全文获取类型
收费全文 | 2920篇 |
免费 | 185篇 |
专业分类
3105篇 |
出版年
2023年 | 22篇 |
2022年 | 40篇 |
2021年 | 75篇 |
2020年 | 47篇 |
2019年 | 63篇 |
2018年 | 79篇 |
2017年 | 61篇 |
2016年 | 96篇 |
2015年 | 150篇 |
2014年 | 178篇 |
2013年 | 174篇 |
2012年 | 235篇 |
2011年 | 192篇 |
2010年 | 134篇 |
2009年 | 103篇 |
2008年 | 147篇 |
2007年 | 153篇 |
2006年 | 125篇 |
2005年 | 115篇 |
2004年 | 115篇 |
2003年 | 108篇 |
2002年 | 103篇 |
2001年 | 25篇 |
2000年 | 15篇 |
1999年 | 11篇 |
1998年 | 33篇 |
1997年 | 14篇 |
1996年 | 15篇 |
1995年 | 25篇 |
1994年 | 13篇 |
1993年 | 14篇 |
1992年 | 25篇 |
1991年 | 15篇 |
1990年 | 14篇 |
1989年 | 16篇 |
1988年 | 16篇 |
1987年 | 12篇 |
1986年 | 10篇 |
1985年 | 14篇 |
1984年 | 12篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1978年 | 10篇 |
1976年 | 9篇 |
1973年 | 14篇 |
1971年 | 9篇 |
1967年 | 7篇 |
1966年 | 7篇 |
1935年 | 7篇 |
1917年 | 7篇 |
排序方式: 共有3105条查询结果,搜索用时 0 毫秒
991.
Cryptic coloration is an adaptative defensive mechanism against predators. Color patterns can become cryptic through background coloration‐matching and disruptive coloration. Disruptive coloration may evolve in visually heterogeneous microhabitats, whereas background matching could be favored in chromatically homogeneous microhabitats. In this work, we used digital photography to explore the potential use of disruptive coloration and background matching in males and females of two grasshopper species of the Sphenarium genus in different habitats. We found chromatic differences in the two grasshopper species that may be explained by local adaptation. We also found that the females and males of both species are dichromatic and seem to follow different color cryptic strategies, males are more disruptive than females, whereas females have a high background matching with less disruptive elements. The selective pressures of the predators in different microhabitats and the differences in mobility between sexes may explain the color pattern divergence between females and males. Nevertheless, more field experiments are needed in order to understand the relative importance of disruptive and background matching coloration in the evolution of sexual dichromatism in these grasshoppers. 相似文献
992.
993.
Lima RT Seca H Soares P Nascimento MS Vasconcelos MH 《Journal of cellular biochemistry》2011,112(1):200-210
Burkitt lymphoma (BL) commonly exhibits Epstein-Barr virus (EBV) positivity associated with latent chronic infection. Models of acute EBV infection have been associated with cellular resistance to apoptosis. However, the effect of latent long-term EBV infection on apoptosis induced by drugs is not well defined. To determine this, we have studied the response of the Akata EBV+ cell line (type I latency) to etoposide, before and after downregulating EBV gene expression. We observed that downregulating EBV nuclear antigen-1 (EBNA-1) expression with siRNAs reverted cellular sensitivity to etoposide. In accordance with this finding, Akata EBV+ cells showed increased sensitivity to etoposide, when compared to the Akata EBV- cells. We also observed that Akata EBV+ cells presented increased apoptosis levels and decreased Bcl-xL mRNA and protein levels, when compared to the Akata EBV- cells. In addition, Akata EBV+ cells contained less endoplasmic reticulum (ER) than EBV- cells. Finally, downregulation of EBV with EBNA-1 siRNAs caused an increase in the expression of Bcl-xL indicating that EBV is responsible for the differences found between the Akata EBV+ and EBV- cell lines. 相似文献
994.
β-adrenoceptors (βAR) play a central role in the regulation of cAMP synthesis and cardiac contractility. Nucleoside diphosphate kinase B (NDPK B) regulates cAMP signalling by complex formation with Gβγ dimers thereby activating and stabilizing heterotrimeric Gs proteins, key transducer of βAR signals into the cell. Here, we explored the requirement of NDPK B for basal and βAR-stimulated cAMP synthesis and analysed the underlying mechanisms by comparing wild-type NDPK B (WT) and its catalytically inactive H118N mutant. Stable overexpression of both WT- and H118N-NDPK B in cardiomyocyte derived H10 cells increased the plasma membrane content of Gs and caveolin-1 and thus enhanced the isoproterenol (ISO)-stimulated cAMP-synthesis by about 2-fold. Conversely, the loss of NDPK B in embryonic fibroblasts from NDPK A/B-depleted mice was associated with a severe reduction in membranous Gs protein and carveolin-1 content causing a marked decrease in basal and ISO-induced cAMP formation. Re-expression of NDPK B, but not of NDPK A, was able to rescue this phenotype. Both, re-expression of WT- and H118N-NDPK B induced the re-appearance of Gs and caveolin-1 at the plasma membrane to a similar extent. Accordingly, WT- and H118N-NDPK B similarly enhanced ISO-induced cAMP formation. In contrast, the catalytically inactive H118N-NDPK B was less potent and less effective in rescuing basal cAMP production. Identical results were obtained in neonatal rat cardiac myocytes after siRNA-induced knockdown and adenoviral re-expression of NDPK B.Our data reveal that NDPK B regulates Gs function by two different mechanisms. The complex formation of NDPK B with Gs is required for the stabilization of the G protein content at the plasma membrane. In addition, the NDPK B-dependent phosphotransfer reaction, which requires the catalytic activity, specifically allows a receptor-independent, basal Gs activation. 相似文献
995.
Cavazza C Martin L Laffly E Lebrette H Cherrier MV Zeppieri L Richaud P Carrière M Fontecilla-Camps JC 《FEBS letters》2011,585(4):711-715
Escherichia coli require nickel for the synthesis of [NiFe] hydrogenases under anaerobic growth conditions. Nickel import depends on the specific ABC-transporter NikABCDE encoded by the nik operon, which deletion causes the complete abolition of hydrogenase activity. We have previously postulated that the periplasmic binding protein NikA binds a natural metallophore containing three carboxylate functions that coordinate a Ni(II) ion, the fourth ligand being His416, the only direct metal-protein contact, completing a square-planar coordination for the metal. The crystal structure of the H416I mutant showed no electron density corresponding to a metal-chelator complex. In vivo experiments indicate that the mutation causes a significant decrease in nickel uptake and hydrogenase activity. These results confirm the essential role of His416 in nickel transport by NikA. 相似文献
996.
The morphology of the Olduvai Hominid (OH) 8 foot and the sequence of metatarsal epiphyseal fusion in modern humans and chimpanzees support the hypothesis that OH 8 belonged to an individual of approximately the same relative age as the OH 7 subadult, the holotype of Homo habilis. Modern humans and chimpanzees exhibit a variety of metatarsal epiphyseal fusion patterns, including one identical to that observed in OH 8 in which metatarsal 1 fuses before metatarsals 2-5. More than the metatarsal fusion sequence, however, the principal evidence of the youthful age of OH 8 lies in the morphology of metatarsals 1, 2, and 3. Because both OH 8 and OH 7 come from the same stratum at the FLK NN type site, the most parsimonious explanation of the OH 8 and OH 7 data is that this material belonged to the same individual, as originally proposed by Louis Leakey. The proposition that OH 8 belonged to an adult is unsupported by morphology, including radiographic evidence, and the fusion sequences in human and chimpanzee skeletal material reported here and in the literature. 相似文献
997.
Hugo Cruz-Hipolito Antonia Rojano-Delgado Jos�� A. Dom��nguez-Valenzuela Antonio Heredia Mar��a Dolores Luque de Castro Rafael De Prado 《Plant and Soil》2011,347(1-2):221-230
Glyphosate tolerance by Clitoria ternatea, Neonotonia wightii and Amaranthus hybridus was studied in whole plants from Mexico. Experiments in a controlled growth chamber showed both legumes to be highly tolerant of glyphosate, with and ED50 values of 600.18 g ae ha?C1 for C. ternatea and 362.94 g ae ha?C1 for N. wightii. On the other hand, A. hybridus was highly susceptible to the herbicide (ED50?=?42.22 g ae ha?C1). Shikimate accumulation peaked 96 h after treatment in the tolerant plants and the susceptible weed under 500 g ae ha?C1 glyphosate. The shikimic acid content of whole leaves was 4.0 and 5.0 times higher in the susceptible weed than in N. wightii and C. ternatea, respectively. 14C-glyphosate absorption and translocation tests showed A. hybridus to absorb 30% more herbicide than the legumes 24 h after glyphosate foliar application. 14C-glyphosate translocation as measured by quantified autoradiography revealed increased translocation of the herbicide to untreated leaves and roots in A. hybridus relative to the two legumes. The cuticular surface of A. hybridus exhibited very low wax coverage relative to the epicuticular surface of N. wightii and, especially, C. ternatea. No significant degradation of glyphosate to aminomethylphosphonic acid and glyoxylate metabolites was detected among the tolerant leguminous plants or the susceptible weed population. These results indicate that the high glyphosate tolerance of Clitoria ternatea and Neonotonia wightii is mainly a result of poor penetration and translocation of the herbicide to apical growing points in their plants. 相似文献
998.
Henriques ES Brito RM Soares H Ventura S de Oliveira VL Parkhouse RM 《Protein science : a publication of the Protein Society》2011,20(2):247-255
African swine fever virus (ASFV) is a large double-stranded DNA virus responsible for a lethal pig disease, to which no vaccine has ever been obtained. Its genome encodes a number of proteins involved in virus survival and transmission in its hosts, in particular proteins that inhibit signaling pathways in infected macrophages and, thus, interfere with the host's innate immune response. A recently identified novel ASFV viral protein (pI329L) was found to inhibit the Toll-like receptor 3 (TLR3) signaling pathway, TLR3 being a crucial "danger detector." pI329L has been predicted to be a transmembrane protein containing extracellular putative leucine-rich repeats similar to TLR3, suggesting that pI329L might act as a TLR3 decoy. To explore this idea, we used comparative modeling and other structure prediction protocols to propose (a) a model for the TLR3-Toll-interleukin-1 receptor homodimer and (b) a structural fold for pI329L, detailed at atomistic level for its cytoplasmic domain. As this later domain shares only remote sequence relationships with the available TLR3 templates, a more complex modeling strategy was employed that combines the iterative implementation of (multi)threading/assembly/refinement (I-TASSER) structural prediction with expertise-guided posterior refinement. The final pI329L model presents a plausible fold, good structural quality, is consistent with the available experimental data, and it corroborates our hypothesis of pI329L being a TLR3 antagonist. 相似文献
999.
Catherine L. Wram Cedar N. Hesse Zafar A. Handoo F. Hugo Pacheco Inga A. Zasada 《Journal of nematology》2022,54(1)
Heterodera carotae, the carrot cyst nematode, is a significant pest affecting carrot globally. Here we present the draft genome of H. carotae, which was generated from short read libraries from Illumina HiSeq technology, and the corresponding genome annotation. 相似文献
1000.
Filipe Manuel Clemente Gibson Moreira Praa Rodrigo Aquino Daniel Castillo Javier Raya-Gonzlez Markel Rico-Gonzlez Jos Afonso Hugo Sarmento Ana Filipa Silva Rui Silva Rodrigo Ramirez-Campillo 《Biology of sport / Institute of Sport》2023,40(1):111
One of the most often-used task constraints in designing small-sided games (SSGs) is the manipulation of pitch size to promote increases or decreases in the relative area per player. Such adjustments cause changes in the acute responses during SSGs. This systematic review with meta-analysis aimed to compare the effects of smaller vs. larger pitch sizes on soccer players’ physiological, physical, technical, and tactical responses during SSGs. Comparisons between smaller and larger pitches were not considered based on a specific size, but also between using at least two dimensions in the same comparative study, aiming to understand differences between using smaller and larger (independently of the specific dimensions). The data sources utilized were PubMed, PsycINFO, Scielo, Scopus, SPORTDiscus, and Web of Science. The database search initially yielded 249 titles. From those, 41 articles were eligible for the systematic review and meta-analysis. Results revealed that, compared to smaller pitches, SSGs played on larger pitches induced greater values for heart rate (p < 0.001; ES = 0.50), rate of perceived exertion (p < 0.001; ES = 0.70), total distance (p < 0.001; ES = 1.95), high-speed running (p < 0.001; ES = 1.20), stretch index (p < 0.001; ES = 1.02) and surface area (p < 0.001; ES = 1.54). No significant differences were found between pitch size regarding the numbers of accelerations (p = 0.232; ES = 0.45), decelerations (p = 0.111; ES = 0.85), passes (p = 0.897; ES = 0.02), dribbles (p = 0.823; ES = -0.05), or positional centroid (p = 0.053; ES = 0.56). Larger pitch sizes can be implemented as a meaningful task constraint to increase the internal and external load experienced by soccer players during SSGs, as well as to increase the dispersion of players while acting together. These results were found independent of format and age group. 相似文献