全文获取类型
收费全文 | 8405篇 |
免费 | 899篇 |
国内免费 | 3篇 |
专业分类
9307篇 |
出版年
2023年 | 58篇 |
2022年 | 144篇 |
2021年 | 273篇 |
2020年 | 136篇 |
2019年 | 180篇 |
2018年 | 208篇 |
2017年 | 202篇 |
2016年 | 347篇 |
2015年 | 536篇 |
2014年 | 534篇 |
2013年 | 566篇 |
2012年 | 716篇 |
2011年 | 736篇 |
2010年 | 478篇 |
2009年 | 371篇 |
2008年 | 523篇 |
2007年 | 518篇 |
2006年 | 392篇 |
2005年 | 385篇 |
2004年 | 393篇 |
2003年 | 339篇 |
2002年 | 305篇 |
2001年 | 72篇 |
2000年 | 45篇 |
1999年 | 63篇 |
1998年 | 79篇 |
1997年 | 36篇 |
1996年 | 50篇 |
1995年 | 34篇 |
1994年 | 37篇 |
1993年 | 33篇 |
1992年 | 33篇 |
1991年 | 31篇 |
1990年 | 36篇 |
1989年 | 24篇 |
1988年 | 23篇 |
1987年 | 14篇 |
1986年 | 17篇 |
1985年 | 28篇 |
1984年 | 33篇 |
1983年 | 17篇 |
1982年 | 21篇 |
1981年 | 27篇 |
1980年 | 23篇 |
1979年 | 17篇 |
1978年 | 17篇 |
1977年 | 19篇 |
1976年 | 16篇 |
1974年 | 16篇 |
1967年 | 15篇 |
排序方式: 共有9307条查询结果,搜索用时 15 毫秒
61.
Rachel Sacks-Davis Galina Daraganova Campbell Aitken Peter Higgs Lilly Tracy Scott Bowden Rebecca Jenkinson David Rolls Philippa Pattison Garry Robins Jason Grebely Alyssa Barry Margaret Hellard 《PloS one》2012,7(10)
It is hypothesized that social networks facilitate transmission of the hepatitis C virus (HCV). We tested for association between HCV phylogeny and reported injecting relationships using longitudinal data from a social network design study. People who inject drugs were recruited from street drug markets in Melbourne, Australia. Interviews and blood tests took place three monthly (during 2005–2008), with participants asked to nominate up to five injecting partners at each interview. The HCV core region of individual isolates was then sequenced and phylogenetic trees were constructed. Genetic clusters were identified using bootstrapping (cut-off: 70%). An adjusted Jaccard similarity coefficient was used to measure the association between the reported injecting relationships and relationships defined by clustering in the phylogenetic analysis (statistical significance assessed using the quadratic assignment procedure). 402 participants consented to participate; 244 HCV infections were observed in 238 individuals. 26 genetic clusters were identified, with 2–7 infections per cluster. Newly acquired infection (AOR = 2.03, 95% CI: 1.04–3.96, p = 0.037, and HCV genotype 3 (vs. genotype 1, AOR = 2.72, 95% CI: 1.48–4.99) were independent predictors of being in a cluster. 54% of participants whose infections were part of a cluster in the phylogenetic analysis reported injecting with at least one other participant in that cluster during the study. Overall, 16% of participants who were infected at study entry and 40% of participants with newly acquired infections had molecular evidence of related infections with at least one injecting partner. Likely transmission clusters identified in phylogenetic analysis correlated with reported injecting relationships (adjusted Jaccard coefficient: 0.300; p<0.001). This is the first study to show that HCV phylogeny is associated with the injecting network, highlighting the importance of the injecting network in HCV transmission. 相似文献
62.
Sean V. Murphy Rebecca Lim Philip Heraud Marian Cholewa Mark Le Gros Martin D. de Jonge Daryl L. Howard David Paterson Courtney McDonald Anthony Atala Graham Jenkin Euan M. Wallace 《PloS one》2012,7(9)
Cystic fibrosis, an autosomal recessive disorder caused by a mutation in a gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR), remains a leading cause of childhood respiratory morbidity and mortality. The respiratory consequences of cystic fibrosis include the generation of thick, tenacious mucus that impairs lung clearance, predisposing the individual to repeated and persistent infections, progressive lung damage and shortened lifespan. Currently there is no cure for cystic fibrosis. With this in mind, we investigated the ability of human amnion epithelial cells (hAECs) to express functional CFTR. We found that hAECs formed 3-dimensional structures and expressed the CFTR gene and protein after culture in Small Airway Growth Medium (SAGM). We also observed a polarized CFTR distribution on the membrane of hAECs cultured in SAGM, similar to that observed in polarized airway cells in vivo. Further, hAECs induced to express CFTR possessed functional iodide/chloride (I−/Cl−) ion channels that were inhibited by the CFTR-inhibitor CFTR-172, indicating the presence of functional CFTR ion channels. These data suggest that hAECs may be a promising source for the development of a cellular therapy for cystic fibrosis. 相似文献
63.
Kyle R. Siegel Muskanjot Kaur A. Calvin Grigal Rebecca A. Metzler Gary H. Dickinson 《Ecology and evolution》2022,12(6)
Crustaceans comprise an ecologically and morphologically diverse taxonomic group. They are typically considered resilient to many environmental perturbations found in marine and coastal environments, due to effective physiological regulation of ions and hemolymph pH, and a robust exoskeleton. Ocean acidification can affect the ability of marine calcifying organisms to build and maintain mineralized tissue and poses a threat for all marine calcifying taxa. Currently, there is no consensus on how ocean acidification will alter the ecologically relevant exoskeletal properties of crustaceans. Here, we present a systematic review and meta‐analysis on the effects of ocean acidification on the crustacean exoskeleton, assessing both exoskeletal ion content (calcium and magnesium) and functional properties (biomechanical resistance and cuticle thickness). Our results suggest that the effect of ocean acidification on crustacean exoskeletal properties varies based upon seawater pCO2 and species identity, with significant levels of heterogeneity for all analyses. Calcium and magnesium content was significantly lower in animals held at pCO2 levels of 1500–1999 µatm as compared with those under ambient pCO2. At lower pCO2 levels, however, statistically significant relationships between changes in calcium and magnesium content within the same experiment were observed as follows: a negative relationship between calcium and magnesium content at pCO2 of 500–999 µatm and a positive relationship at 1000–1499 µatm. Exoskeleton biomechanics, such as resistance to deformation (microhardness) and shell strength, also significantly decreased under pCO2 regimes of 500–999 µatm and 1500–1999 µatm, indicating functional exoskeletal change coincident with decreases in calcification. Overall, these results suggest that the crustacean exoskeleton can be susceptible to ocean acidification at the biomechanical level, potentially predicated by changes in ion content, when exposed to high influxes of CO2. Future studies need to accommodate the high variability of crustacean responses to ocean acidification, and ecologically relevant ranges of pCO2 conditions, when designing experiments with conservation‐level endpoints. 相似文献
64.
The Value of Artificial Stimuli in Behavioral Research: Making the Case for Egg Rejection Studies in Avian Brood Parasitism 下载免费PDF全文
Márk E. Hauber Lainga Tong Miklós Bán Rebecca Croston Tomáš Grim Geoffrey I. N. Waterhouse Matthew D. Shawkey Andrew B. Barron Csaba Moskát 《Ethology : formerly Zeitschrift fur Tierpsychologie》2015,121(6):521-528
Experimentation is at the heart of classical and modern behavioral ecology research. The manipulation of natural cues allows us to establish causation between aspects of the environment, both internal and external to organisms, and their effects on animals' behaviors. In recognition systems research, including the quest to understand the coevolution of sensory cues and decision rules underlying the rejection of foreign eggs by hosts of avian brood parasites, artificial stimuli have been used extensively, but not without controversy. In response to repeated criticism about the value of artificial stimuli, we describe four potential benefits of using them in egg recognition research, two each at the proximate and ultimate levels of analysis: (1) the standardization of stimuli for developmental studies and (2) the disassociation of correlated traits of egg phenotypes used for sensory discrimination, as well as (3) the estimation of the strength of selection on parasitic egg mimicry and (4) the establishment of the evolved limits of sensory and cognitive plasticity. We also highlight constraints of the artificial stimulus approach and provide a specific test of whether responses to artificial cues can accurately predict responses to natural cues. Artificial stimuli have a general value in ethological research beyond research in brood parasitism and may be especially critical in field studies involving the manipulation of a single parameter, where other, confounding variables are difficult or impossible to control experimentally or statistically. 相似文献
65.
66.
Undomesticated strains of Bacillus subtilis exhibit extensive colony spreading on certain soft agarose media: first the formation of dendritic clusters of cells, followed by spreading (pellicle-like) growth to cover the entire surface. These phases of colonization are dependent on the level of potassium ion (K(+)) but independent of flagella, as verified with a mutant with a hag gene replacement; this latter finding highlights the importance of sliding motility in colony spreading. Exploring the K(+) requirement, directed mutagenesis of the higher-affinity K(+) transporter KtrAB, but not the lower-affinity transporter KtrCD, was found to inhibit surface colonization unless sufficient KCl was added. To identify other genes involved in K(+)-dependent colony spreading, transposon insertion mutants in wild-type strain 3610 were screened. Disruption of genes for pyrimidine (pyrB) or purine (purD, purF, purH, purL, purM) biosynthetic pathways abolished the K(+)-dependent spreading phase. Consistent with a requirement for functional nucleic acid biosynthesis, disruption of purine synthesis with the folic acid antagonist sulfamethoxazole also inhibited spreading. Other transposon insertions disrupted acetoin biosynthesis (the alsS gene), acidifying the growth medium, glutamine synthetase (the glnA gene), and two surfactin biosynthetic genes (srfAA, srfAB). This work identified four classes of surface colonization mutants with defective (i) potassium transport, (ii) surfactin formation, (iii) growth rate or yield, or (iv) pH control. Overall, the ability of B. subtilis to colonize surfaces by spreading is highly dependent on balanced nucleotide biosynthesis and nutrient assimilation, which require sufficient K(+) ions, as well as growth conditions that promote sliding motility. 相似文献
67.
Trey K. Sato Mary Tremaine Lucas S. Parreiras Alexander S. Hebert Kevin S. Myers Alan J. Higbee Maria Sardi Sean J. McIlwain Irene M. Ong Rebecca J. Breuer Ragothaman Avanasi Narasimhan Mick A. McGee Quinn Dickinson Alex La Reau Dan Xie Mingyuan Tian Jeff S. Piotrowski Jennifer L. Reed Yaoping Zhang Joshua J. Coon Chris Todd Hittinger Audrey P. Gasch Robert Landick 《PLoS genetics》2016,12(11)
68.
Honious Sarah A. S. Hale Rebecca L. Guilinger James J. Crosby Benjamin T. Baxter Colden V. 《Ecosystems》2022,25(2):422-440
Ecosystems - Stream ecosystem metabolism contributes to global carbon cycling, yet predicting metabolism across ecosystems remains elusive. Even within stream segments, spatial variation in... 相似文献
69.
Rebecca D. Fink Catherine A. Lindell Emily B. Morrison Rakan A. Zahawi and Karen D. Holl 《Restoration Ecology》2009,17(4):479-486
Efforts to restore tropical forest in abandoned pasture are likely to be more successful when bird visitation is promoted because birds disperse seeds and eat herbivorous arthropods that damage leaves. Thus, it is critical to understand bird behavior in relation to different restoration strategies. We measured the likelihood of visitation, number of visits, and duration of visits for all birds and for Cherrie's Tanager ( Ramphocelus costaricensis ), a common seed disperser, in five sizes of forest restoration patches planted with four tree species in southern Costa Rica. We predicted that the largest patches, and the tree species with the greatest canopy cover, would be visited most frequently and have the longest visits because we assumed that these patch types had the greatest food resources and the lowest predation risk. We found that birds were more likely to visit large patches and the tree species with the highest canopy cover ( Inga edulis ). Birds visited Inga trees more often and stayed in Inga and Erythrina poeppigiana trees for longer periods of time than in other tree species. We found similar results for Cherrie's Tanagers. Thus, we identified two factors, tree species and patch size, which may be manipulated in restoration projects to increase bird visitation. 相似文献
70.
Lee Taylor Adrian W. Midgley Bryna Chrismas Angela R. Hilman Leigh A. Madden Rebecca V. Vince Lars R. McNaughton 《Amino acids》2011,40(2):393-401
Heat shock protein 72 (HSP72) performs vital roles within the body at rest and during periods of stress. In vitro, research
demonstrates HSP72 induction in response to hypoxia. Recently, in vivo, an acute hypoxic exposure (75 min at 2,980 m) was
sufficient to induce significant increases in monocyte expressed HSP72 (mHSP72) and a marker of oxidative stress in healthy human subjects. The purpose of the current study was to identify the impact
of 10 consecutive days of hypoxic exposures (75 min at 2,980 m) on mHSP72 and erythropoietin (EPO) expression, markers of oxidative stress, and maximal oxygen consumption in graded incremental aerobic
exercise. Eight male subjects were exposed to daily normobaric hypoxic exposures for 75 min at 2,980 m for 10 consecutive
days, commencing and ceasing at 0930 and 1045, respectively. This stressor was sufficient to induce significant increases
in mHSP72, which was significantly elevated from day 2 of the hypoxic exposures until 48 h post-final exposure. Notably, this increase
had an initial rapid (30% day on day compared to baseline) and final slow phase (16% day on day compared to baseline) of expression.
The authors postulate that 7-day hypoxic exposure in this manner would be sufficient to induce near maximum hypoxia-mediated
basal mHSP72 expression. Elevated levels of mHSP72 are associated with acquired thermotolerance and provide cross tolerance to non-related stressors in vivo, the protocol used
here may provide a useful tool for elevating mHSP72 in vivo. Aside from these major findings, significant transient daily elevations were seen in a marker of oxidative stress,
alongside sustained increases in EPO expression. However, no physiologically significant changes were seen in maximal oxygen
consumption or time to exhaustion. 相似文献