全文获取类型
收费全文 | 13738篇 |
免费 | 1261篇 |
国内免费 | 877篇 |
出版年
2024年 | 47篇 |
2023年 | 274篇 |
2022年 | 296篇 |
2021年 | 414篇 |
2020年 | 479篇 |
2019年 | 731篇 |
2018年 | 611篇 |
2017年 | 605篇 |
2016年 | 568篇 |
2015年 | 534篇 |
2014年 | 649篇 |
2013年 | 1334篇 |
2012年 | 407篇 |
2011年 | 577篇 |
2010年 | 525篇 |
2009年 | 729篇 |
2008年 | 815篇 |
2007年 | 680篇 |
2006年 | 691篇 |
2005年 | 547篇 |
2004年 | 572篇 |
2003年 | 473篇 |
2002年 | 413篇 |
2001年 | 307篇 |
2000年 | 312篇 |
1999年 | 288篇 |
1998年 | 263篇 |
1997年 | 218篇 |
1996年 | 215篇 |
1995年 | 192篇 |
1994年 | 152篇 |
1993年 | 127篇 |
1992年 | 127篇 |
1991年 | 112篇 |
1990年 | 78篇 |
1989年 | 69篇 |
1988年 | 57篇 |
1987年 | 66篇 |
1986年 | 54篇 |
1985年 | 57篇 |
1984年 | 44篇 |
1983年 | 19篇 |
1982年 | 31篇 |
1981年 | 34篇 |
1980年 | 22篇 |
1979年 | 10篇 |
1978年 | 17篇 |
1977年 | 11篇 |
1976年 | 9篇 |
1973年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
This research presents the results of constructing and parameterizing an individual-based model of spatiotemporal dynamics of mixed forest stands. The model facilitates computerized experiments with forest stands having different combinations of species and age structures. These forest stands grow on temperate areas where light is the main system-forming factor that shapes and develops forest ecosystems. The model TEMFORM (TEMperate FORests Model) is developed with few equations and parameters, most of which can be estimated using standard forest inventory data. Parameterization of the model used the growth tables of a set of basic forest-forming species in Far East Russia. Simulation results of the development of the natural single- and mixed-species stands and the effects of different types of disturbances on the stand dynamics and compositions are presented. 相似文献
22.
Siddhartha Kumar Bhaumik Manoj Kumar Singh Subir Karmakar Tripti De 《Glycoconjugate journal》2009,26(6):663-673
As compared to cutaneous leishmaniasis, vaccination against visceral leishmaniasis (VL) has received limited attention. In
this study, we demonstrate for the first time that an UDP-Galactose: N-acetylglucosamine β 1–4 galactosyltransferase (GenBank Accession No. EF159943) expressing attenuated LD clonal population (A-LD) is able to confer protection against the experimental challenge with the virulent LD AG83 parasite. A-LD was also effective in established leishmania infection. The vaccinated animals showed both cell mediated
(in vitro T-cell proliferation, and DTH response) and humoral responses (Th1 type). These results demonstrate the potential of the
attenuated clones as an immunotherapeutic and immunoprophylactic agent against visceral leishmaniasis. 相似文献
23.
24.
YONG-JIANG ZHANG FREDERICK C. MEINZER GUANG-YOU HAO FABIAN G. SCHOLZ SANDRA J. BUCCI FREDERICO S. C. TAKAHASHI RANDOL VILLALOBOS-VEGA JUAN P. GIRALDO KUN-FANG CAO WILLIAM A. HOFFMANN & GUILLERMO GOLDSTEIN 《Plant, cell & environment》2009,32(10):1456-1466
Size-related changes in hydraulic architecture, carbon allocation and gas exchange of Sclerolobium paniculatum (Leguminosae), a dominant tree species in Neotropical savannas of central Brazil (Cerrado), were investigated to assess their potential role in the dieback of tall individuals. Trees greater than ∼6-m-tall exhibited more branch damage, larger numbers of dead individuals, higher wood density, greater leaf mass per area, lower leaf area to sapwood area ratio (LA/SA), lower stomatal conductance and lower net CO2 assimilation than small trees. Stem-specific hydraulic conductivity decreased, while leaf-specific hydraulic conductivity remained nearly constant, with increasing tree size because of lower LA/SA in larger trees. Leaves were substantially more vulnerable to embolism than stems. Large trees had lower maximum leaf hydraulic conductance ( K leaf ) than small trees and all tree sizes exhibited lower K leaf at midday than at dawn. These size-related adjustments in hydraulic architecture and carbon allocation apparently incurred a large physiological cost: large trees received a lower return in carbon gain from their investment in stem and leaf biomass compared with small trees. Additionally, large trees may experience more severe water deficits in dry years due to lower capacity for buffering the effects of hydraulic path-length and soil water deficits. 相似文献
25.
26.
Plants often suffer reductions in fecundity due to insect herbivory. Whether this loss of seeds has population-level consequences
is much debated and often unknown. For many plants, particularly those with long-lived seedbanks, it is frequently asserted
that herbivores have minimal impacts on plant abundance because safe-site availability rather than absolute seed number determines
the magnitude of future plant recruitment and hence population abundance. However, empirical tests of this assertion are generally
lacking and the interplay between herbivory, spatio-temporal variability in seed- or safe-site-limited recruitment, and seedbank
dynamics is likely to be complex. Here we use a stochastic simulation model to explore how changes in the spatial and temporal
frequency of seed-limited recruitment, the strength of density-dependent seedling survival, and longevity of seeds in the
soil influence the population response to herbivory. Model output reveals several surprising results. First, given a seedbank,
herbivores can have substantial effects on mean population abundance even if recruitment is primarily safe-site-limited in
either time or space. Second, increasing seedbank longevity increases the population effects of herbivory, because annual
reductions in seed input due to herbivory are accumulated in the seedbank. Third, population impacts of herbivory are robust
even in the face of moderately strong density-dependent seedling mortality. These results imply that the conditions under
which herbivores influence plant population dynamics may be more widespread than heretofore expected. Experiments are now
needed to test these predictions.
Received: 3 November 1999 / Accepted: 15 February 2000 相似文献
27.
The adsorption and immobilisation of human insulin onto the bio-compatible nanosheets including graphene monoxide, silicon carbide and boron nitride nanosheets were studied by molecular dynamics simulation at the temperature of 310 K. After equilibration, heating and 100 ns production molecular dynamic runs, it was found that the insulin was adsorbed and immobilised onto the considered surfaces in a native folded state. The structural parameters, including root-mean-square deviation and fluctuation, surface accessible solvent area, radius of gyration (Rg) and the distance between the centre of the mass of immobilised protein and the surface of the considered nanosheets, were measured, analysed and discussed. The energetics of the studied systems such as the interaction energy between protein and nanosheet was also measured and addressed. The discussions were centred on the structural and energetic parameters of the protein and nanosheets, including charge density, hydrophobicity, hydrophilicity and residue polarity. The results also showed that the active site of C-termini of chain B played an important role in the adsorption process and this could be helpful in the protection of insulin in its smart delivery and release applications. 相似文献
28.
Miguel R. Lugo Ravikiran Ravulapalli Debajyoti Dutta 《Journal of biomolecular structure & dynamics》2016,34(12):2537-2560
C3larvin toxin is a new member of the C3 class of the mono-ADP-ribosyltransferase toxin family. The C3 toxins are known to covalently modify small G-proteins, e.g. RhoA, impairing their function, and serving as virulence factors for an offending pathogen. A full-length X-ray structure of C3larvin (2.3 Å) revealed that the characteristic mixed α/β fold consists of a central β-core flanked by two helical regions. Topologically, the protein can be separated into N and C lobes, each formed by a β-sheet and an α-motif, and connected by exposed loops involved in the recognition, binding, and catalysis of the toxin/enzyme, i.e. the ADP-ribosylation turn–turn and phosphate–nicotinamide PN loops. Herein, we provide two new C3larvin X-ray structures and present a systematic study of the toxin dynamics by first analyzing the experimental variability of the X-ray data-set followed by contrasting those results with theoretical predictions based on Elastic Network Models (GNM and ANM). We identify residues that participate in the stability of the N-lobe, putative hinges at loop residues, and energy-favored deformation vectors compatible with conformational changes of the key loops and 3D-subdomains (N/C-lobes), among the X-ray structures. We analyze a larger ensemble of known C3bot1 conformations and conclude that the characteristic ‘crab-claw’ movement may be driven by the main intrinsic modes of motion. Finally, via computational simulations, we identify harmonic and anharmonic fluctuations that might define the C3larvin ‘native state.’ Implications for docking protocols are derived. 相似文献
29.
Takashi Nagai 《人类与生态风险评估》2014,20(3):641-657
An algal population growth model integrated with toxicokinetics was developed for assessing the effect of pesticides on population dynamics. This model is a simple one-compartment, first-order kinetic model in which toxicity (growth inhibition and mortality) depends on the intracellular effective concentration of a pesticide at a target site. The model's parameters were derived using an experimental study that investigated the effects of pretilachlor, bensulfuron-methyl, pentoxazone, and quinoclamine on the growth, mortality, and subsequent population recovery of the green alga Pseudokirchneriella subcapitata. Modeled and measured trajectories of algal population dynamics agreed well. The effect on population recovery was underestimated by the model that ignored the toxicokinetics. The four tested herbicides had a variety of toxicity characteristics and physicochemical properties, indicating the wide range of the model's applicability. Moreover, the developed model and the obtained model's parameters were extrapolated to predict long-term algal population dynamics under time-varying herbicide exposure. The calculated integral biomass lost compared with the control was considered a quantitative index of the population-level ecological risk. The model's prediction showed that the same exposure level (peak concentration is equivalent to EC50) indicated much different population-level effect depending on the herbicide. 相似文献
30.
《Bioorganic & medicinal chemistry letters》2014,24(22):5330-5336
Free fatty acid receptor 1 (FFAR1) is a member of a previously characterized cluster of orphan G protein-coupled receptors (GPCRs). Later, this orphan receptor was identified as a target of medium- to long-chain free fatty acids in β-cells of the pancreas. Administration of FFAR1 agonists has been proved to potentiate glucose-stimulated insulin secretion from pancreatic β-cells. It was reported that some thiazolidinediones (TZDs), the best studied PPARγ agonists, are also able to stimulate FFAR1 in a dose-dependent manner. In the present study, a homology model of the human FFAR1 was constructed and inserted into a pre-equilibrated DPPC/TIP3P membrane system. This system was then simulated for 20 ns in complex with the FFAR1 agonist GW9085, as well as rosiglitazone and pioglitazone. We noticed that the salt bridge between Glu172 and Arg258 and the H bond between Glu145 and His153 could be responsible for the stabilization of the receptor in the inactive state. Moreover, we described for the first time the binding mode of TZDs in the binding site of FFAR1. The thiazolidinedione head forms a hydrogen bonding network with the critical polar residues in the binding site, Arg258 and Asn244, while the rest of the molecule is embedded into the receptor hydrophobic pocket. Based on this modeling study, we arrived at a proposal of the pharmacophore required for binding to both PPARγ and FFAR1. Insights gained from this investigation should provide future directions for the design of novel dual acting antidiabetic agents. 相似文献