全文获取类型
收费全文 | 3895篇 |
免费 | 335篇 |
国内免费 | 2篇 |
出版年
2022年 | 12篇 |
2021年 | 86篇 |
2020年 | 31篇 |
2019年 | 67篇 |
2018年 | 69篇 |
2017年 | 61篇 |
2016年 | 92篇 |
2015年 | 166篇 |
2014年 | 174篇 |
2013年 | 233篇 |
2012年 | 308篇 |
2011年 | 292篇 |
2010年 | 180篇 |
2009年 | 142篇 |
2008年 | 231篇 |
2007年 | 230篇 |
2006年 | 198篇 |
2005年 | 164篇 |
2004年 | 178篇 |
2003年 | 168篇 |
2002年 | 169篇 |
2001年 | 110篇 |
2000年 | 92篇 |
1999年 | 72篇 |
1998年 | 51篇 |
1997年 | 31篇 |
1996年 | 30篇 |
1995年 | 33篇 |
1994年 | 41篇 |
1993年 | 23篇 |
1992年 | 57篇 |
1991年 | 40篇 |
1990年 | 33篇 |
1989年 | 35篇 |
1988年 | 25篇 |
1987年 | 25篇 |
1986年 | 25篇 |
1985年 | 19篇 |
1984年 | 25篇 |
1983年 | 14篇 |
1982年 | 12篇 |
1981年 | 12篇 |
1979年 | 14篇 |
1977年 | 12篇 |
1976年 | 13篇 |
1975年 | 11篇 |
1974年 | 9篇 |
1973年 | 10篇 |
1972年 | 9篇 |
1971年 | 9篇 |
排序方式: 共有4232条查询结果,搜索用时 333 毫秒
61.
R.M. de Figueroa I.L. Benito de Cárdenas F. Sesma F. Alvarez A. P. de Ruiz Holgado G. Oliver 《Journal of applied microbiology》1996,81(4):348-354
Lactobacillus rhamnosus ATCC 7469 exhibited diauxie when grown in a medium containing both glucose and citrate as energy source. Glucose was used as the primary energy source during the glucose-citrate diauxie. Uptake of citrate was carried out by an inducible citrate transport system. The induction of citrate uptake system was repressed in the presence of glucose. This repression was reversible and mediated by cAMP. 相似文献
62.
Mutational analysis of U1 function in Schizosaccharomyces pombe: pre-mRNAs differ in the extent and nature of their requirements for this snRNA in vivo. 总被引:2,自引:1,他引:1 下载免费PDF全文
The U1 snRNP is known to play a critical role in spliceosome assembly, at least in part through base pairing of its RNA moiety to the substrate, but many details remain to be elucidated. To further dissect U1 snRNA function, we have analyzed 14 single point mutations in the six nucleotides complementary to the 5' splice site for their effects on growth and splicing in the fission yeast Schizosaccharomyces pombe. Three of the four alleles previously found to support growth of Saccharomyces cerevisiae are lethal in S. pombe, implying a more critical role for the 5' end of U1 in fission yeast. Furthermore, a comparison of phenotypes for individual nucleotide substitutions suggests that the two yeasts use different strategies to modulate the extent of pairing between U1 and the 5' splice site. The importance of U1 function in S. pombe is further underscored by the lethality of several single point mutants not examined previously in S. cerevisiae. In total, only three alleles complement the U1 gene disruption, and these strains are temperature-sensitive for growth. Each viable mutant was tested for impaired splicing of three different S. pombe introns. Among these, only the second intron of the cdc2 gene (cdc2-I2) showed dramatic accumulation of linear precursor. Notably, cdc2-I2 is spliced inefficiently even in cells containing wild-type U1, at least in part due to the presence of a stable hairpin encompassing its 5' splice site. Although point mutations at the 5' end of U1 have no discernible effect on splicing of pre-U6, significant accumulation of unspliced RNA is observed in a metabolic depletion experiment. Taken together, these observations indicate that the repertoire of U1 activities is used to varying extents for splicing of different pre-mRNAs in fission yeast. 相似文献
63.
64.
Edgar Vázquez-Contreras Nora Vázquez-Laslop Georges Dreyfus 《Journal of bioenergetics and biomembranes》1995,27(1):109-116
A functional F0F1 ATP synthase that contains the endogenous inhibitor protein (F0F1I) was isolated by the use of two combined techniques [Adolfsen, R., McClung, J.A., and Moudrianakis, E. N. (1975).Biochemistry
14, 1727–1735; Dreyfus, G., Celis, H., and Ramirez, J. (1984).Anal. Biochem.
142, 215–220]. The preparation is composed of 18 subunits as judged by SDS-PAGE. A steady-state kinetic analysis of the latent ATP synthase complex at various concentrations of ATP showed aV
max of 1.28mol min–1 mg–1, whereas theV
max of the complex without the inhibitor was 8.3mol min–1 mg–1. In contrast, theK
m
for Mg-ATP of F0F1 I was 148M, comparable to theK
m
value of 142M of the F0F1 complex devoid of IF1. The hydrolytic activity of the F0F1I increased severalfold by incubation at 60C at pH 6.8, reaching a maximal ATPase activity of 9.5mol min–1 mg–1; at pH 9.0 a rapid increase in the specific activity of hydrolysis was followed by a sharp drop in activity. The latent ATP synthase was reconstituted into liposomes by means of a column filtration method. The proteoliposomes showed ATP-Pi exchange activity which responded to phosphate concentration and was sensitive to energy transfer inhibitors like oligomycin and the uncouplerp-trifluoromethoxyphenylhydrazone. 相似文献
65.
Verónica Alvarez Maubecin Viviana N. Sanchez Marcelo D. Rosato Siri Bruce D. Cherksey Mutzuyuki Sugimori Rodolfo Llinás Osvaldo D. Uchitel 《Journal of neurochemistry》1995,64(6):2544-2551
Abstract: The voltage-dependent calcium channels present in mammalian and chicken brain synaptosomes were characterized pharmacologically using specific blockers of L-type channels (1,4-dihydropyridines), N-type channels (ω-conotoxin GVIA), and P-type channels [funnel web toxin (FTX) and ω-agatoxin IVA]. K+-induced Ca2+ uptake by chicken synaptosomes was blocked by ω-conotoxin GVIA (IC50 = 250 nM). This toxin at 5 µM did not block Ca2+ entry into rat frontal cortex synaptosomes. FTX and ω-agatoxin IVA blocked Ca2+ uptake by rat synaptosomes (IC50 = 0.17 µl/ml and 40 nM, respectively). Likewise, in chicken synaptosomes, FTX and ω-agatoxin IVA affected Ca2+ uptake. FTX (3 µl/ml) exerted a maximal inhibition of 40% with an IC50 similar to the one obtained in rat preparations, whereas with ω-agatoxin IVA saturation was not reached even at 5 µM. In chicken preparations, the combined effect of saturating concentrations of FTX (1 µl/ml) and different concentrations of ω-conotoxin GVIA showed no additive effects. However, the effect of saturating concentrations of FTX and ω-conotoxin GVIA was never greater than the one observed with ω-conotoxin GVIA. We also found that 60% of the Ca2+ uptake by rat and chicken synaptosomes was inhibited by ω-conotoxin MVIID (1 µM), a toxin that has a high index of discrimination against N-type channels. Conversely, nitrendipine (10 µM) had no significant effect on Ca2+ uptake in either the rat or the chicken. In conclusion, Ca2+ uptake by rat synaptosomes is potently inhibited by different P-type Ca2+ channel blockers, thus indicating that P-type channels are predominant in this preparation. In contrast, Ca2+ uptake by chicken synaptosomes is sensitive to ω-conotoxin GVIA, FTX, ω-agatoxin IVA, and ω-conotoxin MVIID. This suggests that a channel subtype with a mixed pharmacology is present in chicken synaptosomes. 相似文献
66.
Monitoring dynamic changes in free Ca2+ concentration in the endoplasmic reticulum of intact cells. 总被引:7,自引:3,他引:4 下载免费PDF全文
M Montero M Brini R Marsault J Alvarez R Sitia T Pozzan R Rizzuto 《The EMBO journal》1995,14(22):5467-5475
Direct monitoring of the free Ca2+ concentration in the lumen of the endoplasmic reticulum (ER) is an important but still unsolved experimental problem. We have shown that a Ca(2+)-sensitive photoprotein, aequorin, can be addressed to defined subcellular compartments by adding the appropriate targeting sequences. By engineering a new aequorin chimera with reduced Ca2+ affinity, retained in the ER lumen via interaction of its N-terminus with the endogenous resident protein BiP, we show here that, after emptying the ER, Ca2+ is rapidly re-accumulated up to concentrations of > 100 microM, thus consuming most of the reporter photoprotein. An estimate of the steady-state Ca2+ concentration was obtained using Sr2+, a well-known Ca2+ surrogate which elicits a significantly slower rate of aequorin consumption. Under conditions in which the rate and extent of Sr2+ accumulation in the ER closely mimick those of Ca2+, the steady-state mean lumenal Sr2+ concentration ([Sr2+]er) was approximately 2 mM. Receptor stimulation causes, in a few seconds, a 3-fold decrease of the [Sr2+]er, whereas specific inhibition of the ER Ca2+ ATPase leads to an approximately 10-fold drop in a few minutes. 相似文献
67.
68.
69.
Ecological surveys were carried out to investigate the distribution and characterization of remaining mangrove stands in Hong
Kong. The field studies indicate that 43 mangrove stands, excluding Mai Po Nature Reserve, still remained along the coastline
of Hong Kong despite tremendous reclamation and development which occurred in the past 40 years. Most mangrove stands were
found in Deep Bay (western part)and Sai Kung District (eastern coasts). The total areas occupied by these mangrove stands
were 178 ha,varying from a very small stand (with 1–2 mangrove shrubs) to fairly extensive mangroves in Deep Bay (> 10 ha).
It appeared that mangrove stands located in Deep Bay area were larger than those in the eastern coasts. Twenty plant species
were identified from these stands, with 13 being exclusive or associate mangrove species. The major constituent species were
Kandelia candel, Aegiceras corniculatum, Excoecaria agallocha and Avicennia marina. Rare species such as Heritiera littoralis
were only found in a few mangrove stands. Out of the 43remaining mangrove stands, 23 were more worthwhile for conservation
and their plant community structures were further investigated by transect and quadrat analyses. The importance values (sum
of relative abundance,frequency and dominance) show that K. candel was the most dominant species. Species richness and Simpson's
indices together with tree height, tree density and canopy area fluctuated significantly between mangrove stands. These values
were used to prioritize the conservation potential of the remaining mangrove stands in Hong Kong.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
70.
PCR for bioaerosol monitoring: sensitivity and environmental interference. 总被引:20,自引:8,他引:12 下载免费PDF全文
The PCR technique has potential for use in detection of low concentrations of airborne microorganisms. In this study, the sensitivity of PCR and its susceptibility to environmental interference were assessed with Escherichia coli DH1 as the target organism. Air samples, containing environmental bioaerosols, were collected with AGI-30 samplers and seeded with E. coli DH1 cells. Parallel studies were performed with cells seeded into the sampler prior to collection of air samples to determine the effects of environmental inhibition and sampling stress on the PCR assay. Baseline studies were also performed without environmental challenge or sampling stress to compare two protocols for cell lysis, solid phase and freeze-thaw. Amplification of a plasmid target sequence resulted in a detection limit of a single bacterial cell by the freeze-thaw and solid-phase methods within 5 and 9 h, respectively. With a genomic target, the sensitivity of the solid-phase method was 10-fold lower than that of freeze-thaw. Samples which contained 10(3) to 10(4) CFU of environmental organisms per m3 inhibited amplification; however, a 1/10 dilution of these samples resulted in successful amplifications. No difference in sensitivity of the PCR assay was obtained as a result of sampling stress, although a 10-fold decrease in culturability was observed. A field validation of the protocol with genomic primers demonstrated the presence of airborne E. coli and/or Shigella spp. in outdoor samples. This study indicates that the PCR method for detection of airborne microorganisms is rapid and sensitive and can be used as an alternative method for air quality monitoring. 相似文献