全文获取类型
收费全文 | 33880篇 |
免费 | 3006篇 |
国内免费 | 3875篇 |
专业分类
40761篇 |
出版年
2024年 | 152篇 |
2023年 | 705篇 |
2022年 | 983篇 |
2021年 | 1251篇 |
2020年 | 1363篇 |
2019年 | 1568篇 |
2018年 | 1329篇 |
2017年 | 1375篇 |
2016年 | 1324篇 |
2015年 | 1351篇 |
2014年 | 1780篇 |
2013年 | 2457篇 |
2012年 | 1420篇 |
2011年 | 1702篇 |
2010年 | 1328篇 |
2009年 | 1759篇 |
2008年 | 1827篇 |
2007年 | 1873篇 |
2006年 | 1650篇 |
2005年 | 1401篇 |
2004年 | 1282篇 |
2003年 | 1182篇 |
2002年 | 985篇 |
2001年 | 887篇 |
2000年 | 850篇 |
1999年 | 762篇 |
1998年 | 656篇 |
1997年 | 568篇 |
1996年 | 526篇 |
1995年 | 458篇 |
1994年 | 448篇 |
1993年 | 407篇 |
1992年 | 381篇 |
1991年 | 338篇 |
1990年 | 297篇 |
1989年 | 257篇 |
1988年 | 225篇 |
1987年 | 219篇 |
1986年 | 217篇 |
1985年 | 201篇 |
1984年 | 176篇 |
1983年 | 120篇 |
1982年 | 178篇 |
1981年 | 130篇 |
1980年 | 111篇 |
1979年 | 93篇 |
1978年 | 71篇 |
1977年 | 35篇 |
1976年 | 23篇 |
1975年 | 23篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
(+)-2,9 alpha-Dimethyl-5-(m-hydroxyphenyl)morphan is the only phenylmorphan analog whose affinity for opioid kappa-receptors is greater than its affinity for opioid mu-receptors. Pharmacologically, the compound is a pure opioid antagonist devoid of agonist activity in in vivo assays of antinociception. The absolute configuration of the compound has been determined to be (1R,5S,9R) from an X-ray crystallographic study of the chloride salt. Thus, the absolute configuration corresponds to that of the atypical opioid agonist (-)-phenylmorphan while the weak atypical agonist (-)-2,9 alpha-dimethyl-5-(m- hydroxyphenyl)morphan corresponds to the potent morphine-like (+)-phenylmorphan. The preferred orientations of the phenyl ring for the two stereoisomers were determined using the molecular mechanics program MM2-87 and found to vary from that of the two parent compounds. The atypical properties of the two 9 alpha-methyl analogs is consistent with an opioid ligand model which proposes that morphine-like properties require a particular range of phenyl orientations. There was good agreement between the structure obtained from X-ray crystallography and computed with the MM2-87 program. 相似文献
2.
Defective transducing phages carrying aroG, the structural gene for phenylalanine (phe)-inhibitable phospho-2-keto-heptonate aldolase (EC 4.1.2.15; previously known as 3-deoxy-D-arabinoheptulosonate-7-phosphate synthetase[phe]), have been isolated, and DNA from two of these phages has been used to construct a restriction map of the region from att lambda to aroG. A 7.6-kb PstI-HindIII fragment from one of these phages was cloned into pBR322 and shown to contain aroG. The location of aroG within the 7.6 kb was established by subcloning and Tn3 transpositional mutagenesis. A fragment carrying the aroG promoter and operator has been cloned into a high copy number promoter-cloning vector (pMC489), and the resulting aroGpo-LacZ' (alpha) fusion subcloned in a low copy number vector. Strains with this fusion on the low copy number vector exhibit negative regulation of beta-galactosidase expression by both phenylalanine and tryptophan and positive regulation by tyrosine in a tyrR+ background. 相似文献
3.
It has been proposed that amplification of genes for esterase that provide resistance to insecticides may originate from
transposition events. To test this hypothesis, we have constructed a minigene coding for a soluble acetylcholinesterase under
the control of a nontissue-specific promoter (hsp70). When introduced into Drosophila, the gene is expressed in all tissues and the extra acetylcholinesterase produced confers a low level of insecticide resistance
(twofold). The minigene was mobilized by crossing the initial transformant with a strain providing a source of P-element transposase.
After 34 generations of exposure to the organophosphate parathion, we obtained a strain with a higher resistance (fivefold).
This strain had only one extra Ace gene, which overexpressed acetylcholinesterase. Thus, following transposition, resistance resulted from the overexpression
of a single copy of the gene and not from gene amplification.
Received: 9 August 1996 / Accepted: 27 May 1997 相似文献
4.
David R. Nelson 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2013,368(1612)
The world we live in is a biosphere influenced by all organisms who inhabit it. It is also an ecology of genes, with some having rather startling effects. The premise put forth in this issue is cytochrome P450 is a significant player in the world around us. Life and the Earth itself would be visibly different and diminished without cytochrome P450s. The contributions to this issue range from evolution on the billion year scale to the colour of roses, from Darwin to Rachel Carson; all as seen through the lens of cytochrome P450. 相似文献
5.
In this contribution we investigate the impact of the forcing waveform on the productivity of a continuous bioreactor governed by an unstructured, nonlinear kinetic model. The (periodic) forcing is applied on the substrate concentration in the feed. To this end, some alternative waveforms commonly encountered in practice are evaluated and their performance is compared. An analytical/numerical approach is used. The preliminary analytical step is based on the π‐criterion that gives useful information for small amplitudes. The extension to larger amplitudes, when significant improvements are expected, is then performed through a continuation‐optimization procedure. It is found that the choice of the specific waveform has an impact on the performance of the process and there is no unique best forcing for any process condition, but its choice depends on the operating parameters and the forcing amplitude and frequency values. Further, the influence of the waveform functions on the wash‐out conditions are extensively examined. The analysis shows that all the waveforms examined in this work may lead to significant enlargement of the nontrivial regime with respect to a steady state operation. In particular, square‐wave forcing leads in practice to the extinction of the wash‐out conditions for any feed substrate concentration and for a well defined choice of the forcing parameters. © 2010 American Institute of Chemical Engineers Biotechnol. Prog., 2010 相似文献
6.
Abstract Permeabilized cells of Haemophilus influenzae incorporate wall precursors into murein material in an ampicillin-sensitive reaction. In resistant transformants that contain the low antibiotic affinity penicillin-binding proteins (PBPs) 4 and 5, the sensitivity of this incorporation reaction to ampicillin is proportionally lower, suggesting a catalytic role for these proteins in wall synthesis. We conclude that, analogous to the reaction in Escherichia coli , PBPs 4 and 5 of H. influenzae have transpeptidase activity. 相似文献
7.
EMMA DESPLAND MARIA GUNDERSEN SIMON P. DAOUST BRIAN J. MADER NATHALIE DELVAS PAUL J. ALBERT ERIC BAUCE 《Physiological Entomology》2011,36(1):39-46
The pattern of feeding of Eastern spruce budworm Choristoneura fumiferana (Clem.) (Lepidoptera, Tortricidae) is compared on foliage from white spruce Picea glauca (Moench) Voss. (Pinaceae) trees previously determined to be susceptible and resistant to defoliation by budworm. No differences are observed in electrophysiological responses from taste sensilla to aqueous extracts of the two foliage types, nor is there a preference for either extract type in a choice test. Acetone extracts from the two foliage types are both preferred to a control sucrose solution, although neither elicits a preference relative to the other. These results suggest that there is no difference in phagostimulatory power of internal leaf contents of the two foliage types. Longer‐term observation of feeding behaviour in a no‐choice situation shows no difference in meal duration, confirming the lack of difference in phagostimulatory power. However, on average, intermeal intervals are twice as long on the resistant foliage, leading to an overall lower food consumption during the assay. This result suggests an anti‐digestive or toxic effect of the resistant foliage that slows behaviour and limits food intake. Previous research has shown that waxes of the resistant foliage deter initiation of feeding by the spruce budworm and that this foliage contains higher levels of tannins and monoterpenes. The data suggest that the resistant foliage contains a post‐ingestive second line of defence against the spruce budworm. 相似文献
8.
9.
Maria João Feio Trefor B. Reynoldson Verónica Ferreira Manuel Augusto S. Graça 《Hydrobiologia》2007,579(1):55-68
We sampled macroinvertebrates at 75 locations in the Mondego river catchment, Central Portugal, and developed a predictive
model for water quality assessment of this basin, based on the Reference Condition Approach. Sampling was done from June to
September 2001. Fifty-five sites were identified as “Reference sites” and 20 sites were used as “Test sites” to test the model.
At each site we also measured 40 habitat variables to characterize water physics and chemistry, habitat type, land use, stream
hydrology and geographic location. Macroinvertebrates were generally identified to species or genus level; a total of 207
taxa were found. By Unweighted Pair Group Method with Arithmetic mean (UPGMA) clustering and analysis of species contribution
to similarities percentage (SIMPER), two groups of reference sites were established. Using Discriminant Analysis (stepwise
forward), four variables correctly predicted 78% of the reference sites to the appropriate group: stream order, pool quality,
substrate quality and current velocity. Test sites’ environmental quality was established from their relative distance to
reference sites, in MDS ordination space, using a series of bands (BEAST methodology). The model performed well at upstream
sites, but at downstream sites it was compromised by the lack of reference sites. As with the English RIVPACS predictive model,
the Mondego model should be continually improved with the addition of new reference sites. The adaptation of the Mondego model
methodology to the Water Framework Directive is possible and would consist mainly of the integration of the WFD typology and
increasing the number of ellipses that define quality bands.
Handling editor: K. Martens 相似文献
10.
Charles L. Nunn Peter H. Thrall Kelly Stewart Alexander H. Harcourt 《Evolutionary ecology》2008,22(4):519-543
Emerging infectious diseases threaten a wide diversity of animals, and important questions remain concerning disease emergence
in socially structured populations. We developed a spatially explicit simulation model to investigate whether—and under what
conditions—disease-related mortality can impact rates of pathogen spread in populations of polygynous groups. Specifically,
we investigated whether pathogen-mediated dispersal (PMD) can occur when females disperse after the resident male dies from
disease, thus carrying infections to new groups. We also examined the effects of incubation period and virulence, host mortality
and rates of background dispersal, and we used the model to investigate the spread of the virus responsible for Ebola hemorrhagic
fever, which currently is devastating African ape populations. Output was analyzed using regression trees, which enable exploration
of hierarchical and non-linear relationships. Analyses revealed that the incidence of disease in single-male (polygynous)
groups was significantly greater for those groups containing an average of more than six females, while the total number of
infected hosts in the population was most sensitive to the number of females per group. Thus, as expected, PMD occurs in polygynous
groups and its effects increase as harem size (the number of females) increases. Simulation output further indicated that
population-level effects of Ebola are likely to differ among multi-male–multi-female chimpanzees and polygynous gorillas,
with larger overall numbers of chimpanzees infected, but more gorilla groups becoming infected due to increased dispersal
when the resident male dies. Collectively, our results highlight the importance of social system on the spread of disease
in wild mammals. 相似文献