全文获取类型
收费全文 | 91149篇 |
免费 | 7637篇 |
国内免费 | 5080篇 |
专业分类
103866篇 |
出版年
2024年 | 151篇 |
2023年 | 989篇 |
2022年 | 2215篇 |
2021年 | 3905篇 |
2020年 | 2526篇 |
2019年 | 3049篇 |
2018年 | 3140篇 |
2017年 | 2257篇 |
2016年 | 3219篇 |
2015年 | 5143篇 |
2014年 | 5837篇 |
2013年 | 6654篇 |
2012年 | 7719篇 |
2011年 | 7124篇 |
2010年 | 4297篇 |
2009年 | 3840篇 |
2008年 | 4752篇 |
2007年 | 4217篇 |
2006年 | 3671篇 |
2005年 | 3146篇 |
2004年 | 2776篇 |
2003年 | 2414篇 |
2002年 | 2141篇 |
2001年 | 1953篇 |
2000年 | 1895篇 |
1999年 | 1755篇 |
1998年 | 990篇 |
1997年 | 891篇 |
1996年 | 909篇 |
1995年 | 859篇 |
1994年 | 780篇 |
1993年 | 616篇 |
1992年 | 986篇 |
1991年 | 847篇 |
1990年 | 738篇 |
1989年 | 686篇 |
1988年 | 555篇 |
1987年 | 493篇 |
1986年 | 445篇 |
1985年 | 432篇 |
1984年 | 328篇 |
1983年 | 287篇 |
1982年 | 171篇 |
1981年 | 183篇 |
1980年 | 148篇 |
1979年 | 217篇 |
1978年 | 156篇 |
1977年 | 158篇 |
1975年 | 164篇 |
1974年 | 165篇 |
排序方式: 共有10000条查询结果,搜索用时 7 毫秒
41.
42.
K T Chen 《Acta cytologica》1984,28(2):133-135
Localized amyloidosis of the respiratory tract is seldom diagnosed in cytologic specimens. This report describes a case of the tracheobronchial form of amyloidosis in which diagnostic material was present in a cytologic specimen obtained during bronchoscopy. 相似文献
43.
Chien-Hung Liu Wen-Ming Chen Jo-Shu Chang 《World journal of microbiology & biotechnology》2007,23(5):633-640
Acidic lipase finds its commercial values in medical applications and bioremediation of food wastes. In this work, approaches
for rapid screening of lipase-producing bacteria were developed and the feasibility assessment of the screening methods was
performed. From food waste samples, the proposed screening procedures allowed isolation of sixteen pure bacterial strains
expressing higher lipase activity at acidic pH (pH 6.0) than at alkaline pH (pH 9.0). To enhance the accuracy of lipase activity
determination under acidic conditions, a novel assay procedure was also developed by deactivating lipase activity by microwave
treatment prior to back titration. This additional step could minimize interferences arising from residual lipase activity
during conventional direct back-titration methods in measuring lipase activity at acidic pH. Using the four strategies proposed
in this work, the best acidic-lipase-producing isolate was obtained by strategy C (SSC) and was identified as Aeromonas sp. C14, displaying an optimal lipase activity of 0.7 U/ml at an acidic pH of 6.0. 相似文献
44.
Mycoplasmas exhibit a novel, substrate-dependent gliding motility that is driven by ∼400 “leg” proteins. The legs interact with the substrate and transmit the forces generated by an assembly of ATPase motors. The velocity of the cell increases linearly by nearly 10-fold over a narrow temperature range of 10-40°C. This corresponds to an Arrhenius factor that decreases from ∼45 kBT at 10°C to ∼10 kBT at 40°C. On the other hand, load-velocity curves at different temperatures extrapolate to nearly the same stall force, suggesting a temperature-insensitive force-generation mechanism near stall. In this article, we propose a leg-substrate interaction mechanism that explains the intriguing temperature sensitivity of this motility. The large Arrhenius factor at low temperature comes about from the addition of many smaller energy barriers arising from many substrate-binding sites at the distal end of the leg protein. The Arrhenius dependence attenuates at high temperature due to two factors: 1), the reduced effective multiplicity of energy barriers intrinsic to the multiple-site binding mechanism; and 2), the temperature-sensitive weakly facilitated leg release that curtails the power stroke. The model suggests an explanation for the similar steep, sub-Arrhenius temperature-velocity curves observed in many molecular motors, such as kinesin and myosin, wherein the temperature behavior is dominated not by the catalytic biochemistry, but by the motor-substrate interaction. 相似文献
45.
Transfer RNAs, isolated from Escherichia coli F cells infected with T5 bacteriophage, were charged with radioactive amino acids and used in RNA-DNA hybridization studies to detect and locate T5 tRNA cistrons in the T5 DNA chromosome. Hybridization of 14 3H-aminoacyl-tRNA species, including purified T5 [35S]Met-tRNAm and [35S]Met-tRNAf, to the separated strands of T5+ DNA indicates that most, if not all, of the T5 tRNAs are transcribed from the continuous heavy strand of T5 DNA. Heteroduplex mapping of eight mutant T5 DNA deletions has enabled us to locate and determine the size of these deleted segments. By correlating this information with the presence and absence of specific tDNA sequences in these mutants, as determined by tRNA-DNA hybridization, we were able to define the physical limits of four tDNA-containing loci along the T5 DNA molecule. A physical map for 15 tRNA species examined indicates that the structural genes for these tRNAs are clustered within a segment length of T5 DNA that represents approximately 11.2% of the total wild type T5 DNA. The existence of the deletion mutants indicates that T5 tRNAs are dispensable for T5 replication under the growth conditions and for the host employed. 相似文献
46.
G. Matthew Hebdon Harry Le Vine Naji E. Sahyoun Claus J. Schmitges Pedro Cuatrecasas 《Life sciences》1980,26(17):1385-1396
3T3C2 mouse fibroblasts rendered permeable to (α?32P)NAD+ show cholera toxin-dependent labeling of a 45,000 m.w. protein and of a doublet of polypeptides around 52,000 m.w. These same bands are ADP-ribosylated in broken cells. Membranes prepared from pigeon erythrocytes pretreated with choleragen show a decrease in subsequent cholera toxin-specific ADP-ribosylation of a 43,000 m.w. polypeptide. Both whole cell and broken cell adenylate cyclase activation and toxin-specific ADP-ribosylation are reversed specifically by low pH and high concentrations of toxin and nicotinamide in all systems. Thus ADP-ribosylation appears to be relevant to the molecular action of choleragen in whole cells as well as in broken cells. 相似文献
47.
48.
49.
M T Hsieh H C Chen P H Hsu T Shibuya 《Proceedings of the National Science Council, Republic of China. Part B, Life sciences》1986,10(1):43-48
Previously, it was found that the ancient Chinese remedy of Suanzaorentang could be a promising anxiolytic drug (Chen and Hsieh, 1985a, Chen and Hsieh, 1985b). To understand the mechanism of the action of Suanzaorentang, the effects of Suanzaorentang on behavior changes and central monoamines and their metabolites were studied in rats. It was found that Suanzaorentang significantly (1) prolonged the period from the onset of clonic to tonic convulsions induced by pentylenetetrazol or picrotoxin, (2) prolonged the sleep duration induced by hexobarbital, (3) reduced locomotor activity, (4) enhanced the hypomotility induced by alpha-MT, (5) reduced the locomotor stimulation produced by levodopa plus benserazide, and (6) reduced central HVA, VMA, and 5-HIAA, but had no significant effects on central DA, NA, and 5-HT. These facts implied that Suanzaorentang decreased the turnover rate of central monoamines and central catecholaminergic activity. 相似文献
50.
Nucleotide sequence of the mouse ornithine decarboxylase gene. 总被引:6,自引:3,他引:3