全文获取类型
收费全文 | 4541篇 |
免费 | 329篇 |
国内免费 | 1篇 |
出版年
2023年 | 20篇 |
2022年 | 46篇 |
2021年 | 89篇 |
2020年 | 58篇 |
2019年 | 65篇 |
2018年 | 94篇 |
2017年 | 88篇 |
2016年 | 155篇 |
2015年 | 201篇 |
2014年 | 222篇 |
2013年 | 312篇 |
2012年 | 369篇 |
2011年 | 353篇 |
2010年 | 225篇 |
2009年 | 193篇 |
2008年 | 287篇 |
2007年 | 301篇 |
2006年 | 262篇 |
2005年 | 248篇 |
2004年 | 211篇 |
2003年 | 190篇 |
2002年 | 196篇 |
2001年 | 43篇 |
2000年 | 21篇 |
1999年 | 37篇 |
1998年 | 61篇 |
1997年 | 25篇 |
1996年 | 37篇 |
1995年 | 45篇 |
1994年 | 23篇 |
1993年 | 39篇 |
1992年 | 26篇 |
1991年 | 22篇 |
1990年 | 15篇 |
1989年 | 9篇 |
1988年 | 12篇 |
1987年 | 22篇 |
1986年 | 16篇 |
1985年 | 16篇 |
1984年 | 15篇 |
1983年 | 15篇 |
1982年 | 14篇 |
1981年 | 17篇 |
1980年 | 17篇 |
1977年 | 10篇 |
1976年 | 12篇 |
1975年 | 16篇 |
1974年 | 10篇 |
1973年 | 10篇 |
1972年 | 11篇 |
排序方式: 共有4871条查询结果,搜索用时 62 毫秒
941.
Rita Skoda-F ldes Zita Cs kai L szl Koll r G bor Szalontai Judit Horv th Zolt n Tuba 《Steroids》1995,60(12):786-790
17-Iodo-Δ16 steroids undergo selective dimerization and carbonylative dimerization in the presence of palladium catalysts in dimethylformamide which result in 16–17′-coupled dienes and 17-carboxylic anhydrides, respectively. Moderate to good yields have been obtained for both types of dimers. 相似文献
942.
943.
Gilda Nardi Antonio Canziani Palmira Striani Nicola Santini Carla Coccia Laura Seghetti Rita Kranic 《Aerobiologia》1996,12(1):269-271
The aim of this work is to study the incidence of pollinosis in the Health District of Ascoli Piceno, Central Italy (U.S.L.24). this being an underestimated pathology from the clinical point of view and also as a result of the recent introduction of this taxa in the National Aeroallergen Network. Since 1990, 5055 patients of both sexes with respiratory symptomatology of suspected IgE mediated aetiology have been examined in our Centre with the Skin Prick Test (SPT) using allergen panels including Cypress; 171 (3.38%) patients were found to be positive to this allergen. These results show that the subjects with symptoms in the period January–March in most cases have a sensitization toCupressus pollen and new studies will evaluate the possibility of specific immunotherapy. 相似文献
944.
Reef Sites
Coral survivors of the 1982–83 El Niño-Southern Oscillation, Galápagos Islands, Ecuador 相似文献945.
Sven Hammerschmidt Astrid Müller Hanna Sillmann Martina Miihlenhoff Raymond Borrow rew Fox Jos van Putten Wendell D. Zollinger Rita Gerardy-Schahn Matthias Frosch 《Molecular microbiology》1996,20(6):1211-1220
A mechanism of capsular polysaccharide phase variation in Neisseria meningitidis is described. Meningococcal cells of an encapsulated serogroup B strain were used in invasion assays. Only unencapsulated variants were found to enter epithelial cells. Analysis of one group of capsule-deficient variants indicated that the capsular polysaccharide was re-expressed at a frequency of 10?3. Measurement of enzymatic activities involved in the biosynthesis of the α-2,8 polysialic acid capsule showed that polysialyltransferase (PST) activity was absent in these capsule-negative variants. Nucleotide sequence analysis of siaD revealed an insertion or a deletion of one cytidine residue within a run of (dC)7 residues at position 89, resulting in a frameshift and premature termination of translation. We analysed unencapsulated isolates from carriers and encapsulated case isolates collected during an outbreak of meningococcal disease. Further paired blood-culture isolates and unencapsulated nasopharyngeal isolates from patients with meningococcal meningitis were examined. In all unencapsulated strains analysed we found an insertion or deletion within the oligo-(dC) stretch within siaD, resulting in a frameshift and loss of capsule formation. All encapsulated isolates, however, had seven dC residues at this position, indicating a correlation between capsule phase variation and bacterial invasion and the out-break of meningococcal disease. 相似文献
946.
Wim Van den Ende ré Van Laere Katrien Le Roy Rudy Vergauwen David Boogaerts Rita C. L. Figueiredo-Ribeiro Maria Angela Machado de Carvalho 《Physiologia plantarum》2005,125(4):419-429
Inulin-type fructans are stored in the tuberous roots of the Brazilian cerrado plant Viguiera discolor Baker (Asteraceae). In Cynara scolymus (artichoke) and Echinops ritro (globe thistle), the fructans have a considerably higher degree of polymerization (DP) than in Cichorium intybus (chicory) and Helianthus tuberosus (Jerusalem artichoke). It was shown before that the higher DP in some species can be attributed to the properties of their fructan: fructan 1-fructosyl transferases (1-FFTs; EC 2.4.1.100), enzymes responsible for chain elongation. Here, we describe the cloning of a high DP (hDP) 1-FFT cDNA from V. discolor and its heterologous expression in Pichia pastoris . Starting from 1-kestose and Neosugar P (a mixture of oligo-inulins from microbial origin) as substrates, the recombinant enzyme produces a typical hDP inulin profile in vitro, closely resembling the one observed in vivo. The enzyme shows no invertase activity and sucrose: sucrose 1-fructosyl transferase (1-SST; EC 2.4.1.99) activity in vitro. Pattern evolution during incubation suggests that inulins with DP ≥ 6 are much better substrates than sucrose or lower DP oligo-fructans. Because hDP inulin-type fructans show superior properties for specific food and non-food applications, the hDP 1-FFT gene from V. discolor has potential for the production of hDP inulin in vitro or in transgenic crops. 相似文献
947.
Rita Krumscheid Rüdiger Ettrich Zofie Sovová Klára Susánková Zdenek Lánsky Katerina Hofbauerová Holger Linnertz Jan Teisinger Evzen Amler Wilhelm Schoner 《European journal of biochemistry》2004,271(19):3923-3936
The structural stability of the large cytoplasmic domain (H(4)-H(5) loop) of mouse alpha(1) subunit of Na(+)/K(+) ATPase (L354-I777), the number and the location of its binding sites for 2'-3'-O-(trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) and p-nitrophenylphosphate (pNPP) were investigated. C- and N-terminal shortening revealed that neither part of the phosphorylation (P)-domain are necessary for TNP-ATP binding. There is no indication of a second ATP site on the P-domain of the isolated loop, even though others reported previously of its existence by TNP-N(3)ADP affinity labeling of the full enzyme. Fluorescein isothiocyanate (FITC)-anisotropy measurements reveal a considerable stability of the nucleotide (N)-domain suggesting that it may not undergo a substantial conformational change upon ATP binding. The FITC modified loop showed only slightly diminished phosphatase activity, most likely due to a pNPP site on the N-domain around N398 whose mutation to D reduced the phosphatase activity by 50%. The amino acids forming this pNPP site (M384, L414, W411, S400, S408) are conserved in the alpha(1-4) isoforms of Na(+)/K(+) ATPase, whereas N398 is only conserved in the vertebrates' alpha(1) subunit. The phosphatase activity of the isolated H(4)-H(5) loop was neither inhibited by ATP, nor affected by mutation of D369, which is phosphorylated in native Na(+)/K(+) ATPase. 相似文献
948.
Winter severity determines functional trait composition of phytoplankton in seasonally ice‐covered lakes
下载免费PDF全文
![点击此处可从《Global Change Biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Deniz Özkundakci Alena S. Gsell Thomas Hintze Helgard Täuscher Rita Adrian 《Global Change Biology》2016,22(1):284-298
How climate change will affect the community dynamics and functionality of lake ecosystems during winter is still little understood. This is also true for phytoplankton in seasonally ice‐covered temperate lakes which are particularly vulnerable to the presence or absence of ice. We examined changes in pelagic phytoplankton winter community structure in a north temperate lake (Müggelsee, Germany), covering 18 winters between 1995 and 2013. We tested how phytoplankton taxa composition varied along a winter‐severity gradient and to what extent winter severity shaped the functional trait composition of overwintering phytoplankton communities using multivariate statistical analyses and a functional trait‐based approach. We hypothesized that overwintering phytoplankton communities are dominated by taxa with trait combinations corresponding to the prevailing winter water column conditions, using ice thickness measurements as a winter‐severity indicator. Winter severity had little effect on univariate diversity indicators (taxon richness and evenness), but a strong relationship was found between the phytoplankton community structure and winter severity when taxon trait identity was taken into account. Species responses to winter severity were mediated by the key functional traits: motility, nutritional mode, and the ability to form resting stages. Accordingly, one or the other of two functional groups dominated the phytoplankton biomass during mild winters (i.e., thin or no ice cover; phototrophic taxa) or severe winters (i.e., thick ice cover; exclusively motile taxa). Based on predicted milder winters for temperate regions and a reduction in ice‐cover durations, phytoplankton communities during winter can be expected to comprise taxa that have a relative advantage when the water column is well mixed (i.e., need not be motile) and light is less limiting (i.e., need not be mixotrophic). A potential implication of this result is that winter severity promotes different communities at the vernal equinox, which may have different nutritional quality for the next trophic level and ecosystem‐scale effects. 相似文献
949.
Evaluating an unmanned aerial vehicle‐based approach for assessing habitat extent and condition in fine‐scale early successional mountain mosaics
下载免费PDF全文
![点击此处可从《应用植被学》网站下载免费的PDF全文](/ch/ext_images/free.gif)
950.
The transcription factor Pax6 contributes to the induction of GLT‐1 expression in astrocytes through an interaction with a distal enhancer element
下载免费PDF全文
![点击此处可从《Journal of neurochemistry》网站下载免费的PDF全文](/ch/ext_images/free.gif)