全文获取类型
收费全文 | 13094篇 |
免费 | 1117篇 |
国内免费 | 7篇 |
专业分类
14218篇 |
出版年
2022年 | 84篇 |
2021年 | 167篇 |
2020年 | 88篇 |
2019年 | 146篇 |
2018年 | 131篇 |
2017年 | 117篇 |
2016年 | 262篇 |
2015年 | 451篇 |
2014年 | 499篇 |
2013年 | 693篇 |
2012年 | 820篇 |
2011年 | 839篇 |
2010年 | 532篇 |
2009年 | 509篇 |
2008年 | 725篇 |
2007年 | 733篇 |
2006年 | 640篇 |
2005年 | 703篇 |
2004年 | 678篇 |
2003年 | 615篇 |
2002年 | 608篇 |
2001年 | 115篇 |
2000年 | 96篇 |
1999年 | 121篇 |
1998年 | 171篇 |
1997年 | 112篇 |
1996年 | 104篇 |
1995年 | 91篇 |
1994年 | 92篇 |
1993年 | 88篇 |
1992年 | 96篇 |
1991年 | 86篇 |
1990年 | 82篇 |
1989年 | 70篇 |
1988年 | 75篇 |
1987年 | 84篇 |
1986年 | 86篇 |
1985年 | 81篇 |
1984年 | 123篇 |
1983年 | 99篇 |
1982年 | 123篇 |
1981年 | 130篇 |
1980年 | 144篇 |
1979年 | 111篇 |
1978年 | 103篇 |
1977年 | 108篇 |
1976年 | 85篇 |
1975年 | 84篇 |
1974年 | 93篇 |
1973年 | 74篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
Plants,microorganisms, and soil temperatures contribute to a decrease in methane fluxes on a drained Arctic floodplain 下载免费PDF全文
Min Jung Kwon Felix Beulig Iulia Ilie Marcus Wildner Kirsten Küsel Lutz Merbold Miguel D. Mahecha Nikita Zimov Sergey A. Zimov Martin Heimann Edward A. G. Schuur Joel E. Kostka Olaf Kolle Ines Hilke Mathias Göckede 《Global Change Biology》2017,23(6):2396-2412
As surface temperatures are expected to rise in the future, ice‐rich permafrost may thaw, altering soil topography and hydrology and creating a mosaic of wet and dry soil surfaces in the Arctic. Arctic wetlands are large sources of CH4, and investigating effects of soil hydrology on CH4 fluxes is of great importance for predicting ecosystem feedback in response to climate change. In this study, we investigate how a decade‐long drying manipulation on an Arctic floodplain influences CH4‐associated microorganisms, soil thermal regimes, and plant communities. Moreover, we examine how these drainage‐induced changes may then modify CH4 fluxes in the growing and nongrowing seasons. This study shows that drainage substantially lowered the abundance of methanogens along with methanotrophic bacteria, which may have reduced CH4 cycling. Soil temperatures of the drained areas were lower in deep, anoxic soil layers (below 30 cm), but higher in oxic topsoil layers (0–15 cm) compared to the control wet areas. This pattern of soil temperatures may have reduced the rates of methanogenesis while elevating those of CH4 oxidation, thereby decreasing net CH4 fluxes. The abundance of Eriophorum angustifolium, an aerenchymatous plant species, diminished significantly in the drained areas. Due to this decrease, a higher fraction of CH4 was alternatively emitted to the atmosphere by diffusion, possibly increasing the potential for CH4 oxidation and leading to a decrease in net CH4 fluxes compared to a control site. Drainage lowered CH4 fluxes by a factor of 20 during the growing season, with postdrainage changes in microbial communities, soil temperatures, and plant communities also contributing to this reduction. In contrast, we observed CH4 emissions increased by 10% in the drained areas during the nongrowing season, although this difference was insignificant given the small magnitudes of fluxes. This study showed that long‐term drainage considerably reduced CH4 fluxes through modified ecosystem properties. 相似文献
162.
Raghupathy R Witzenburg C Lake SP Sander EA Barocas VH 《Journal of biomechanical engineering》2011,133(9):091011
In a previous work (Raghupathy and Barocas, 2010, "Generalized Anisotropic Inverse Mechanics for Soft Tissues,"J. Biomech. Eng., 132(8), pp. 081006), a generalized anisotropic inverse mechanics method applicable to soft tissues was presented and tested against simulated data. Here we demonstrate the ability of the method to identify regional differences in anisotropy from full-field displacements and boundary forces obtained from biaxial extension tests on soft tissue analogs. Tissue heterogeneity was evaluated by partitioning the domain into homogeneous subdomains. Tests on elastomer samples demonstrated the performance of the method on isotropic materials with uniform and nonuniform properties. Tests on fibroblast-remodeled collagen cruciforms indicated a strong correlation between local structural anisotropy (measured by polarized light microscopy) and the evaluated local mechanical anisotropy. The results demonstrate the potential to quantify regional anisotropic material behavior on an intact tissue sample. 相似文献
163.
Patel G Johnson DS Sun B Pandey M Yu X Egelman EH Wang MD Patel SS 《The Journal of biological chemistry》2011,286(23):20490-20499
The helicase and primase activities of the hexameric ring-shaped T7 gp4 protein reside in two separate domains connected by a linker region. This linker region is part of the subunit interface between monomers, and point mutations in this region have deleterious effects on the helicase functions. One such linker region mutant, A257T, is analogous to the A359T mutant of the homologous human mitochondrial DNA helicase Twinkle, which is linked to diseases such as progressive external opthalmoplegia. Electron microscopy studies show that A257T gp4 is normal in forming rings with dTTP, but the rings do not assemble efficiently on the DNA. Therefore, A257T, unlike the WT gp4, does not preassemble on the unwinding DNA substrate with dTTP without Mg(II), and its DNA unwinding activity in ensemble assays is slow and limited by the DNA loading rate. Single molecule assays measured a 45 times slower rate of A257T loading on DNA compared with WT gp4. Interestingly, once loaded, A257T has almost WT-like translocation and DNA unwinding activities. Strikingly, A257T preassembles stably on the DNA in the presence of T7 DNA polymerase, which restores the ensemble unwinding activity of A257T to ~75% of WT, and the rescue does not require DNA synthesis. The DNA loading rate of A257T, however, remains slow even in the presence of the polymerase, which explains why A257T does not support T7 phage growth. Similar types of defects in the related human mitochondrial DNA helicase may be responsible for inefficient DNA replication leading to the disease states. 相似文献
164.
Nuclear factor-kappa B (NF-κB) is a critical regulator of multiple biological functions including innate and adaptive immunity and cell survival. Activation of NF-κB is tightly regulated to preclude chronic signaling that may lead to persistent inflammation and cancer. Ubiquitination of key signaling molecules by E3 ubiquitin ligases has emerged as an important regulatory mechanism for NF-κB signaling. Deubiquitinases (DUBs) counteract E3 ligases and therefore play a prominent role in the downregulation of NF-κB signaling and homeostasis. Understanding the mechanisms of NF-κB downregulation by specific DUBs such as A20 and CYLD may provide therapeutic opportunities for the treatment of chronic inflammatory diseases and cancer. 相似文献
165.
Evidence suggests that humans might have neurological specializations for music processing, but a compelling adaptationist
account of music and dance is lacking. The sexual selection hypothesis cannot easily account for the widespread performance
of music and dance in groups (especially synchronized performances), and the social bonding hypothesis has severe theoretical
difficulties. Humans are unique among the primates in their ability to form cooperative alliances between groups in the absence
of consanguineal ties. We propose that this unique form of social organization is predicated on music and dance. Music and
dance may have evolved as a coalition signaling system that could, among other things, credibly communicate coalition quality,
thus permitting meaningful cooperative relationships between groups. This capability may have evolved from coordinated territorial
defense signals that are common in many social species, including chimpanzees. We present a study in which manipulation of
music synchrony significantly altered subjects’ perceptions of music quality, and in which subjects’ perceptions of music
quality were correlated with their perceptions of coalition quality, supporting our hypothesis. Our hypothesis also has implications
for the evolution of psychological mechanisms underlying cultural production in other domains such as food preparation, clothing
and body decoration, storytelling and ritual, and tools and other artifacts.
Edward Hagen is a research scientist at the Institute for Theoretical Biology, Humboldt University, Berlin. Gregory Bryant
is a doctoral candidate in the Department of Psychology, University of California, Santa Cruz. 相似文献
166.
Santoro F Greenstone HL Insinga A Liszewski MK Atkinson JP Lusso P Berger EA 《The Journal of biological chemistry》2003,278(28):25964-25969
Human herpesvirus 6 (HHV-6) employs the complement regulator CD46 (membrane cofactor protein) as a receptor for fusion and entry into target cells. Like other known herpesviruses, HHV-6 encodes multiple glycoproteins, several of which have been implicated in the entry process. In this report, we present evidence that glycoprotein H (gH) is the viral component responsible for binding to CD46. Antibodies to CD46 co-immunoprecipitated an approximately 110-kDa protein band specifically associated with HHV-6-infected cells. This protein was identified as gH by selective depletion with an anti-gH monoclonal antibody, as well as by immunoblot analysis with a rabbit hyperimmune serum directed against a gH synthetic peptide. In reciprocal experiments, a monoclonal antibody against HHV-6 gH was found to co-immunoprecipitate CD46. Studies using monoclonal antibodies directed against specific CD46 domains, as well as engineered constructs lacking defined CD46 regions, demonstrated a close correspondence between the CD46 domains involved in the interaction with gH and those previously shown to be critical for HHV-6 fusion (i.e. short consensus repeats 2 and 3). 相似文献
167.
Brooke EW Davies SG Mulvaney AW Okada M Pompeo F Sim E Vickers RJ Westwood IM 《Bioorganic & medicinal chemistry letters》2003,13(15):2527-2530
The synthesis and inhibitory activity of a series of 5-substituted-(1,1-dioxo-2,3-dihydro-1H-1 lambda(6)-benzo[e][1,2]thiazin-4-ylidene)-thiazolidine-2,4-dione derivatives as competitive inhibitors of recombinant bacterial arylamine-N-acetyltransferases (NATs) are described. The most potent NAT inhibitors are those that contain planar hydrophobic substituents on the sultam nitrogen. 相似文献
168.
169.
Sujay K. Singh Edward K. Wakeland Ivica Vučak Zoltan A. Nagy Jan Klein 《Immunogenetics》1981,14(3-4):273-281
The B10.STA62 strain carries the H-2
w27 haplotype derived from a wild mouse captured in the vicinity of Ann Arbor, Michigan. Products of two class II loci composing this haplotype, A
and A
, are serologically, biochemically (by tryptic peptide mapping), and functionally indistinguishable from products controlled by the A
b
and A
/b
genes of the B10.A(5R) strain. In contrast, the polypeptide chain controlled by the third class II locus, E
, is different from that controlled by the E
/b
gene. This E
/w27
chain lacks an antigenic determinant present on the Eb molecule and carries determinants lacking on the Eb molecule, the E
/b
and E
/w27
peptide maps differ in at least six peptides, and cytotoxic T cells specific for the E
b
chains do not react with B10.STA62 target cells. This great difference between the E
/b
and E
/w27
chains suggests that the corresponding genes have not been derived from one another by a direct mutational conversion; instead, H-2
w27 appears to be a recombinant haplotype derived by crossing-over between the A
A
duplex and the E
locus. This is the first recombinant discovered separating these class II loci. 相似文献
170.
Justin R. Prigge James A. Wiley Emily A. Talago Elise M. Young Laura L. Johns Jean A. Kundert Katherine M. Sonsteng William P. Halford Mario R. Capecchi Edward E. Schmidt 《Mammalian genome》2013,24(9-10):389-399
Cre-responsive dual-fluorescent alleles allow in situ marking of cell lineages or genetically modified cells. Here we report a dual-fluorescent allele, ROSA nT-nG , which directs nuclear accumulation of tdTomato in Cre-naïve lineages. Cre converts the allele to ROSA nG , which drives nuclear EGFP accumulation. Conditions were established for analyzing marked nuclei by flow cytometry on the basis of red–green fluorescence and ploidy, with a particular focus on liver nuclei. Hydrodynamic delivery of a Cre-expression plasmid was used to time-stamp arbitrary hepatocytes for lineage tracing. The distinct green fluorescence of nuclei from Cre-exposed lineages facilitated analyses of ploidy transitions within clones. To assess developmental transitions in liver nuclei, ROSA nT-nG was combined with the hepatocyte-specific AlbCre transgene, facilitating discrimination between hepatocyte and nonhepatocyte nuclei. Nuclei extracted from postnatal day 2 (P2) livers were 41 % green and 59 % red and reached a stable level of 84 % green by P22. Until P20, green nuclei were >98 % diploid (2N); at P40 they were ~56 % 2N, 43 % 4N, and <1 % 8N; and by P70 they reached a stable distribution of ~46 % 2N, 45 % 4N, and 9 % 8N. In conclusion, ROSA nT-nG will facilitate in vivo and ex vivo studies on liver and will likely be valuable for studies on tissues like muscle, kidney, or brain in which cells are refractory to whole-cell flow cytometry, or like trophectoderm derivatives or cancers in which cells undergo ploidy transitions. 相似文献