全文获取类型
收费全文 | 10601篇 |
免费 | 794篇 |
国内免费 | 2篇 |
出版年
2023年 | 62篇 |
2022年 | 72篇 |
2021年 | 158篇 |
2020年 | 133篇 |
2019年 | 137篇 |
2018年 | 334篇 |
2017年 | 307篇 |
2016年 | 392篇 |
2015年 | 546篇 |
2014年 | 590篇 |
2013年 | 745篇 |
2012年 | 880篇 |
2011年 | 816篇 |
2010年 | 477篇 |
2009年 | 399篇 |
2008年 | 553篇 |
2007年 | 569篇 |
2006年 | 487篇 |
2005年 | 487篇 |
2004年 | 434篇 |
2003年 | 411篇 |
2002年 | 398篇 |
2001年 | 332篇 |
2000年 | 310篇 |
1999年 | 254篇 |
1998年 | 94篇 |
1997年 | 60篇 |
1996年 | 58篇 |
1995年 | 42篇 |
1994年 | 44篇 |
1993年 | 43篇 |
1992年 | 81篇 |
1991年 | 82篇 |
1990年 | 71篇 |
1989年 | 56篇 |
1988年 | 56篇 |
1987年 | 42篇 |
1986年 | 36篇 |
1985年 | 41篇 |
1984年 | 42篇 |
1983年 | 28篇 |
1982年 | 19篇 |
1981年 | 15篇 |
1980年 | 20篇 |
1979年 | 22篇 |
1978年 | 14篇 |
1977年 | 16篇 |
1975年 | 14篇 |
1974年 | 13篇 |
1968年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Fernández-Muñoz B Yurrita MM Martín-Villar E Carrasco-Ramírez P Megías D Renart J Quintanilla M 《The international journal of biochemistry & cell biology》2011,43(6):886-896
Podoplanin is a transmembrane glycoprotein that is upregulated in cancer and was reported to induce an epithelial-mesenchymal transition (EMT) in MDCK cells. The promotion of EMT was dependent on podoplanin binding to ERM (ezrin, radixin, moesin) proteins through its cytoplasmic (CT) domain, which led to RhoA-associated kinase (ROCK)-dependent ERM phosphorylation. Using detergent-resistant membrane (DRM) assays, as well as transmembrane (TM) interactions and ganglioside GM1 binding, we present evidence supporting the localization of podoplanin in raft platforms important for cell signalling. Podoplanin mutant constructs harbouring a heterologous TM region or lacking the CT tail were unable to associate with DRMs, stimulate ERM phosphorylation and promote EMT or cell migration. Similar effects were observed upon disruption of a GXXXG motif within the TM domain, which is involved in podoplanin self-assembly. In contrast, deletion of the extracellular (EC) domain did not affect podoplanin DRM association. Together, these data suggest that both the CT and TM domains are required for podoplanin localization in raft platforms, and that this association appears to be necessary for podoplanin-mediated EMT and cell migration. 相似文献
992.
Phenotypic plasticity in life history traits favors the establishment of invaders and may magnify their ecological impacts.
Pomacea canaliculata, the only freshwater snail listed among the 100 worst invaders worldwide, is able to complete its life cycle within a wide
range of conditions, a capacity attributed to its life history plasticity. Using snails from their native range in Argentina
we investigated the changes in fecundity, egg mass traits, offspring quality, and efficiency of food conversion into eggs
in response to different levels of food availability throughout different life stages. Pre-maturity mortality was not affected
by chronic reductions of up to 80% in food availability. Females fed ad libitum demonstrated no significant reproductive output
differences when mated with males raised at different food availability levels. For females, the number and total weight of
eggs and the size of egg masses decreased at high levels of food deprivation. Their efficiency of conversion into eggs of
the food ingested during the reproductive period increased with deprivation, as did the survival time of their offspring.
In contrast, the egg mass laying rate and the individual egg weight did not differ under different food availability regimes.
Reductions in food availability have been suggested as a control method but our results indicate that fecundity would be lessened
only at deprivation levels higher than 50% and would be partially compensated by an increase in hatchling survival. 相似文献
993.
Arceo-Gómez G Martínez ML Parra-Tabla V García-Franco JG 《Plant biology (Stuttgart, Germany)》2011,13(Z1):19-24
Enantiostyly (mirror-image flowers) is usually associated with buzz pollination. In buzz-pollinated flowers, pollen is released through terminal pores after bees vibrate the stamens. Several studies have evaluated the function of 'buzzing' in pollen release, but less attention has been paid to the effect of buzzing on pollen capture and deposition on stigmas. Evaluating the mechanism of pollen dispersion in buzz-pollinated flowers is important because it may affect mating patterns and reproductive success. In this study, we analysed the morphology of sexual organs (anther and stigma) using electron microscopy, and determined the relationship between sexual organ structure and pollen capture function through experimental manipulations of buzz-pollinated flowers of Chamaecrista chamaecristoides, as well as vibration frequencies on floral visitors. Pollen release occurs through two terminal pores at the tip of the stamens. However, unlike most angiosperms that have their stigmatic surface exposed, C.?chamaecristoides presents a stigmatic surface inside a cavity covered by trichomes. Experimental manipulations showed that effective fertilisation is only achieved when the style is vibrated, suggesting that buzzing is not only important for pollen release but also for pollen capture and deposition on the stigma. This result, in addition to vibration frequency analysis, suggests that although all floral visitors buzz flowers only those that buzz at higher frequencies achieve effective fertilisation. The anatomical features of sexual organs in flowers of C.?chamaecristoides demonstrate that this species possesses a highly specialised, elaborate morphology, with both genders selected for traits that promote buzz pollination. 相似文献
994.
The species Balantidium coli is the only ciliate that parasitizes humans. It has been described in other primates, and it has been proposed that the species B. suis from pigs and B. struthionis from ostriches are synonyms of B. coli. Previous genetic analysis of pig and ostrich Balantidium isolates found a genetic polymorphism in the ITS region but its taxonomic relevance was not established. We have extended the genetic analysis to Balantidium isolates of pig, gorilla, human and ostrich origin. We have PCR-amplified and sequenced the ITS region of individual Balantidium cells. The predicted ITS secondary structures of the sequences obtained were transferred by homology modelling to the sequences of other Trichostomatia ciliates (Isotricha, Troglodytella, Lacrymaria and Spathidium) and compared to determine the importance of the differences in the primary sequences. The results show that the ITS2 secondary structure of the species considered follows the general pattern of other ciliates, although with some deviations. There are at least two main types of ITS sequence variants in B. coli which could be present in the same cell and they are common to the mammal and avian hosts studied. These data do not support B. suis and B. struthionis as distinct species. 相似文献
995.
Obesity is a pandemic disorder that can be defined as a chronic excess of adipose tissue that increases the risk of suffering chronic diseases such as, diabetes, arterial hypertension, stroke and some forms of cancer. We now know that adipose tissue, aside from being an energy store, is also an important endocrine and metabolic organ. Recently, new mechanisms that control obesity have been identified, such as the equilibrium between white and brown adipose tissue, the localization of adipose mass (visceral or ventral), and the presence of adipose and mesenchymal stem cells. In this review, we describe the implication of these stem cell types in the normal physiology and dysfunction of adipose tissue. These stem cells provide a potential target for modulating the response of the body to obesity and diabetes, as well as a potential tool for regenerative medicine. 相似文献
996.
C. Lopez y Royo G. Pergent T. Alcoverro M.C. Buia G. Casazza B. Martínez-Crego M. Pérez F. Silvestre J. Romero 《Ecological Indicators》2011,11(2):557-563
The pervasive use of ecological indices is increasingly requiring actions of harmonisation. Specifically, within the EU Water Framework Directive, an important effort in methods intercalibration is being done. However, a significant limitation in comparability assessment arises from the datasets used, which have different geographic origins. The purpose of our study was to perform an experimental intercalibration, where data were collected specifically on a set of common sites and following all the requirements of the methods being assessed. Three indices based on the marine angiosperm Posidonia oceanica, the POMI, the BiPo and the PoSte, were applied to sites in three different geographical areas of the western Mediterranean: Catalonia, Corsica and Southern Italy (Ischia), distant between hundreds and more than thousands of kilometers. Two indices, POMI and BiPo, showed not only a very good relationship with human pressures (measured on a common scale for all sites) but also a high comparability, in all aspects investigated. The differences found for the third one (PoSte) are hypothesised as being due to a different rationale used to define reference conditions, the different metrics used in the index, and in particular to a different definition of ecological status in relation to the time scale of the response to anthropogenic pressures. Our study demonstrates that indices with very different approaches can provide fully reliable and comparable results. 相似文献
997.
Background
The gene transformer-2, which is involved in sex determination, has been studied in Drosophila, Musca, Ceratitis, Anastrepha and Lucilia. All these members of Diptera belong to the suborder Brachycera. In this work, it is reported the isolation and characterisation of genes transformer-2 of the dipterans Sciara ocellaris and Bradysia coprophila (formerly Sciara coprophila), which belong to the much less extensively analysed Sciaridae Family of the Suborder Nematocera, which is paraphyletic with respect to Suborder Brachycera. 相似文献998.
Sempere F Martínez-Soria V Palau J Penya-Roja JM San-Valero P Gabaldón C 《Bioprocess and biosystems engineering》2011,34(7):859-867
The removal of styrene-polluted air emissions by biotrickling filtration was performed to evaluate the influence of using
nitrate and urea as a nitrogen source in the nutrient solution supplied to two bioreactors run in parallel under the same
operational conditions for 3 months. The use of urea resulted in less biomass content along the packed bed and better performance
of the process, with a maximum elimination capacity (EC) of 57.6 g C m−3 h−1 (removal efficiency (RE) of 88.3% and empty bed residence time (EBRT) of 60 s), which was around 54% higher than when using
nitrate. EBRTs of 60, 30 and 15 s were evaluated with a urea-based nutrient supply. By decreasing the EBRT from 60 to 30 s
the styrene concentration that could be treated with REs above 80% was almost the half, from 1,100 to 600 mg C m−3, resulting in ECs of 52.8 g C m−3 h−1. Working at 15 s was not possible to obtain REs higher than 40% with a maximum EC of 28.5 g C m−3 h−1. 相似文献
999.
Christian Eichholz Martín Silvestre Matthias Franzreb Hermann Nirschl 《Engineering in Life Science》2011,11(1):75-83
A new concept for the improvement of the downstream processing and purification is the so‐called magnetic separation. By using surface functionalized magnetic substrate particles, which selectively adsorb the target product, it can be directly separated out of a crude bioprocess stream. These methods are already used for analytical purposes, where only small amounts of functionalized particles are necessary. To apply the same concept at a larger scale, effective and economical procedures have to be provided. First, suitable process equipment has to be developed. Second, the magnetic particles have to be manufactured with a stable surface functionalization and long‐term stability for their reuse. Up to now mainly high‐gradient magnetic separation filter devices are applied for selective magnetic separation. They consist of a magnetic matrix in which the magnetic particles are trapped. In this work, a new magnetic filter is introduced that overcomes the capacity limitations of the current high‐gradient magnetic separation technology. The principle is demonstrated by selective recovery of lysozyme from hen egg white. Prior to the separation experiments magnetic beads with a strong acid cation‐exchange surface functionalization are synthesized. The separation procedure is implemented in only one unit operation. With the implementation of the displacement elution sequence lysozyme can be separated out of a hen egg white solution with a purification factor of PF=36 and a purity P=0.83. 相似文献
1000.