全文获取类型
收费全文 | 40061篇 |
免费 | 4151篇 |
国内免费 | 11篇 |
专业分类
44223篇 |
出版年
2023年 | 175篇 |
2022年 | 352篇 |
2021年 | 671篇 |
2020年 | 488篇 |
2019年 | 571篇 |
2018年 | 704篇 |
2017年 | 632篇 |
2016年 | 1046篇 |
2015年 | 1737篇 |
2014年 | 1889篇 |
2013年 | 2341篇 |
2012年 | 2872篇 |
2011年 | 2796篇 |
2010年 | 1857篇 |
2009年 | 1613篇 |
2008年 | 2254篇 |
2007年 | 2241篇 |
2006年 | 2126篇 |
2005年 | 1902篇 |
2004年 | 1890篇 |
2003年 | 1682篇 |
2002年 | 1654篇 |
2001年 | 651篇 |
2000年 | 607篇 |
1999年 | 620篇 |
1998年 | 433篇 |
1997年 | 353篇 |
1996年 | 339篇 |
1995年 | 351篇 |
1994年 | 308篇 |
1993年 | 306篇 |
1992年 | 413篇 |
1991年 | 355篇 |
1990年 | 351篇 |
1989年 | 363篇 |
1988年 | 364篇 |
1987年 | 334篇 |
1986年 | 268篇 |
1985年 | 298篇 |
1984年 | 303篇 |
1983年 | 270篇 |
1982年 | 257篇 |
1981年 | 231篇 |
1980年 | 209篇 |
1979年 | 215篇 |
1978年 | 181篇 |
1977年 | 183篇 |
1976年 | 182篇 |
1975年 | 198篇 |
1973年 | 164篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Voituron Y Paaschburg L Holmstrup M Barré H Ramløv H 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》2009,179(2):223-230
Freeze tolerance and changes in metabolism during freezing were investigated in the moor frog (Rana arvalis) under laboratory conditions. The data show for the first time a well-developed freeze tolerance in juveniles of a European
frog capable of surviving a freezing exposure of about 72 h with a final body temperature of −3°C. A biochemical analysis
showed an increase in liver and muscle glucose in response to freezing (respectively, 14-fold and 4-fold between 4 and −1°C).
Lactate accumulation was only observed in the liver (4.1 ± 0.8 against 16.6 ± 2.4 μmol g−1 fresh weight (FW) between 4 and −1°C). The quantification of the respiratory metabolism of frozen frogs showed that the aerobic
metabolism persists under freezing conditions (1.4 ± 0.7 μl O2 g−1 FW h−1 at −4°C) and decreases with body temperature. After thawing, the oxygen consumption rose rapidly during the first hour (6-fold
to 16-fold) and continued to increase for 24 h, but at a lower rate. In early winter, juvenile R. arvalis held in an outdoor enclosure were observed to emerge from ponds and hibernate in the upper soil and litter layers. Temperature
recordings in the substratum of the enclosure suggested that the hibernacula of these juvenile frogs provided sheltering from
sub-zero air temperatures and reduced the time spent in a frozen state corresponding well with the observed freeze tolerance
of the juveniles. This study strongly suggests that freeze tolerance of R. arvalis is an adaptive trait necessary for winter survival. 相似文献
992.
Stefania Piantavigna Patricia Czihal Adam Mechler Monique Richter Ralf Hoffmann Lisandra L. Martin 《International journal of peptide research and therapeutics》2009,15(2):139-146
Apidaecin peptides from Apis mellifera hemolymph are believed to attack intracellular bacterial targets. Our in vivo results for apidaecins 1a and 1b confirm that
bacterial activity is non-lytic, however, the manner in which these peptides pass through the cell membrane to exert this
activity is unknown. These data are combined with fluorescence (dye leakage) and quartz crystal microbalance studies to investigate
the membrane interaction for these two wildtype peptides. It was found that the peptides penetrate the membrane in a trans-membrane
manner. The amount of peptide uptake by the membrane is proportional to the concentration of the peptide, however, this appears
to be a dynamic equilibrium which can be almost completely reversed by addition of buffer medium. Interestingly, a small residual
mass remains within the membrane and the amount of peptide remaining in the membrane is a function of the buffer-salt concentration
viz. in high salt, the residual peptide mass remaining is small whereas at low salt concentration, a larger mass of peptide
remains bound. These results support a direct membrane penetration mechanism by the wild type apidaecins 1a and 1b. In both
cases the peptide–membrane interaction has a negligible effect on the membrane, although, in high salt a permanent change
in the membrane does occur at the highest peptide concentration which does not recover following peptide removal.
Stefania Piantavigna and Patricia Czihal contributed equally to this article. 相似文献
993.
Dynamics and functional relevance of ammonia-oxidizing archaea in two agricultural soils 总被引:4,自引:0,他引:4
Kristina Schauss reas Focks Sven Leininger Anja Kotzerke Holger Heuer Sören Thiele-Bruhn Shilpi Sharma Berndt-Michael Wilke Michael Matthies Kornelia Smalla Jean Charles Munch Wulf Amelung Martin Kaupenjohann Michael Schloter Christa Schleper 《Environmental microbiology》2009,11(2):446-456
Crucial steps in geochemical cycles are in many cases performed by more than one group of microorganisms, but the significance of this functional redundancy with respect to ecosystem functioning is poorly understood. Ammonia-oxidizing archaea (AOA) and their bacterial counterparts (AOB) are a perfect system to address this question: although performing the same transformation step, they belong to well-separated phylogenetic groups. Using pig manure amended with different concentrations of sulfadiazine (SDZ), an antibiotic that is frequently used in veterinary medicine, it was possible to affect AOB and AOA to different degrees. Addition of manure stimulated growth of AOB in both soils and, interestingly, also growth of AOA was considerably stimulated in one of the soils. The antibiotic treatments decreased the manure effect notably on AOB, whereas AOA were affected to a lower extent. Model calculations concerning the respective proportions of AOA and AOB in ammonia oxidation indicate a substantial contribution of AOA in one of the soils that further increased under the influence of SDZ, hence indicating functional redundancy between AOA and AOB. 相似文献
994.
Timothy S. Church Corby K. Martin Angela M. Thompson Conrad P. Earnest Catherine R. Mikus Steven N. Blair 《PloS one》2009,4(2)
Background
It has been suggested that exercise training results in compensatory mechanisms that attenuate weight loss. However, this has only been examined with large doses of exercise. The goal of this analysis was to examine actual weight loss compared to predicted weight loss (compensation) across different doses of exercise in a controlled trial of sedentary, overweight or obese postmenopausal women (n = 411).Methodology/Principal Findings
Participants were randomized to a non-exercise control (n = 94) or 1 of 3 exercise groups; exercise energy expenditure of 4 (n = 139), 8 (n = 85), or 12 (n = 93) kcal/kg/week (KKW). Training intensity was set at the heart rate associated with 50% of each woman''s peak VO2 and the intervention period was 6 months. All exercise was supervised. The main outcomes were actual weight loss, predicted weight loss (exercise energy expenditure/ 7700 kcal per kg), compensation (actual minus predicted weight loss) and waist circumference. The study sample had a mean (SD) age 57.2 (6.3) years, BMI of 31.7 (3.8) kg/m2, and was 63.5% Caucasian. The adherence to the intervention was >99% in all exercise groups. The mean (95% CI) weight loss in the 4, 8 and 12 KKW groups was −1.4 (−2.0, −0.8), −2.1 (−2.9, −1.4) and −1.5 (−2.2, −0.8) kg, respectively. In the 4 and 8 KKW groups the actual weight loss closely matched the predicted weight loss of −1.0 and −2.0 kg, respectively, resulting in no significant compensation. In the 12 KKW group the actual weight loss was less than the predicted weight loss (−2.7 kg) resulting in 1.2 (0.5, 1.9) kg of compensation (P<0.05 compared to 4 and 8 KKW groups). All exercise groups had a significant reduction in waist circumference which was independent of changes in weight.Conclusion
In this study of previously sedentary, overweight or obese, postmenopausal women we observed no difference in the actual and predicted weight loss with 4 and 8 KKW of exercise (72 and 136 minutes respectively), while the 12 KKW (194 minutes) produced only about half of the predicted weight loss. However, all exercise groups had a significant reduction in waist circumference which was independent of changes in weight.Trial Registration
ClinicalTrials.gov NCT 00011193相似文献995.
Matt Turner Seth Eidemiller Bryan Martin Andrew Narver Joshua Marshall Logan Zemp Kenneth A. Cornell J.M. McIntosh Owen M. McDougal 《Bioorganic & medicinal chemistry》2009,17(16):5894-5899
Parkinson’s disease is a debilitating movement disorder characterized by altered levels of α6β21 (1 indicates the possible presence of additional subunits) nicotinic acetylcholine receptors (nAChRs) localized on presynaptic striatal catecholaminergic neurons. α-Conotoxin MII (α-CTx MII) is a highly useful ligand to probe α6β2 nAChRs structure and function, but it does not discriminate among closely related α61 nAChR subtypes. Modification of the α-CTx MII primary sequence led to the identification of α-CTx MII[E11A], an analog with 500–5300-fold discrimination between α61 subtypes found in both human and non-human primates. α-CTx MII[E11A] binds most strongly (femtomolar dissociation constant) to the high affinity α6 nAChR, a subtype that is selectively lost in Parkinson’s disease. Here, we present the three-dimensional solution structure for α-CTx MII[E11A] as determined by two-dimensional 1H NMR spectroscopy to 0.13 ± 0.09 ? backbone and 0.45 ± 0.08 ? heavy atom root-mean-square deviation from mean structure. Structural comparisons suggest that the increased hydrophobic area of α-CTx MII[E11A] relative to other members of the α-CTx family may be responsible for its exceptionally high affinity for α6α4β21 nAChR as well as discrimination between α6β2 and α3β2 containing nAChRs. This finding may enable the rational design of novel peptide analogs that demonstrate enhanced specificity for α61 nAChR subunit interfaces and provide a means to better understand nAChR structural determinants that modulate brain dopamine levels and the pathophysiology of Parkinson’s disease. 相似文献
996.
997.
The interactions among plants and pollinators are crucial for plant reproduction because they structure gene flow among individual plants and among populations of plants. Throughout the Neotropics there has been a strong increase in recent years in the use of artificial nectar feeders in private nature reserves to encourage ecotourism through prolonged observations of hummingbirds. Currently, there is considerable uncertainty whether artificial feeders have a detrimental effect on plant reproduction through competition or a beneficial effect through facilitation. This uncertainty is disconcerting given that nature reserves harbour many rare and endangered plants whose successful reproduction is a conservation goal. To assess whether nectar feeders affected hummingbird visitation to flowering plants, we determined visitation rates in ten flowering species in five Andean nature reserves in Ecuador. We found that visitation rates tended to be higher within 5 m around the feeders than they were at 100 m, 500 m, or 1.5 km distance to the feeders, therefore indicating that nectar feeders tended to facilitate flower visitation at close distance. Because visitation rates at 100 m and 500 m distance from the feeders did not differ from those at 1.5 km, we suggest that feeders do not draw hummingbirds away from flower resources, but if they have an effect on flower visitation at all, they tend to facilitate flower visitation rather than reduce plant reproduction. 相似文献
998.
999.
For many targets of pharmaceutical importance conformational changes of the receptor protein are relevant during the ligand binding process. A new docking approach, ReFlexIn (Receptor Flexibility by Interpolation), that combines receptor flexibility with the computationally efficient potential grid representation of receptor molecules has been evaluated on the retroviral HIV-1 (Human Immunodeficiency Virus 1) protease system. An approximate inclusion of receptor flexibility is achieved by using interpolation between grid representations of individual receptor conformations. For the retroviral protease the method was tested on an ensemble of protease structures crystallized in the presence of different ligands and on a set of structures obtained from morphing between the unbound and a ligand-bound protease structure. Docking was performed on ligands known to bind to the protease and several non-binders. For the binders the ReFlexIn method yielded in almost all cases ligand placements in similar or closer agreement with experiment than docking to any of the ensemble members without degrading the discrimination with respect to non-binders. The improved docking performance compared to docking to rigid receptors allows for systematic virtual screening applications at very small additional computational cost. 相似文献
1000.