全文获取类型
收费全文 | 6654篇 |
免费 | 452篇 |
国内免费 | 4篇 |
专业分类
7110篇 |
出版年
2022年 | 38篇 |
2021年 | 72篇 |
2020年 | 37篇 |
2019年 | 62篇 |
2018年 | 66篇 |
2017年 | 59篇 |
2016年 | 103篇 |
2015年 | 195篇 |
2014年 | 233篇 |
2013年 | 306篇 |
2012年 | 346篇 |
2011年 | 375篇 |
2010年 | 297篇 |
2009年 | 254篇 |
2008年 | 349篇 |
2007年 | 372篇 |
2006年 | 365篇 |
2005年 | 344篇 |
2004年 | 359篇 |
2003年 | 347篇 |
2002年 | 357篇 |
2001年 | 96篇 |
2000年 | 81篇 |
1999年 | 106篇 |
1998年 | 129篇 |
1997年 | 85篇 |
1996年 | 85篇 |
1995年 | 99篇 |
1994年 | 76篇 |
1993年 | 88篇 |
1992年 | 71篇 |
1991年 | 62篇 |
1990年 | 76篇 |
1989年 | 55篇 |
1988年 | 56篇 |
1987年 | 39篇 |
1986年 | 53篇 |
1985年 | 38篇 |
1984年 | 64篇 |
1983年 | 53篇 |
1982年 | 68篇 |
1981年 | 73篇 |
1980年 | 60篇 |
1979年 | 56篇 |
1978年 | 45篇 |
1977年 | 43篇 |
1976年 | 34篇 |
1975年 | 34篇 |
1974年 | 38篇 |
1973年 | 18篇 |
排序方式: 共有7110条查询结果,搜索用时 15 毫秒
971.
Both mating and reproduction strongly affect the physiology of insect females. In the ant Cardiocondyla obscurior, a comparison among virgin queens, mated queens, and queens mated with sterilized males ("sham-mated") allows to separate the different effects of mating and egg laying. Here, we investigate whether and how different mating status is reflected in the cuticular lipid profiles of queens, i.e., the blend of chemicals that is thought to signal a queen's fertility. Surprisingly, discriminant analyses failed to reliably distinguish among virgin, mated, and sham-mated queens. A generalized linear model on individual substances showed only very subtle differences. While mating appeared to be positively associated with the proportions of 3-MeC(25,) 11-/13-MeC(27), 5-MeC(27), 3-MeC(27), and 12-/14-MeC(28) and negatively with C(27:1), fecundity was negatively associated with C(29:1), C(31:1), and a sterol derivative. We discuss these results in the light of the special life history of C. obscurior, with completely sterile workers and low egg laying rates in queens. 相似文献
972.
973.
Recent investigations suggest that genes that confer risk for panic disorder (PD) may moderate response to panicogenic agents in healthy volunteers. Given the potential role of the central cholecystokinin receptor (CCKBR) (CT) polymorphism alleles 26 and 27 in PD, the present study attempted to discern if these alleles moderated panicogenic sensitivity to the CCKBR agonist, CCK-tetrapeptide (CCK-4), in healthy volunteers. The study group consisted of 92 men and women with no personal or family history of psychiatric illness. Participants provided blood samples for genotyping of the CCKBR alleles and they received a 25 μg bolus injection of CCK-4. Behavioral, cardiovascular and hormonal responses to the peptide were assessed and analyzed with adjusted linear regression models. Carriers of the CCKBR alleles tended to have higher levels of pre-challenge anxiety and significantly higher levels of anxiety sensitivity and introversion than those without the alleles. However, they did not exhibit an enhanced panicogenic response to CCK-4. Overall, our findings do not demonstrate a role of these alleles in modulating CCK-4's panicogenicity. The significant association between the risk alleles and anxiety-related personality traits is intriguing and further exploration of this association is merited. 相似文献
974.
Kawabe Y Weening KE Marquay-Markiewicz J Schaap P 《Development (Cambridge, England)》2012,139(7):1336-1345
Dictyostelium discoideum amoebas coordinate aggregation and morphogenesis by secreting cyclic adenosine monophosphate (cAMP) pulses that propagate as waves through fields of cells and multicellular structures. To retrace how this mechanism for self-organisation evolved, we studied the origin of the cAMP phosphodiesterase PdsA and its inhibitor PdiA, which are essential for cAMP wave propagation. D. discoideum and other species that use cAMP to aggregate reside in group 4 of the four major groups of Dictyostelia. We found that groups 1-3 express a non-specific, low affinity orthologue of PdsA, which gained cAMP selectivity and increased 200-fold in affinity in group 4. A low affinity group 3 PdsA only partially restored aggregation of a D. discoideum pdsA-null mutant, but was more effective at restoring fruiting body morphogenesis. Deletion of a group 2 PdsA gene resulted in disruption of fruiting body morphogenesis, but left aggregation unaffected. Together, these results show that groups 1-3 use a low affinity PdsA for morphogenesis that is neither suited nor required for aggregation. PdiA belongs to a family of matrix proteins that are present in all Dictyostelia and consist mainly of cysteine-rich repeats. However, in its current form with several extensively modified repeats, PdiA is only present in group 4. PdiA is essential for initiating spiral cAMP waves, which, by organising large territories, generate the large fruiting structures that characterise group 4. We conclude that efficient cAMP-mediated aggregation in group 4 evolved by recruitment and adaptation of a non-selective phosphodiesterase and a matrix component into a system for regulated cAMP degradation. 相似文献
975.
George E. Cutsail III Peter E. Doan Brian M. Hoffman Jacques Meyer Joshua Telser 《Journal of biological inorganic chemistry》2012,17(8):1137-1150
We have employed EPR and a set of recently developed electron nuclear double resonance (ENDOR) spectroscopies to characterize a suite of [2Fe?C2S] ferredoxin clusters from Aquifex aeolicus (Aae Fd1, Fd4, and Fd5). Antiferromagnetic coupling between the FeII, S?=?2, and FeIII, S?=?5/2, sites of the [2Fe?C2S]+ cluster in these proteins creates an S?=?1/2 ground state. A complete discussion of the spin-Hamiltonian contributions to g includes new symmetry arguments along with references to related FeS model compounds and their symmetry and EPR properties. Complete 57Fe hyperfine coupling (hfc) tensors for each iron, with respective orientations relative to g, have been determined by the use of ??stochastic?? continuous wave and/or ??random hopped?? pulsed ENDOR, with the relative utility of the two approaches being emphasized. The reported hyperfine tensors include absolute signs determined by a modified pulsed ENDOR saturation and recovery (PESTRE) technique, RD-PESTRE??a post-processing protocol of the ??raw data?? that comprises an ENDOR spectrum. The 57Fe hyperfine tensor components found by ENDOR are nicely consistent with those previously found by M?ssbauer spectroscopy, while accurate tensor orientations are unique to the ENDOR approach. These measurements demonstrate the capabilities of the newly developed methods. The high-precision hfc tensors serve as a benchmark for this class of FeS proteins, while the variation in the 57Fe hfc tensors as a function of symmetry in these small FeS clusters provides a reference for higher-nuclearity FeS clusters, such as those found in nitrogenase. 相似文献
976.
Wang W Ortiz Rdel C Jacques FM Xiang XG Li HL Lin L Li RQ Liu Y Soltis PS Soltis DE Chen ZD 《The New phytologist》2012,195(2):470-478
? Modern tropical rainforests have the highest biodiversity of terrestrial biomes and are restricted to three low-latitude areas. However, the actual timeframe during which tropical rainforests began to appear on a global scale has been intensely disputed. Here, we used the moonseed family (Menispermaceae), an important physiognomic and structural component of tropical rainforests on a worldwide basis, to obtain new insights into the diversification of this biome. ? We integrated phylogenetic, biogeographic and molecular dating methods to analyse temporal and spatial patterns of global diversification in Menispermaceae. ? Importantly, a burst of moonseed diversification occurred in a narrow window of time, which coincides with the Cretaceous-Paleogene (K-Pg) boundary. Our data also suggest multiple independent migrations from a putative ancestral area of Indo-Malay into other tropical regions. ? Our data for Menispermaceae suggest that modern tropical rainforests may have appeared almost synchronously throughout the three major tropical land areas close to, or immediately following, the K-Pg mass extinction. 相似文献
977.
Xenopus laevis provides a unique animal model, alternative to mouse, to study immunology. Even though, several methodologies have been developed for the generation of transgenic Xenopus, to date none have been adapted for the X. laevis/gilli (LG) isogenetic clones that are essential for immunological studies. Since LG clones are generated via gynogenesis, transgenic methods using transgene integration into the sperm nuclei are not suited. Therefore, we have tested three alternative methods for LG transgenesis: the phiC31 integrase, the Sleeping Beauty transposase, and the I-SceI meganuclease. All three techniques produced transgenic LG clones; however, the I-SceI meganuclease was most effective. It resulted in high transgenesis efficiency (35-50%), bright nonmosaic GFP expression as well as stable germline transmission with 100% of the progeny carrying the transgene. Production of transgenic LG clones will allow us to modulate immune gene expression and further strengthen X. laevis as a biomedical model. 相似文献
978.
Müller AC Breitwieser FP Fischer H Schuster C Brandt O Colinge J Superti-Furga G Stingl G Elbe-Bürger A Bennett KL 《Journal of proteome research》2012,11(7):3715-3727
Aberrations in skin morphology and functionality can cause acute and chronic skin-related diseases that are the focus of dermatological research. Mechanically induced skin suction blister fluid may serve as a potential, alternative human body fluid for quantitative mass spectrometry (MS)-based proteomics in order to assist in the understanding of the mechanisms and causes underlying skin-related diseases. The combination of abundant-protein removal with iTRAQ technology and multidimensional fractionation techniques improved the number of identified protein groups. A relative comparison of a cohort of 8 healthy volunteers was thus recruited in order to assess the net variability encountered in a healthy scenario. The technology enabled the identification, to date, of the highest number of reported protein groups (739) with concomitant relative quantitative data for over 90% of all proteins with high reproducibility and accuracy. The use of iTRAQ 8-plex resulted in a 66% decrease in protein identifications but, despite this, provided valuable insight into interindividual differences of the healthy control samples. The geometric mean ratio was close to 1 with 95% of all ratios ranging between 0.45 and 2.05 and a calculated mean coefficient of variation of 15.8%, indicating a lower biological variance than that reported for plasma or urine. By applying a multistep sample processing, the obtained sensitivity and accuracy of quantitative MS analysis demonstrates the prospective value of the approach in future research into skin diseases. 相似文献
979.
980.
Brasme JF Grill J Doz F Lacour B Valteau-Couanet D Gaillard S Delalande O Aghakhani N Puget S Chalumeau M 《PloS one》2012,7(4):e33415