全文获取类型
收费全文 | 7672篇 |
免费 | 781篇 |
国内免费 | 5篇 |
专业分类
8458篇 |
出版年
2022年 | 62篇 |
2021年 | 119篇 |
2020年 | 75篇 |
2019年 | 101篇 |
2018年 | 103篇 |
2017年 | 94篇 |
2016年 | 178篇 |
2015年 | 266篇 |
2014年 | 302篇 |
2013年 | 351篇 |
2012年 | 496篇 |
2011年 | 501篇 |
2010年 | 294篇 |
2009年 | 285篇 |
2008年 | 420篇 |
2007年 | 424篇 |
2006年 | 354篇 |
2005年 | 356篇 |
2004年 | 357篇 |
2003年 | 320篇 |
2002年 | 294篇 |
2001年 | 159篇 |
2000年 | 154篇 |
1999年 | 137篇 |
1998年 | 92篇 |
1997年 | 90篇 |
1996年 | 69篇 |
1995年 | 69篇 |
1994年 | 66篇 |
1993年 | 74篇 |
1992年 | 91篇 |
1991年 | 109篇 |
1990年 | 100篇 |
1989年 | 89篇 |
1988年 | 67篇 |
1987年 | 70篇 |
1986年 | 58篇 |
1985年 | 78篇 |
1984年 | 51篇 |
1983年 | 61篇 |
1982年 | 50篇 |
1981年 | 50篇 |
1979年 | 84篇 |
1978年 | 55篇 |
1977年 | 49篇 |
1976年 | 52篇 |
1975年 | 37篇 |
1974年 | 42篇 |
1973年 | 42篇 |
1972年 | 54篇 |
排序方式: 共有8458条查询结果,搜索用时 15 毫秒
991.
Energy drinks are frequently consumed by athletes prior to competition to improve performance. This study examined the effect
of Red Bull™ on repeated sprint performance in women athletes. Fifteen collegiate soccer players participated, with mean age,
height, and body mass equal to 19.5 ± 1.1 year, 168.4 ± 5.8 cm, and 63.4 ± 6.1 kg, respectively. After performing a familiarization
trial, subjects performed three sets of eight bouts of the modified t test after ingestion of 255 mL of placebo or Red Bull 1 h pre-exercise in a randomized, placebo-controlled crossover design.
Throughout testing, sprint time, heart rate (HR), and rating of perceived exertion (RPE) were continuously obtained. Repeated
measures analysis of variance was used to examine differences in variables between drink conditions. Across athletes, t test time ranged from 10.4 to 12.7 s. Mean sprint time was similar (p > 0.05) between Red Bull (11.31 ± 0.61 s) and placebo (11.35 ± 0.61 s). HR and RPE increased (p < 0.05) during the bouts, but there was no effect (p > 0.05) of Red Bull on either variable versus placebo. Findings indicate that 255 mL of Red Bull containing 1.3 mg/kg of
caffeine and 1 g of taurine does not alter repeated sprint performance, RPE, or HR in women athletes versus placebo. One serving
of this energy drink provides no ergogenic benefit for women athletes engaging in sprint-based exercise. 相似文献
992.
993.
Canales E Carlson JS Appleby T Fenaux M Lee J Tian Y Tirunagari N Wong M Watkins WJ 《Bioorganic & medicinal chemistry letters》2012,22(13):4288-4292
The use of a tri-substituted acylhydrazine as an isostere of a tertiary amide was explored in a series of HCV NS5B thumb site II inhibitors. Direct replacement generated an analog with similar conformational and physicochemical properties. The series was extended to produce compounds with potent binding affinities and encouraging levels of cellular potency. 相似文献
994.
Variation in the stable carbon and oxygen isotope composition (δ(13) C, Δ(18) O) of co-occurring plant species may reflect the functional diversity of water use strategies present in natural plant communities. We investigated the patterns of water use among 10 coexisting plant species representing diverse taxonomic groups and life forms in semiarid southeast Spain by measuring their leaf δ(13) C and Δ(18) O, the oxygen isotope ratio of stem water and leaf gas exchange rates. Across species, Δ(18) O was tightly negatively correlated with stomatal conductance (g(s) ), whereas δ(13) C was positively correlated with intrinsic water use efficiency (WUE(i) ). Broad interspecific variation in Δ(18) O, δ(13) C and WUE(i) was largely determined by differences in g(s) , as indicated by a strong positive correlation between leaf δ(13) C and Δ(18) O across species The 10 co-occurring species segregated along a continuous ecophysiological gradient defined by their leaf δ(13) C and Δ(18) O, thus revealing a wide spectrum of stomatal regulation intensity and contrasting water use strategies ranging from 'profligate/opportunistic' (high g(s) , low WUE(i) ) to 'conservative' (low g(s) , high WUE(i) ). Coexisting species maintained their relative isotopic rankings in 2?yr with contrasting rainfall, suggesting the existence of species-specific 'isotopic niches' that reflect ecophysiological niche segregation in dryland plant communities. 相似文献
995.
Halling RE Nuhn M Fechner NA Osmundson TW Soytong K Arora D Hibbett DS Binder M 《Mycologia》2012,104(4):951-961
Sutorius is described as a new genus of Boletaceae to accommodate Boletus robustus originally named illegitimately by C.C. Frost from eastern North America. The legitimate name, Boletus eximius, provided by C.H. Peck, has been used since for a dark purple to chocolate brown bolete with finely scaly stipe and reddish brown spore deposit. This iconic taxon has been documented on five continents. Despite the straightforward species identification from morphology, the interpretation of stipe macro-morphology and spore color has led to equivocal generic placement. Phylogenetic analyses of genes encoding large subunit rRNA and translation elongation factor 1α confirm Sutorius as a unique generic lineage in the Boletaceae. Two species are recognized based on multiple accessions: S. eximius, represented by collections from North America, Costa Rica, Guyana, Indonesia and Japan (molecular data are lacking for only the Guyanan and Japanese material); and S. australiensis, represented by material from Queensland, Australia. Additional collections from Zambia and Thailand represent independent lineages, but sampling is insufficient to describe new species for these entities. 相似文献
996.
? Sexual reproduction in mosses requires that sperm be released freely into the environment before finding and fertilizing a receptive female. After release from the male plant, moss sperm may experience a range of abiotic stresses; however, few data are available examining stress tolerance of moss sperm and whether there is genetic variation for stress tolerance in this important life stage. ? Here, we investigated the effects of environmental desiccation and recovery on the sperm cells of three moss species (Bryum argenteum, Campylopus introflexus, and Ceratodon purpureus). ? We found that a fraction of sperm cells were tolerant to environmental desiccation for extended periods (d) and that tolerance did not vary among species. We found that this tolerance occurs irrespective of ambient dehydration conditions, and that the addition of sucrose during dry-down improved cell recovery. Although we observed no interspecific variation, significant variation among individuals within species in sperm cell tolerance to environmental desiccation was observed, suggesting selection could potentially act on this basic reproductive trait. ? The observation of desiccation-tolerant sperm in multiple moss species has important implications for understanding bryophyte reproduction, suggesting the presence of a significant, uncharacterized complexity in the ecology of moss mating systems. 相似文献
997.
TL Mollan B Abraham MB Strader Y Jia JN Lozier JS Olson AI Alayash 《Protein science : a publication of the Protein Society》2012,21(10):1444-1455
Hemoglobin Brigham (β Pro100 to Leu) was first reported in a patient with familial erythrocytosis. Erythrocytes of an affected individual from the same family contain both HbA and Hb Brigham and exhibit elevated O(2) affinity compared with normal cells (P(50) = 23 mm Hg vs. 31 mmHg at pH 7.4 at 37°C). O(2) affinities measured for hemolysates were sensitive to changes in pH or chloride concentrations, indicating little change in the Bohr and Chloride effects. Hb Brigham was separated from normal HbA by nondenaturing cation exchange liquid chromatography, and the amino acid substitution was verified by mass spectrometry. The properties of Hb Brigham isolated from the patient's blood were then compared with those of recombinant Hb Brigham expressed in Escherichia coli. Kinetic experiments suggest that the rate constants for ligand binding and release in the high (R) and low (T) affinity quaternary states of Hb Brigham are similar to those of native hemoglobin. However, the Brigham mutation decreases the T to R equilibrium constant (L) which accelerates the switch to the R state during ligand binding to deoxy-Hb, increasing the rate of association by approximately twofold, and decelerates the switch during ligand dissociation from HbO(2) , decreasing the rate approximately twofold. These kinetic data help explain the high O(2) affinity characteristics of Hb Brigham and provide further evidence for the importance of the contribution of Pro100 to intersubunit contacts and stabilization of the T quaternary structure. 相似文献
998.
999.
The Collaboration for AIDS Vaccine Discovery/Comprehensive Antibody-Vaccine Immune Monitoring Consortium (CAVD/CA-VIMC) assisted an international network of laboratories in transferring a validated assay used to judge HIV-1 vaccine immunogenicity in compliance with Good Clinical Laboratory Practice (GCLP) with the goal of adding quality to the conduct of endpoint assays for Human Immunodeficiency Virus I (HIV-1) vaccine human clinical trials. Eight Regional Laboratories in the international setting (Regional Laboratories), many located in regions where the HIV-1 epidemic is most prominent, were selected to implement the standardized, GCLP-compliant Neutralizing Antibody Assay for HIV-1 in TZM-bl Cells (TZM-bl NAb Assay). Each laboratory was required to undergo initial training and implementation of the immunologic assay on-site and then perform partial assay re-validation, competency testing, and undergo formal external audits for GCLP compliance. Furthermore, using a newly established external proficiency testing program for the TZM-bl NAb Assay has allowed the Regional Laboratories to assess the comparability of assay results at their site with the results of neutralizing antibody assays performed around the world. As a result, several of the CAVD/CA-VIMC Regional Laboratories are now in the process of conducting or planning to conduct the GCLP-compliant TZM-bl NAb Assay as an indicator of vaccine immunogenicity for ongoing human clinical trials. 相似文献
1000.
Taxonomic assignment of sequence reads is a challenging task in metagenomic data analysis, for which the present methods mainly use either composition- or homology-based approaches. Though the homology-based methods are more sensitive and accurate, they suffer primarily due to the time needed to generate the Blast alignments. We developed the MetaBin program and web server for better homology-based taxonomic assignments using an ORF-based approach. By implementing Blat as the faster alignment method in place of Blastx, the analysis time has been reduced by severalfold. It is benchmarked using both simulated and real metagenomic datasets, and can be used for both single and paired-end sequence reads of varying lengths (≥45 bp). To our knowledge, MetaBin is the only available program that can be used for the taxonomic binning of short reads (<100 bp) with high accuracy and high sensitivity using a homology-based approach. The MetaBin web server can be used to carry out the taxonomic analysis, by either submitting reads or Blastx output. It provides several options including construction of taxonomic trees, creation of a composition chart, functional analysis using COGs, and comparative analysis of multiple metagenomic datasets. MetaBin web server and a standalone version for high-throughput analysis are available freely at http://metabin.riken.jp/. 相似文献