos of western larch 总被引:11,自引:0,他引:11
Somatic embryogenesis was initiated from immature embryos of western larch (Larix occidentalis Nutt.) on media containing 2,4-dichlorophenoxyacetic acid and N6- benzyladenine. The effects of explant type and ammonium nitrate and glutamine concentrations on initiation were tested. Although 21–93% of explants rendered cultures in various experiments, only 3% yielded sustainable embryogenic lines. Excised embryos at the early cotyledonary stage were optimal for initiation. Maturation of somatic embryos was promoted by abscisic acid. Response to abscisic acid concentrations and duration of exposure to abscisic acid varied with genotype. Maximal results were obtained with 0.025 M abscisic acid for 1 to 2 weeks followed by individual culture on medium without growth regulators. Mature somatic embryos developed into shoots with roots. Plantlets have been established in peat.Abbreviations 2,4-D
2,4-dichlorophenoxyacetic acid
- BA
N6-benzyladenine
- IBA
indole-3-butyric acid
- ABA
abscisic acid 相似文献
79.
80.
Calling song and selective phonotaxis in the field crickets,Gryllus firmus andG. pennsylvanicus (Orthoptera: Gryllidae) 总被引:1,自引:1,他引:0
The field cricket species, Gryllus firmusand G. pennsylvanicus,occur in a mosaic hybrid zone that roughly parallels the eastern slope of the Appalachian mountains in the northeastern United States. It is important to know what role, if any, the calling song plays in mate choice in sympatric and allopatric populations. In this report, we present results on the variability of calling song properties along transects across this hybrid zone. We also present the results of experiments on phonotactic selectivity of females from an allopatric population of G. firmus.The male calling song of allopatric G. firmuswas significantly slower in temporal rhythm (i. e., chirp and pulse repetition rates) and lower in pitch (i.e., dominant frequency) than that of allopatric G. pennsylvanicus.Calling song properties of males recorded in the hybrid zone varied considerably in temporal and spectral properties. In two-stimulus (choice) phonotaxis experiments, allopatric females of G. firmuspreferred synthetic calling songs with conspecific pulse repetition rates over songs that had lower and higher pulse rates. This preference persisted even when the sound pressure levels of alternative stimuli were unequal. Therefore, allopatric females of G. firmuscan discriminate between conspecific and heterospecific calling songs. Whether or not this same selectivity is present in sympatric populations remains unclear. Investigations of phonotactic selectivity in other allopatric and sympatric populations of both species are currently under way. 相似文献