全文获取类型
收费全文 | 55796篇 |
免费 | 4797篇 |
国内免费 | 4693篇 |
出版年
2024年 | 100篇 |
2023年 | 645篇 |
2022年 | 1597篇 |
2021年 | 2676篇 |
2020年 | 1767篇 |
2019年 | 2205篇 |
2018年 | 2019篇 |
2017年 | 1540篇 |
2016年 | 2205篇 |
2015年 | 3389篇 |
2014年 | 3953篇 |
2013年 | 4242篇 |
2012年 | 5015篇 |
2011年 | 4664篇 |
2010年 | 2855篇 |
2009年 | 2645篇 |
2008年 | 3034篇 |
2007年 | 2705篇 |
2006年 | 2355篇 |
2005年 | 2022篇 |
2004年 | 1737篇 |
2003年 | 1557篇 |
2002年 | 1320篇 |
2001年 | 1057篇 |
2000年 | 919篇 |
1999年 | 881篇 |
1998年 | 573篇 |
1997年 | 549篇 |
1996年 | 516篇 |
1995年 | 442篇 |
1994年 | 446篇 |
1993年 | 346篇 |
1992年 | 481篇 |
1991年 | 408篇 |
1990年 | 350篇 |
1989年 | 262篇 |
1988年 | 243篇 |
1987年 | 220篇 |
1986年 | 151篇 |
1985年 | 196篇 |
1984年 | 129篇 |
1983年 | 114篇 |
1982年 | 86篇 |
1981年 | 66篇 |
1980年 | 56篇 |
1979年 | 77篇 |
1978年 | 67篇 |
1977年 | 47篇 |
1976年 | 51篇 |
1973年 | 51篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
161.
主养青鱼池塘生态系统能量转换率的研究 总被引:14,自引:3,他引:11
1985—1987年对苏州市郊区主养青鱼池塘生态系统的能量转换率进行了分析。结果表明,主养青鱼净产7.5、11.25、15t/ha 3个产量级型池塘青鲤团头鲂产出能占养鱼总产出能的比例分别为82.49、78.03、79.34%;总投入能(太阳辐射能+辅助能)转移到鱼的总产出能转换率分别为0.19、0.24、0.31%;太阳辐射能转移到毛和净初级生产力的能量转换率分别为0.76、0.90、0.96%和0.61、0.72、0.77%;净初级生产力转移到滤食性鱼净产量的能量转换率分别为4.02、4.63、5.27%;辅助能转移到鱼净产量的能量转换率分别为12.20、11.33。11.74%。在3个产量级型池塘中,以15t/ha产量级的能量转换率为最佳型。 相似文献
162.
163.
164.
U. Brand L. Brandes V. Koch T. Kullik B. Reinhardt F. Rüther T. Scheper K. Schügerl S. Wang X. Wu R. Ferretti S. Prasad D. Wilhelm 《Applied microbiology and biotechnology》1991,36(2):167-172
Summary Single and multisensor field effect transistors (FET) with a pH-sensitive Si/SiO2/Si3N4/Ta2O5-gate and reference electrode (for single sensor) were developed and used for manufacturing the following biological (Bio)-FETs: for glucose analysis, glucose oxidase-FET (GOD-FET); for urea analysis, urease-FET; and for cephalosporin C analysis, cephalosporinase-FET. The GOD-FETs were integrated into flow injection analysis (FIA) of the Eppendorf variables analyser (EVA) system and used for monitoring the glucose concentration in microbial cultivation and production processes with recombinant Escherichia coli K12 MF, recombinant E. coli JM103, Saccharomyces cerevisiae H620, and Candida boidinii. Urease-FET-FIA was used to monitor the urea concentration in a simulated cultivation of Cephalosporium acremonium and urease-FET-FIA and GOD-FET-FIA for the monitoring of urea and glucose concentrations in simulated S. cerevisiae cultivations. 相似文献
165.
Ding W. Wu John G. Duman Chi-Hing C. Cheng Francis J. Castellino 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1991,161(3):271-278
Summary Four antifreeze proteins (AFPs) were purified from larvae of the beetle Dendroides canadensis. The AFPs are similar in amino acid compositions, having high contents of hydrophilic amino acids (45–55 mol%) and cysteine (16 mol% Cys). Approximately half of the Cys residues form disulfide bridges, and both the disulfide bridges and free sulfhydryls are essential for activity. The N-terminals of the AFPs are blocked. The pH optimum of the AFPs is 7.8, but major loss of activity occurred only at very high pH (12.0). The detergents SDS and Triton X-100 did not inactivate the AFPs. Circular dichroism spectra indicate the presence of both and secondary structures in the AFPs, in addition to a large random structure component.Abbreviations
AFP
antifreeze protein
-
CD
circular dichroism
-
DTT
dithiothreitol
-
HPLC
high pressure liquid chromatography
-
PAGE
polyacrylamide gel electrophoresis
-
PAS
periodic acid Schiff
-
SDS
sodium dodecyl sulfate
-
TFA
trifluoroacetic acid 相似文献
166.
Ding Wen Wu John G. Duman 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1991,161(3):279-283
Summary Purified antifreeze proteins (AFPs) from the larvae of the beetle Dendroides canadensis do not produce the high levels of antifreeze activity seen in the hemolymph of overwintering larvae, even when the purified AFPs are assayed at very high concentrations. However, addition of certain proteins or agar (at concentrations sufficiently low that the gel state does not result) to the Dendroides AFP resulted in a 2–3-fold increase in activity. A 70-kDa protein with AFP-activating capabilities was purified from Dendroides larvae. Addition of this endogenous activator protein to a 4 mg·ml-1 solution of AFP increased the activity of the AFPs to values comparable to those of the hemolymph of overwintering larvae. Data derived from a modified immunoblot technique demonstrate that the activators bind to the AFP, or vice versa. Formation of this association must allow the AFP to block ice crystal growth by binding to the surface of potential seed crystals in the normal fashion. However, because the AFP-activator complex is much larger than the AFP alone, the complex probably blocks a greater surface area of the crystal and is thus a more efficient antifreeze.Abbreviations
AFP
antifreeze protein
-
BSA
bovine serum albumine
-
DEAE
diethylaminoethyl
-
Ig
immunoglubolin
-
LPIN
lipoprotein ice nucleator
-
PIN
protein ice nucleator
-
SDS
sodium dodecyl sulfate
-
PAGE
polyacrylamide gel electrophoresis
-
TH
thermal hysteresis 相似文献
167.
Emetine allows identification of origins of mammalian DNA replication by imbalanced DNA synthesis, not through conservative nucleosome segregation. 总被引:24,自引:0,他引:24 下载免费PDF全文
W C Burhans L T Vassilev J Wu J M Sogo F S Nallaseth M L DePamphilis 《The EMBO journal》1991,10(13):4351-4360
In the presence of emetine, an inhibitor of protein synthesis, nascent DNA on forward arms of replication forks in hamster cell lines containing either single or amplified copies of the DHFR gene region was enriched 5- to 7-fold over nascent DNA on retrograde arms. This forward arm bias was observed on both sides of the specific origin of bidirectional DNA replication located 17 kb downstream of the hamster DHFR gene (OBR-1), consistent with at least 85% of replication forks within this region emanating from OBR-1. However, the replication fork asymmetry induced by emetine does not result from conservative nucleosome segregation, as previously believed, but from preferentially inhibiting Okazaki fragment synthesis on retrograde arms of forks to produce 'imbalanced DNA synthesis'. Three lines of evidence support this conclusion. First, the bias existed in long nascent DNA strands prior to nuclease digestion of non-nucleosomal DNA. Second, the fraction of RNA-primed Okazaki fragments was rapidly diminished. Third, electron microscopic analysis of SV40 DNA replicating in the presence of emetine revealed forks with single-stranded DNA on one arm, and nucleosomes randomly distributed to both arms. Thus, as with cycloheximide, nucleosome segregation in the presence of emetine was distributive. 相似文献
168.
169.
170.
The addition of the proper amount of ammonium to the culture medium containing nitrate as nitrogen source enhanced the growth rate of Anabaena viguieri. The amount of geosmin produced by these cells varied with the concentrations of ammonium added. A negative correlation between the amount of geosmin produced and of the growth rate of cells was revealed. This was also found in cells grown on various forms of nitrogen sources. Without supply of any nitrogen compound, this organism is capable of fixing gaseous nitrogen, and under these conditions the cells grew relatively slowly. However, they produced more geosmin (per unit protein mass) than cells grown in the presence of combined nitrogen. The isolation of heterocysts, in which nitrogen was fixed, showed that these cells produced higher amounts of geosmin than vegetative cells. The possible relation of nitrogen assimilation to the production of geosmin in the cells was discussed. 相似文献