全文获取类型
收费全文 | 5014篇 |
免费 | 374篇 |
国内免费 | 537篇 |
出版年
2024年 | 29篇 |
2023年 | 105篇 |
2022年 | 179篇 |
2021年 | 206篇 |
2020年 | 174篇 |
2019年 | 213篇 |
2018年 | 179篇 |
2017年 | 162篇 |
2016年 | 161篇 |
2015年 | 189篇 |
2014年 | 213篇 |
2013年 | 346篇 |
2012年 | 158篇 |
2011年 | 157篇 |
2010年 | 145篇 |
2009年 | 222篇 |
2008年 | 224篇 |
2007年 | 254篇 |
2006年 | 218篇 |
2005年 | 172篇 |
2004年 | 159篇 |
2003年 | 141篇 |
2002年 | 173篇 |
2001年 | 137篇 |
2000年 | 86篇 |
1999年 | 96篇 |
1998年 | 93篇 |
1997年 | 93篇 |
1996年 | 80篇 |
1995年 | 80篇 |
1994年 | 90篇 |
1993年 | 91篇 |
1992年 | 73篇 |
1991年 | 68篇 |
1990年 | 51篇 |
1989年 | 87篇 |
1988年 | 49篇 |
1987年 | 52篇 |
1986年 | 41篇 |
1985年 | 71篇 |
1984年 | 66篇 |
1983年 | 45篇 |
1982年 | 37篇 |
1981年 | 52篇 |
1980年 | 39篇 |
1979年 | 49篇 |
1978年 | 33篇 |
1977年 | 28篇 |
1976年 | 22篇 |
1973年 | 9篇 |
排序方式: 共有5925条查询结果,搜索用时 296 毫秒
71.
Hurricane Danny resulted in the rapid deposition of 10cm of oxidized, acidic sediment in the Contrary Creek arm of Lake Anna, Virginia. Several biological and geochemical parameters were monitored with time to ascertain how long it took the newly-deposited lake sediments to attain the anaerobic, circumneutral, actively sulfate-reducing state normally observed in this portion of the lake. The sediment platinum-electrode potential dropped from 350 mV to 100 mV within the first week after the storm. The pH of the pore water increased from 4.5 to 5.8 within three weeks, and titratable alkalinity was detected within two weeks and three weeks at 3 cm and 1 cm depths, respectively. Accumulation of reduced products of sulfate reduction (acid volatile sulfide) began by three to four weeks after the storm event. Both methanogens and sulfate reducers were present in high and approximately equal numbers in the freshly deposited material. The rapid neutralization of the acidity in the fresh sediment prior to the onset of sulfate reduction suggests that reactions other than sulfate reduction caused the initial increase in pH and alkalinity in this system. 相似文献
72.
SecA protein: Autoregulated initiator of secretory precursor protein translocation across theE. coli plasma membrane 总被引:10,自引:0,他引:10
Donald B. Oliver Robert J. Cabelli Gregory P. Jarosik 《Journal of bioenergetics and biomembranes》1990,22(3):311-336
Several classes ofsecA mutants have been isolated which reveal the essential role of this gene product forE. coli cell envelope protein secretion. SecA-dependent,in vitro protein translocation systems have been utilized to show that SecA is an essential, plasma membrane-associated, protein translocation factor, and that SecA's ATPase activity appears to play an essential but as yet undefined role in this process. Cell fractionation studies suggested that SecA protein is in a dynamic state within the cell, occurring in soluble, peripheral, and integral membraneous states. These data have been used to argue that SecA is likely to promote the initial insertion of secretory precursor proteins into the plasma membrane in a manner dependent on ATP hydrolysis. The protein secretion capability of the cell has been shown to translationally regulatesecA expression with SecA protein serving as an autogenous repressor, although the exact mechanism and purpose of this regulation need to be defined further. 相似文献
73.
Steven R. Hagen Duane LeTourneau Paul Muneta Janice Brown 《Plant Growth Regulation》1990,9(4):341-345
Callus cultures of 7 potato cultivars were initiated from tuber tissue and maintained on Gelrite-solidified media with 1–20 M picloram as the only PGR. Ten M picloram was the optimal concentration for callus induction. By 4–6 weeks after explanting, there was sufficient callus produced for subculture to maintenance media which contained 1–20 M picloram as the only PGR. When grown in the dark at 25°C, subcultured callus typically increased 10-fold in wet weight in 4–5 weeks. The callus produced was friable and a light grey to cream color. Callus cultures were used to establish cell suspension cultures. Callus and cell suspension cultures have been maintained for over 2 years on the picloram containing media.Abbreviations BA
benzyladenine
- 2,4-D
2,4-dichlorophenoxyacetic acid
- MS
Murashige-Skoog
- NAA
naphthaleneacetic acid
- PGR
plant growth regulator
Research paper #9053 of the Idaho Agricultural Experiment Station. 相似文献
74.
Charlotte L. Branchaud Cynthia G. Goodyer Harvey J. Guyda Yves Lefebvre 《In vitro cellular & developmental biology. Plant》1990,26(9):865-870
Summary We have compared hormone production by early gestation and term human placental trophoblasts cultured in Ham's F10 medium
containing 10% fetal bovine serum with that by cells cultured in serum-free HB102 medium. Mean daily production of progesterone
on Days 3 to 7 was approximately 25% less by both early gestation and term cells cultured in HB102 as compared to Ham's F10,
but production was maintained at a stable level for at least 7 d longer than the cells in Ham's. Estradiol production from
10−6
M dehydroepiandrosterone by both early gestation and term cells was comparable in both media. Human placental lactogen production
on Days 3 to 7 was 40% less by cells cultured in HB102. Human chorionic gonadotropin (hCG) output by early gestation cells
was also 50% less in HB102 but term cells in HB102 produced twice as much hCG as those in Ham's F10. 3B-Hydroxysteroid dehydrogenase
(3BHSD) activity in early gestation and term cells and 11B-hydroxysteroid dehydrogenase (11BHSD) activity of early gestation
cultures was comparable in the two media. 11BHSD activity was decreased in the term cultures, and this decrease was more marked
in Ham's than in HB102. Sulfatase and aromatase activities in the early gestation cultures were comparable in both media;
sulfatase activity was comparable and aromatase activity only 20% less in the term cells cultured in HB102. These results
indicate that serum-free HB102 supports differential function of human trophoblast cells and is useful for studies of placental
activity for as long as 14 d in culture. 相似文献
75.
LEONOR E. ROVAI AGUSTIN AOKI NELIA M. GEREZ DE BURGOS ANTONCO BLANCO 《The Journal of eukaryotic microbiology》1990,37(4):280-286
Bloodstream Trypanosoma cruzi trypomastigotes isolated from infected mice undergo reduction of motility and structural damages after 5 to 45 min exposure to gossypol at concentrations ranging from 5 to 50 μM. When 1% serum albumin is added to the incubation medium, no alterations of parasites are observed, even with 100 μM gossypol. Intracellular T. cruzi amastigotes in infected Vero cell cultures exposed to 5 μM gossypol for 2 h do not show changes. Incubation with 5 μM gossypol for 48 h produces complete disruption of host cells; however, the amastigotes they contain show only mineor alterations. The observations indicate that, in protein-rich media, gossypol is complexed into associations which have no activity on the different forms of the T. cruzi biological cycle. 相似文献
76.
M. Couturier F. Lemonnier M. Conti M. Feneant-Thibault A. Lemonnier 《In vitro cellular & developmental biology. Plant》1990,26(1):29-32
Summary 2-Deoxy-d-glucose (2-DOG) uptake was tested in human fibroblast cultures in the presence and absence of vitamin E. Addition of 10 μg/ml
vitamin E to the culture medium significantly reduced this uptake for 2-DOG concentrations of 0.005, to 10 mmol/liter (P≤0.01). The decrease of 2-DOG uptake was inversely proportional to the rise in 2-DOG concentration (P≤0.01). The presence of vitamin E reduced by 71% the average cellular level of lipid peroxides (expressed as thiobarbituric
acid reactive substances) and caused a small but significant decrease in the cholesterol concentration (P≤0.01). These last results might explain the decrease in 2-DOG uptake observed in the presence of vitamin E. 相似文献
77.
Abstract: Reactive oxygen species have been implicated in neuronal injury associated with various neuropathological disorders. However, little is known regarding the relationship between antioxidant enzyme capacity and resultant toxicity. The antioxidant pathways of primary cerebrocortical cultures were directly examined using a novel technique that measures pentose phosphate pathway (PPP) activity, which is enzymatically coupled to glutathione peroxidase (GPx) detoxification of hydrogen peroxide (H2O2). PPP activity was quantified from data obtained by gas chromatography/mass spectrometry analysis of released labeled lactate following metabolic degradation of [1,6-13C2,6,6-2H2]glucose by cerebrocortical cultures. The antioxidant capacity of these cultures was systematically evaluated using H2O2, and the resultant toxicity was quantified by lactate dehydrogenase release. Exposure of primary mixed and purified astrocytic cultures to H2O2 caused stimulation of PPP activity in a concentration-dependent fashion from 0.25 to 22.2% and from 6.9 to 66.7% of glucose metabolized to lactate through the PPP, respectively. In the mixed cultures, chelation of iron before H2O2 exposure was protective and resulted in a correlation between PPP saturation and toxicity. Conversely, addition of iron, inhibition of GPx, or depletion of glutathione decreased H2O2-induced PPP stimulation and increased toxicity. These results implicate the Fenton reaction, reflect the pivotal role of GPx in H2O2 detoxification, and contribute to our understanding of the etiological role of free radicals in neuropathological conditions. 相似文献
78.
Diversity and phylogeny of rhizobia 总被引:22,自引:1,他引:21
79.
80.
Spectral and kinetic characteristics of fluorescence from isolated reaction centers of photosynthetic purple bacteria Rhodobacter sphaeroides and Rhodobacter capsulatus were measured at room temperature under rectangular shape of excitation at 810 nm. The kinetics of fluorescence at 915 nm reflected redox changes due to light and dark reactions in the donor and acceptor quinone complex of the reaction center as identified by absorption changes at 865 nm (bacteriochlorophyll dimer) and 450 nm (quinones) measured simultaneously with the fluorescence. Based on redox titration and gradual bleaching of the dimer, the yield of fluorescence from reaction centers could be separated into a time-dependent (originating from the dimer) and a constant part (coming from contaminating pigment (detached bacteriochlorin)). The origin was also confirmed by the corresponding excitation spectra of the 915 nm fluorescence. The ratio of yields of constant fluorescence over variable fluorescence was much smaller in Rhodobacter sphaeroides (0.15±0.1) than in Rhodobacter capsulatus (1.2±0.3). It was shown that the changes in fluorescence yield reflected the disappearance of the dimer and the quenching by the oxidized primary quinone. The redox changes of the secondary quinone did not have any influence on the yield but excess quinone in the solution quenched the (constant part of) fluorescence. The relative yields of fluorescence in different redox states of the reaction center were tabulated. The fluorescence of the dimer can be used as an effective tool in studies of redox reactions in reaction centers, an alternative to the measurements of absorption kinetics.Abbreviations Bchl
bacteriochlorophyll
- Bpheo
bacteriopheophytin
- D
electron donor to P+
- P
bacteriochlorophyll dimer
- Q
quinone acceptor
- QA
primary quinone acceptor
- QB
secondary quinone acceptor
- RC
reaction center protein
- UQ6
ubiquinone-30 相似文献