全文获取类型
收费全文 | 8947篇 |
免费 | 588篇 |
国内免费 | 3篇 |
出版年
2023年 | 77篇 |
2022年 | 124篇 |
2021年 | 220篇 |
2020年 | 168篇 |
2019年 | 186篇 |
2018年 | 243篇 |
2017年 | 228篇 |
2016年 | 374篇 |
2015年 | 475篇 |
2014年 | 519篇 |
2013年 | 609篇 |
2012年 | 801篇 |
2011年 | 693篇 |
2010年 | 483篇 |
2009年 | 417篇 |
2008年 | 545篇 |
2007年 | 534篇 |
2006年 | 454篇 |
2005年 | 376篇 |
2004年 | 354篇 |
2003年 | 322篇 |
2002年 | 268篇 |
2001年 | 86篇 |
2000年 | 73篇 |
1999年 | 69篇 |
1998年 | 76篇 |
1997年 | 53篇 |
1996年 | 62篇 |
1995年 | 68篇 |
1994年 | 49篇 |
1993年 | 39篇 |
1992年 | 40篇 |
1991年 | 32篇 |
1990年 | 19篇 |
1989年 | 24篇 |
1988年 | 29篇 |
1987年 | 18篇 |
1986年 | 21篇 |
1985年 | 24篇 |
1984年 | 32篇 |
1983年 | 18篇 |
1982年 | 17篇 |
1981年 | 31篇 |
1980年 | 20篇 |
1979年 | 22篇 |
1978年 | 16篇 |
1976年 | 15篇 |
1974年 | 18篇 |
1973年 | 18篇 |
1968年 | 12篇 |
排序方式: 共有9538条查询结果,搜索用时 15 毫秒
61.
Abstract: Different neurotransmitter receptor agonists [carbachol, serotonin, noradrenaline, histamine, endothelin-1, and trans -(1 S ,3 R )-aminocyclopentyl-1,3-dicarboxylic acid ( trans -ACPD)], known as stimuli of phospholipase C in brain tissue, were tested for phospholipase D stimulation in [32 P]Pi -prelabeled rat brain cortical and hippocampal slices. The accumulation of [32 P]phosphatidylethanol was measured as an index of phospholipase D-catalyzed transphosphatidylation in the presence of ethanol. Among the six neurotransmitter receptor agonists tested, only noradrenaline, histamine, endothelin-1, and trans -ACPD stimulated phospholipase D in hippocampus and cortex, an effect that was strictly dependent of the presence of millimolar extracellular calcium concentrations. The effect of histamine (EC50 18 µ M ) was inhibited by the H1 receptor antagonist mepyramine with a K i constant of 0.7 n M and was resistant to H2 and H3 receptor antagonists (ranitidine and tioperamide, respectively). Endothelin-1-stimulated phospholipase D (EC50 44 n M ) was not blocked by BQ-123, a specific antagonist of the ETA receptor. Endothelin-3 and the specific ETB receptor agonist safarotoxin 6c were also able to stimulate phospholipase D with efficacies similar to that of endothelin-1, and EC50 values of 16 and 3 n M , respectively. These results show that histamine and endothelin-1 stimulate phospholipase D in rat brain through H1 and ETB receptors, respectively. 相似文献
62.
Spatio-temporal distribution of the microphytobenthic biomass in intertidal flats of Tagus Estuary (Portugal) 总被引:3,自引:1,他引:2
Brotas Vanda Cabrita Teresa Portugal Ana Serôdio João Catarino Fernando 《Hydrobiologia》1995,(1):93-104
A study on spatio-temporal distribution of microphytobhethos in intertidal zones of Tagus Estuary was carried out from 1990 to 1992. Near Lisbon, Portugal, Tagus Estuary is a shallow mesotidal estuary, covering an area of 320 km2. The intertidal area ranges from 20 to 40% off the total area and it is constituted mainly by mudflats. Intertidal flats are richly populated by microalgae, diatoms being the most important and ubiquitos group.Spatial variation of microphytobethos was studied in spring 1990, 21 different sites were sampled. Microphytobenthos biomass was evaluated as chlorophyll a content of the surface centimeter, ranging from 10 to 240 mg m–2. A Principal Component Analysis showed that 62% of the total variability found in intertidal flats of Tagus estuary could be attributed to two major factors: sediment type and tidal height. A hierarchical grouping defined 3 major groups of similar stations, each one representing a different strata of the ecosystem.One station from each group was chosen for the study of the temporal variation. A sampling, rogram took place from April 1991 to April 1992, with fortnightly sampling, the Chl a ranged from 20–300 mg m–2. No clear seasonal variation was found, and our results indicated that tidal height of sampledsite played an essential role in temporal biomass evolution, thus upper littoral sites were influenced by climatic parameters, whereas in lower sites action of tides mainly controlled microphytic biomass.
Résumé Une étude sur l'hétérogénéité spatio-temporelle du microphytobenthos dans les sédiments intertidaux de l'Estuaire du Tage a été accompli de 1990 á 1992.L'Estuaire du Tage, prés de Lisbonne (Portugal) est un estuaire peu profond, mesotidal, avec une aire total de 320 km2. L'aire intertidale est comprise entre 20 et 40% du total, et constituéé surtout par des vasiéres. Ces slikkes sont peuplées par une communauté assez riche de microalgues, ou les diatomées sont les plus abundantes.La variation spatialle du microphytobenthos était évalué au Printemps 1990, ou 21 différentes stations étaient échantillonnées. La biomasse était évalué par la concentration enchlorophylle a du premier centimétre de sédiment, qui a varié de 10 á 240 mg Chl a m–2. Une Analyse en Composants Principales a montré que 62% de la variabilité de la biomasse était lié á deux facteurs: le sédiment et l'hauteur vis-á-vis la marée. Une classification hiérarchique des stations par similitude a établi 3 groupes principaux, représantantles différents strates de écecosytéme.Une station de chaque groupement a été choisie pour l'étude de la variation temporelle, qui s'est deroulé d'avril 1991 á avril 1992, avec des prélévements deux fois par mois. Les valeurs de Chl a obtenus vont de 20 á 300 mg m–2. Les variations saisonniéres observées ne sont pas claires: nos résultats indiquent que l'hauteur de la station (m) joue un rôle essentiel dans l'évolution temporel de la biomasse, c'est á dire, la biomasse microalgal des sites du supra-littoral est influencié par les paramétres climatiques, tandis que dans l'infra-littoral c'est l'action des marées le facteur principal.相似文献
63.
Linkage Analysis with an Alternative Formulation for the Mixed Model of Inheritance: The Finite Polygenic Mixed Model 总被引:1,自引:1,他引:0 下载免费PDF全文
This paper presents an extension of the finite polygenic mixed model of FERNANDO et al. (1994) to linkage analysis. The finite polygenic mixed model, extended for linkage analysis, leads to a likelihood that can be calculated using efficient algorithms developed for oligogenic models. For comparison, linkage analysis of 5 simulated 4021-member pedigrees was performed using the usual mixed model of inheritance, approximated by HASSTEDT (1982), and the finite polygenic mixed model extended for linkage analysis presented here. Maximum likelihood estimates of the finite polygenic mixed model could be inferred to be closer to the simulated values in these pedigrees. 相似文献
64.
Xavier Isaac M. de Pádua Antônio Moraes Fernando De Miranda-Neto J. A. 《Molecular Engineering》1997,7(3-4):283-291
Starburst dendrimers are highly branched oligomers. A rigid dendritic hydrocarbon, C1134H1146, has recently been synthesized. It consists of 94 phenylacetylene units displayed in a self-similar two-dimensional skeleton
isomorphous to the three-coordinated Bethe lattice. The three-dimensional representation of phenylacetylene dendrimer shows
a globular architecture with large voids and niches in its interior, characteristic of hyperbolic surfaces. This work investigates
the geometrical scaling behavior of this starburst dendrimer using the symmetry properties of a Bethe lattice embedded in
the hyperbolic plane. The results for C1134H1146 provide its density profile and an upper bound for its macromolecular size.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
65.
66.
Contribution of Mutation and RNA Recombination to the Evolution of a Plant Pathogenic RNA 总被引:10,自引:0,他引:10
Miguel A. Aranda Aurora Fraile Joaquín Dopazo José M. Malpica Fernando García-Arenal 《Journal of molecular evolution》1997,44(1):81-88
The nucleotide sequence of 17 variants of the satellite RNA of cucumber mosaic virus (CMV-satRNA) isolated from field-infected
tomato plants in the springs of 1989, 1990, and 1991 was determined. The sequence of each of the 17 satRNAs was unique and
was between 334 and 340 nucleotides in length; 57 positions were polymorphic. There was much genetic divergence, ranging from
0.006 to 0.141 nucleotide substitutions per site for pairwise comparisons, and averaging 0.074 for any pair. When the polymorphic
positions were analyzed relative to a secondary structure model proposed for CMV-satRNAs, it was found that there were significantly
different numbers of changes in base-paired and non–base-paired positions, and that mutations that did not disrupt base pairing
were preferred at the putatively paired sites. This supports the concept that the need to maintain a functional structure
may limit genetic divergence of CMV-satRNA. Phylogenetic analyses showed that the 17 CMV-satRNA variants clustered into two
subgroups, I and II, and evolutionary lines proceeding by the sequential accumulation of mutations were apparent. Three satRNA
variants were outliers for these two phylogenetic groups. They were shown to be recombinants of subgroup I and II satRNAs
by calculating phylogenies for different molecular regions and by using Sawyer's test for gene conversion. At least two recombination
events were required to produce these three recombinant satRNAs. Thus, recombinants were found to be frequent (∼17%) in natural
populations of CMV-satRNA, and recombination may make an important contribution to the generation of new variants. To our
knowledge this is the first report of data allowing the frequency of recombinant isolates in natural populations of an RNA
replicon to be estimated.
Received: 14 May 1996 / Accepted: 17 July 1996 相似文献
67.
Twenty-five accessions belonging to the genus Cytisus and allies (Genisteae: Leguminosae), potentially useful for agrosylvopastoral systems in the Mediterranean basin, were analyzed using isoenzymes. The aim was to assess the taxonomic usefulness of these characters at the specific and generic levels, and its value as an aid for the taxonomy. A high level of infrageneric and infraspecific variability was detected. Numerical analysis showed that isoenzymes are useful for the characterization both at the specific and generic levels. The obtained dendrogram supports the generic arrangement proposed by Bisby [(1981) Advances in Legume Systematics, Vol. 1, pp. 409–425], therefore helping to elucidate the difficult taxonomy of the studied species. 相似文献
68.
Plasmid rolling circle replication and its control 总被引:10,自引:0,他引:10
Manuel Espinosa Gloria del Solar Fernando Rojo Juan C. Alonso 《FEMS microbiology letters》1995,130(2-3):111-120
Abstract This review summarises current information on rolling circle replicating plasmids originally isolated from Gram-positive bacteria with a low guanine and cytosine content in their DNA. It focuses on the peculiar biological features of these small, high copy number plasmids that replicate via an asymmetric RC mechanism. The regulation of plasmid copy number is also discussed. 相似文献
69.
T. Macedo C. A. Fontes Ribeiro D. Cotrim P. Tavares M. T. Morgadinho M. Caramona M. T. Nunes Vicente L. Rodrigues M. G. Cardoso M. L. Keating 《Molecular neurobiology》1995,11(1-3):21-29
This work evaluated in a population of heroin and heroin plus cocaine human addicts:
- Norepinephrine (NE), epinephrine (Epi), and 3-methoxy-4-hydroxyphenylglycol (MHPG) (the principal metabolite of brain NE) plasma levels;
- Monoamine oxidase (MAO) activity; and
- 3H-imipramine specific binding to the amine carrier in platelets.
70.
To study the effects of prenatal cocaine-exposure on the developing retinal ganglion cell layer of the rat, female Wistar rats were administered subcutaneously (sc) cocaine hydrochloride (60 mg/kg body wt/d) or saline, or were not manipulated from gestational d 8–22. Male offspring were sacrificed at postnatal day 14 and 30. Radial semithin sections of epon-embedded flat mounts of the retinal quadrants were used to evaluate the following parameters along the centroperipheral axis:
- Thickness of ganglion cell plus nerve fiber layer;
- Nuclear size of ganglion cell layer neurons; and
- Linear density (number per unit length) of ganglion cell layer neurons.