全文获取类型
收费全文 | 6837篇 |
免费 | 533篇 |
国内免费 | 2篇 |
专业分类
7372篇 |
出版年
2023年 | 42篇 |
2022年 | 116篇 |
2021年 | 193篇 |
2020年 | 119篇 |
2019年 | 132篇 |
2018年 | 155篇 |
2017年 | 126篇 |
2016年 | 219篇 |
2015年 | 400篇 |
2014年 | 422篇 |
2013年 | 498篇 |
2012年 | 628篇 |
2011年 | 566篇 |
2010年 | 368篇 |
2009年 | 355篇 |
2008年 | 461篇 |
2007年 | 421篇 |
2006年 | 359篇 |
2005年 | 303篇 |
2004年 | 270篇 |
2003年 | 289篇 |
2002年 | 261篇 |
2001年 | 51篇 |
2000年 | 39篇 |
1999年 | 69篇 |
1998年 | 50篇 |
1997年 | 47篇 |
1996年 | 27篇 |
1995年 | 36篇 |
1994年 | 37篇 |
1993年 | 31篇 |
1992年 | 30篇 |
1991年 | 21篇 |
1990年 | 15篇 |
1989年 | 16篇 |
1988年 | 22篇 |
1987年 | 8篇 |
1986年 | 12篇 |
1985年 | 6篇 |
1984年 | 13篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 7篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1974年 | 9篇 |
1973年 | 6篇 |
1969年 | 8篇 |
排序方式: 共有7372条查询结果,搜索用时 28 毫秒
71.
Modern zoos are required to fulfill a growing number of responsibilities including scientific research, wildlife conservation, public recreation, and education. With so many roles and responsibilities and limited funds, zoos have to prioritize their activities in relation to their own specified goals, objectives, and mission statements. Given their desire to nurture community relations and educational opportunities, it is important to determine the extent to which zoos are prioritizing their activities, according to visitors' expectations. This paper presents empirical research of a two staged mixed-methods investigation into zoo priorities from both zoos' and visitors' perspectives. The first stage involved an online questionnaire to which more than 190 zoos across 52 countries reported their priorities and activities. We then undertook in-depth case studies in nine institutions, enabling direct observation of each site within its context, including face-to-face visitor interviews. Our research contrasts the priority given to zoo activities from each perspective and indicates that educating visitors, including school children, is the highest priority activity from the zoos' perspective and that of their visitors. The findings also show that visitors place high value on learning about actions they can take to help conservation efforts. Some inconsistencies between zoos' and visitors' priorities were also evident, such as the different emphasis placed on viewing endemic species, and zoos being a place for people to relax and socialize. The implications of these findings are discussed. 相似文献
72.
Hu XP Heath C Taylor MP Tuffin M Cowan D 《Extremophiles : life under extreme conditions》2012,16(1):79-86
A novel, cold-active and highly alkaliphilic esterase was isolated from an Antarctic desert soil metagenomic library by functional
screening. The 1,044 bp gene sequence contained several conserved regions common to lipases/esterases, but lacked clear classification
based on sequence analysis alone. Moderate (<40%) amino acid sequence similarity to known esterases was apparent (the closest
neighbour being a hypothetical protein from Chitinophaga pinensis), despite phylogenetic distance to many of the lipolytic “families”. The enzyme functionally demonstrated activity towards
shorter chain p-nitrophenyl esters with the optimal activity recorded towards p-nitrophenyl propionate (C3). The enzyme possessed an apparent Topt at 20°C and a pH optimum at pH 11. Esterases possessing such extreme alkaliphily are rare and so this enzyme represents an
intriguing novel locus in protein sequence space. A metagenomic approach has been shown, in this case, to yield an enzyme
with quite different sequential/structural properties to known lipases. It serves as an excellent candidate for analysis of
the molecular mechanisms responsible for both cold and alkaline activity and novel structure–function relationships of esterase
activity. 相似文献
73.
74.
75.
Daniel P. Demarque Sonia Maria F. Fitts Amanda G. Boaretto Júlio César Leite da Silva Maria C. Vieira Vanessa N. P. Franco Caroline B. Teixeira M?nica C. Toffoli-Kadri Carlos A. Carollo 《PloS one》2015,10(2)
Achyrocline alata, known as Jateí-ka-há, is traditionally used to treat several health problems, including inflammations and infections. This study aimed to optimize an active extract against Streptococcus mutans, the main bacteria that causes caries. The extract was developed using an accelerated solvent extraction and chemometric calculations. Factorial design and response surface methodologies were used to determine the most important variables, such as active compound selectivity. The standardized extraction recovered 99% of the four main compounds, gnaphaliin, helipyrone, obtusifolin and lepidissipyrone, which represent 44% of the extract. The optimized extract of A. alata has a MIC of 62.5 μg/mL against S. mutans and could be used in mouth care products. 相似文献
76.
Rawson PM Molette C Videtta M Altieri L Franceschini D Donato T Finocchi L Propato A Paroli M Meloni F Mastroianni CM d'Ettorre G Sidney J Sette A Barnaba V 《Nature medicine》2007,13(12):1431-1439
We found that the proteome of apoptotic T cells includes prominent fragments of cellular proteins generated by caspases and that a high proportion of distinct T cell epitopes in these fragments is recognized by CD8+ T cells during HIV infection. The frequencies of effector CD8+ T cells that are specific for apoptosis-dependent epitopes correlate with the frequency of circulating apoptotic CD4+ T cells in HIV-1-infected individuals. We propose that these self-reactive effector CD8+ T cells may contribute to the systemic immune activation during chronic HIV infection. The caspase-dependent cleavage of proteins associated with apoptotic cells has a key role in the induction of self-reactive CD8+ T cell responses, as the caspase-cleaved fragments are efficiently targeted to the processing machinery and are cross-presented by dendritic cells. These findings demonstrate a previously undescribed role for caspases in immunopathology. 相似文献
77.
Karam H Valdenaire O Belair MF Prigent-Sassy C Rakotosalama A Clozel M Itskovitz J Bruneval P 《Cell and tissue research》1999,295(1):101-109
The endothelin system is composed of three endothelin isoforms (ET-1, ET-2, and ET-3), the endothelin receptors ETA and ETB, and the endothelin-converting enzyme (ECE). Besides having a major vasoactive role, endothelins have roles in different cell types at a local level. We investigated the presence of the different components of the endothelin system in primate ovaries. Human ovaries and gonadotropin-stimulated monkey ovaries were studied using immunohistochemistry for endothelin, and in situ hybridization with probes for ET-1, ET-2, ET-3, ETA and ETB receptors, and ECE. ET-1 and ETA receptors were detected in endothelial cells and vascular smooth muscle cells, respectively, in stromal vessels adjacent to follicles and corpora lutea. ETB receptors and ET-1 were found in the endothelial cells of capillaries of corpora lutea. ECE was present in internal theca cells of secondary, de Graaf, atretic follicles, and in luteinized granulosa cells of the corpora lutea. The endothelin system components are present in or around the follicles of human and monkey ovaries. Although the components are not expressed in the same cell types, they are synthesized, mainly in follicles, by cells that are in close proximity. Thus, the endothelin system could act in a paracrine manner. ECE expression in steroid-producing cells changes its compartmentalization during follicle maturation. 相似文献
78.
79.
Pfefferle S Schöpf J Kögl M Friedel CC Müller MA Carbajo-Lozoya J Stellberger T von Dall'Armi E Herzog P Kallies S Niemeyer D Ditt V Kuri T Züst R Pumpor K Hilgenfeld R Schwarz F Zimmer R Steffen I Weber F Thiel V Herrler G Thiel HJ Schwegmann-Wessels C Pöhlmann S Haas J Drosten C von Brunn A 《PLoS pathogens》2011,7(10):e1002331
Coronaviruses (CoVs) are important human and animal pathogens that induce fatal respiratory, gastrointestinal and neurological disease. The outbreak of the severe acute respiratory syndrome (SARS) in 2002/2003 has demonstrated human vulnerability to (Coronavirus) CoV epidemics. Neither vaccines nor therapeutics are available against human and animal CoVs. Knowledge of host cell proteins that take part in pivotal virus-host interactions could define broad-spectrum antiviral targets. In this study, we used a systems biology approach employing a genome-wide yeast-two hybrid interaction screen to identify immunopilins (PPIA, PPIB, PPIH, PPIG, FKBP1A, FKBP1B) as interaction partners of the CoV non-structural protein 1 (Nsp1). These molecules modulate the Calcineurin/NFAT pathway that plays an important role in immune cell activation. Overexpression of NSP1 and infection with live SARS-CoV strongly increased signalling through the Calcineurin/NFAT pathway and enhanced the induction of interleukin 2, compatible with late-stage immunopathogenicity and long-term cytokine dysregulation as observed in severe SARS cases. Conversely, inhibition of cyclophilins by cyclosporine A (CspA) blocked the replication of CoVs of all genera, including SARS-CoV, human CoV-229E and -NL-63, feline CoV, as well as avian infectious bronchitis virus. Non-immunosuppressive derivatives of CspA might serve as broad-range CoV inhibitors applicable against emerging CoVs as well as ubiquitous pathogens of humans and livestock. 相似文献
80.
Jean-Paul Pais de Barros Thomas Gautier Wahib Sali Christophe Adrie Hélène Choubley Emilie Charron Caroline Lalande Naig Le Guern Valérie Deckert Mehran Monchi Jean-Pierre Quenot Laurent Lagrost 《Journal of lipid research》2015,56(7):1363-1369
Quantitation of plasma lipopolysaccharides (LPSs) might be used to document Gram-negative bacterial infection. In the present work, LPS-derived 3-hydroxymyristate was extracted from plasma samples with an organic solvent, separated by reversed phase HPLC, and quantitated by MS/MS. This mass assay was combined with the limulus amebocyte lysate (LAL) bioassay to monitor neutralization of LPS activity in biological samples. The described HPLC/MS/MS method is a reliable, practical, accurate, and sensitive tool to quantitate LPS. The combination of the LAL and HPLC/MS/MS analyses provided new evidence for the intrinsic capacity of plasma lipoproteins and phospholipid transfer protein to neutralize the activity of LPS. In a subset of patients with systemic inflammatory response syndrome, with documented infection but with a negative plasma LAL test, significant amounts of LPS were measured by the HPLC/MS/MS method. Patients with the highest plasma LPS concentration were more severely ill. HPLC/MS/MS is a relevant method to quantitate endotoxin in a sample, to assess the efficacy of LPS neutralization, and to evaluate the proinflammatory potential of LPS in vivo. 相似文献