全文获取类型
收费全文 | 10887篇 |
免费 | 854篇 |
国内免费 | 6篇 |
专业分类
11747篇 |
出版年
2023年 | 72篇 |
2022年 | 186篇 |
2021年 | 281篇 |
2020年 | 155篇 |
2019年 | 282篇 |
2018年 | 310篇 |
2017年 | 276篇 |
2016年 | 399篇 |
2015年 | 623篇 |
2014年 | 635篇 |
2013年 | 823篇 |
2012年 | 958篇 |
2011年 | 902篇 |
2010年 | 557篇 |
2009年 | 539篇 |
2008年 | 649篇 |
2007年 | 660篇 |
2006年 | 558篇 |
2005年 | 501篇 |
2004年 | 444篇 |
2003年 | 400篇 |
2002年 | 368篇 |
2001年 | 94篇 |
2000年 | 60篇 |
1999年 | 84篇 |
1998年 | 93篇 |
1997年 | 67篇 |
1996年 | 61篇 |
1995年 | 67篇 |
1994年 | 62篇 |
1993年 | 42篇 |
1992年 | 55篇 |
1991年 | 44篇 |
1990年 | 29篇 |
1989年 | 43篇 |
1988年 | 25篇 |
1987年 | 29篇 |
1986年 | 27篇 |
1985年 | 31篇 |
1984年 | 19篇 |
1983年 | 27篇 |
1982年 | 14篇 |
1981年 | 12篇 |
1980年 | 18篇 |
1979年 | 23篇 |
1978年 | 15篇 |
1977年 | 15篇 |
1974年 | 15篇 |
1972年 | 12篇 |
1968年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
De Felice B Argenziano C Guida M Trifuoggi M Russo F Condorelli V Inglese M 《Molecular biotechnology》2009,41(2):123-132
Little is known about pharmaceutical and personal care products pollutants (PPCPs), but there is a growing interest in how they might impact the environment and microbial communities. The widespread use of Viagra (sildenafil citrate) has attracted great attention because of the high usage rate, the unpredictable disposal and the unknown potential effects on wildlife and the environment. Until now information regarding the impact of Viagra on microbial community in water environment has not been reported. In this research, for the first time, the genetic profile of the microbial community, developing in a Viagra polluted water environment, was evaluated by means of the 16S and 18S rRNA genes, for bacteria and fungi, respectively, amplified by polymerase chain reaction (PCR) and separated using the denaturing gradient gel electrophoresis (DGGE) technique. The DGGE results revealed a complex microbial community structure with most of the population persisting throughout the experimental period. DNA sequences from bands observed in the different denaturing gradient gel electrophoresis profiles exhibited the highest degree of identity to uncultured bacteria and fungi found previously mainly in polluted environmental and treating bioreactors. Biotransformation ability of sildenafil citrate by the microbial pool was studied and the capability of these microorganisms to detoxify a polluted water ecosystem was assessed. The bacterial and fungal population was able to degrade sildenafil citrate entirely. Additionally, assays conducted on Daphnia magna, algal growth inhibition assay and cell viability determination on HepG2 human cells showed that biotransformation products obtained from the bacterial growth was not toxic. The higher removal efficiency for sildenafil citrate and the lack of toxicity by the biotransformation products obtained showed that the microbial community identified here represented a composite population that might have biotechnological relevance to retrieve sildenafil citrate contaminated sites. 相似文献
92.
93.
Mathias M. Pires Mauro Galetti Camila I. Donatti Marco A. Pizo Rodolfo Dirzo Paulo R. Guimarães Jr. 《Oecologia》2014,175(4):1247-1256
The late Quaternary megafaunal extinction impacted ecological communities worldwide, and affected key ecological processes such as seed dispersal. The traits of several species of large-seeded plants are thought to have evolved in response to interactions with extinct megafauna, but how these extinctions affected the organization of interactions in seed-dispersal systems is poorly understood. Here, we combined ecological and paleontological data and network analyses to investigate how the structure of a species-rich seed-dispersal network could have changed from the Pleistocene to the present and examine the possible consequences of such changes. Our results indicate that the seed-dispersal network was organized into modules across the different time periods but has been reconfigured in different ways over time. The episode of megafaunal extinction and the arrival of humans changed how seed dispersers were distributed among network modules. However, the recent introduction of livestock into the seed-dispersal system partially restored the original network organization by strengthening the modular configuration. Moreover, after megafaunal extinctions, introduced species and some smaller native mammals became key components for the structure of the seed-dispersal network. We hypothesize that such changes in network structure affected both animal and plant assemblages, potentially contributing to the shaping of modern ecological communities. The ongoing extinction of key large vertebrates will lead to a variety of context-dependent rearranged ecological networks, most certainly affecting ecological and evolutionary processes. 相似文献
94.
We examined vertical migration and colonisation patterns of stream macroinvertebrates within the substratum of an Apennine
creek in NW Italy. Macrobenthos was sampled at three depths in the streambed (0–5, 5–10, 10–15 cm) by means of artificial
baskets filled with natural substratum. We placed 42 traps (5×5×15 cm), i.e. 21 top-opened (T-traps) and 21 bottom-opened
(B-traps), each composed of three overlapping baskets (high-H, medium-M and low-L), to evaluate differences in the vertical
movements. We also collected Surber samples to compare interstitial assemblages with streambed communities. The multilevel
traps yielded 42 taxa, compared with 60 taxa in the natural riverbed. Interstitial traps were rapidly colonised; both taxa
richness and organism number increased during the 42-day study period. We found active migration in both vertical directions,
but there were more invertebrates in the top-opened traps than in the bottom-opened traps. In the T-traps the most colonised
baskets were those placed at the H level, while in the B-traps the L level baskets were more rapidly colonised. The interstitial
assemblages differed markedly from the streambed communities in both composition and functional organisation, with more collector-gatherers
and predators in the interstitial zone and more filterers and scrapers in the natural riverbed. In Apennine lotic systems,
the interstitial zone is an important habitat for stream macrobenthos, although it may not be used by all species. 相似文献
95.
Wood anatomy and carbon‐isotope discrimination support long‐term hydraulic deterioration as a major cause of drought‐induced dieback 下载免费PDF全文
Elena Pellizzari J. Julio Camarero Antonio Gazol Gabriel Sangüesa‐Barreda Marco Carrer 《Global Change Biology》2016,22(6):2125-2137
Hydraulic impairment due to xylem embolism and carbon starvation are the two proposed mechanisms explaining drought‐induced forest dieback and tree death. Here, we evaluate the relative role played by these two mechanisms in the long‐term by quantifying wood‐anatomical traits (tracheid size and area of parenchyma rays) and estimating the intrinsic water‐use efficiency (iWUE) from carbon isotopic discrimination. We selected silver fir and Scots pine stands in NE Spain with ongoing dieback processes and compared trees showing contrasting vigour (declining vs nondeclining trees). In both species earlywood tracheids in declining trees showed smaller lumen area with thicker cell wall, inducing a lower theoretical hydraulic conductivity. Parenchyma ray area was similar between the two vigour classes. Wet spring and summer conditions promoted the formation of larger lumen areas, particularly in the case of nondeclining trees. Declining silver firs presented a lower iWUE than conspecific nondeclining trees, but the reverse pattern was observed in Scots pine. The described patterns in wood anatomical traits and iWUE are coherent with a long‐lasting deterioration of the hydraulic system in declining trees prior to their dieback. Retrospective quantifications of lumen area permit to forecast dieback in declining trees 2–5 decades before growth decline started. Wood anatomical traits provide a robust tool to reconstruct the long‐term capacity of trees to withstand drought‐induced dieback. 相似文献
96.
Bioprocess intensification: a potential aqueous two-phase process for the primary recovery of B-phycoerythrin from Porphyridium cruentum 总被引:1,自引:0,他引:1
Benavides J Rito-Palomares M 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2004,807(1):33-38
A process for the primary recovery of B-phycoerythrin from Porphyridium cruentum exploiting aqueous two-phase systems (ATPS) was developed in order to reduce the number of unit operations and benefit from an increased yield of the protein product. The evaluation of system parameters such as poly(ethylene glycol) (PEG) molecular mass, concentration of PEG as well as salt, system pH and volume ratio was carried out to determine under which conditions the B-phycoerythrin and contaminants concentrate to opposite phases. PEG 1450-phosphate ATPS proved to be suitable for the recovery of B-phycoerythrin because the target protein concentrated to the top phase whilst the protein contaminants and cell debris concentrated in the bottom phase. An extraction ATPS stage comprising volume ratio (Vr) equal to 1.0, PEG 1450 24.9% (w/w), phosphate 12.6% (w/w) and system pH of 8.0 allowed B-phycoerythrin recovery with a purity of 2.9 (estimated as the relation of the 545-280 nm absorbances). The use of ATPS resulted in a primary recovery process that produced a protein purity of 2.9 +/- 0.2 and an overall product yield of 77.0% (w/w). The results reported demonstrated the practical implementation of ATPS for the design of a primary recovery process as a first step for the commercial purification of B-phycoerythrin produced by P. cruentum. 相似文献
97.
98.
Marco Mura Claudia C dos Santos Duncan Stewart Mingyao Liu 《Journal of applied physiology》2004,97(5):1605-1617
VEGFs and their receptors have been implicated in the regulation of vascular permeability in many organ systems, including the lung. Increased permeability and interstitial and pulmonary edema are prominent features of acute lung injury (ALI)/acute respiratory distress syndrome (ARDS). Extrapolating data from other organ systems and animal experiments have suggested that overexpression of VEGF functions primarily as proinjurious molecules in the lung. Recent data, from animal models as well as from patients with ARDS, have shown decreased levels of VEGF in the lung. The role of VEGF and related molecules in ALI/ARDS is, therefore, controversial: what has become clear is that there are many unique features in the regulation of pulmonary vascular permeability and in VEGF expression in the lung. In this review, we explore a growing body of literature looking at the expression and function of VEGF and related molecules in different models of ALI and in patients with ALI/ARDS. Novel evidence points to a potential role of VEGF in promoting repair of the alveolar-capillary membrane during recovery from ALI/ARDS. Understanding the role of VEGF in this disease process is crucial for developing new therapeutic strategies for ALI/ARDS. 相似文献
99.
Human Cytomegalovirus UL131-128 Genes Are Indispensable for Virus Growth in Endothelial Cells and Virus Transfer to Leukocytes 下载免费PDF全文
100.
Cobucci-Ponzano B Zorzetti C Strazzulli A Carillo S Bedini E Corsaro MM Comfort DA Kelly RM Rossi M Moracci M 《Glycobiology》2011,21(4):448-456
The large-scale production of oligosaccharides is a daunting task, hampering the study of the role of glycans in vivo and the testing of the efficacy of novel glycan-based drugs. Glycosynthases, mutated glycosidases that synthesize oligosaccharides in high yields, are becoming important chemo-enzymatic tools for the production of oligosaccharides. However, while β-glycosynthase can be produced with a rather well-established technology, examples of α-glycosynthases are thus far limited only to enzymes from glycoside hydrolase 29 (GH29), GH31 and GH95 families. α-L-Fucosynthases from GH29 use convenient glycosyl azide derivatives as a strategic alternative to glycosyl fluoride donors. However, the general applicability of this method to other α-glycosynthases is not trivial and remains to be confirmed. Here, β-D-galactopyranosyl azide was converted to α-galacto-oligosaccharides with good yields and high regioselectivity, catalyzed by a novel α-galactosynthase based on the GH36 α-galactosidase from the hyperthermophilic bacterium Thermotoga maritima. These results open a new avenue to the practical synthesis of biologically interesting α-galacto-oligosaccharides and demonstrate more widespread use of β-glycosyl-azide as donors, confirming their utility to expand the repertoire of glycosynthases. 相似文献