全文获取类型
收费全文 | 9613篇 |
免费 | 883篇 |
国内免费 | 1篇 |
专业分类
10497篇 |
出版年
2023年 | 75篇 |
2022年 | 126篇 |
2021年 | 306篇 |
2020年 | 149篇 |
2019年 | 191篇 |
2018年 | 254篇 |
2017年 | 220篇 |
2016年 | 348篇 |
2015年 | 568篇 |
2014年 | 609篇 |
2013年 | 743篇 |
2012年 | 804篇 |
2011年 | 874篇 |
2010年 | 517篇 |
2009年 | 400篇 |
2008年 | 609篇 |
2007年 | 596篇 |
2006年 | 470篇 |
2005年 | 445篇 |
2004年 | 412篇 |
2003年 | 321篇 |
2002年 | 305篇 |
2001年 | 84篇 |
2000年 | 76篇 |
1999年 | 75篇 |
1998年 | 64篇 |
1997年 | 52篇 |
1996年 | 46篇 |
1995年 | 60篇 |
1994年 | 40篇 |
1993年 | 33篇 |
1992年 | 32篇 |
1991年 | 38篇 |
1990年 | 36篇 |
1989年 | 29篇 |
1988年 | 29篇 |
1987年 | 28篇 |
1986年 | 27篇 |
1985年 | 22篇 |
1984年 | 41篇 |
1983年 | 21篇 |
1982年 | 15篇 |
1981年 | 23篇 |
1980年 | 15篇 |
1978年 | 15篇 |
1977年 | 20篇 |
1976年 | 18篇 |
1975年 | 19篇 |
1974年 | 15篇 |
1973年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Dawe AS Nylund R Leszczynski D Kuster N Reader T De Pomerai DI 《Bioelectromagnetics》2008,29(2):92-99
Recent data suggest that there might be a subtle thermal explanation for the apparent induction by radiofrequency (RF) radiation of transgene expression from a small heat-shock protein (hsp16-1) promoter in the nematode, Caenorhabditis elegans. The RF fields used in the C. elegans study were much weaker (SAR 5-40 mW kg(-1)) than those routinely tested in many other published studies (SAR approximately 2 W kg(-1)). To resolve this disparity, we have exposed the same transgenic hsp16-1::lacZ strain of C. elegans (PC72) to higher intensity RF fields (1.8 GHz; SAR approximately 1.8 W kg(-1)). For both continuous wave (CW) and Talk-pulsed RF exposures (2.5 h at 25 degrees C), there was no indication that RF exposure could induce reporter expression above sham control levels. Thus, at much higher induced RF field strength (close to the maximum permitted exposure from a mobile telephone handset), this particular nematode heat-shock gene is not up-regulated. However, under conditions where background reporter expression was moderately elevated in the sham controls (perhaps as a result of some unknown co-stressor), we found some evidence that reporter expression may be reduced by approximately 15% following exposure to either Talk-pulsed or CW RF fields. 相似文献
142.
143.
144.
145.
146.
Cellular compartmentation of calcium in mouse fibroblasts in vitro depends on cell density and transformation 总被引:1,自引:0,他引:1
Cellular compartmentation of Ca has been investigated by kinetic analysis of 45Ca efflux from preloaded cells at various states of cell density-dependent proliferation of normal (3T3) and transformed (3T6 and SV40-3T3) mouse cells. Three pools of exchangeable calcium were separated on the basis of their differing exchange kinetics. For each of the cell lines tested, all three compartments decrease with cell density. Significant differences between normal and transformed cells are observed upon quiescence of the normal cells, where the slowly exchanging compartment of normal cells gradually increases, whereas that of the transformed cells continues to decrease (with increasing cell density). Free cytoplasmic Ca2+ concentration as determined by the Quin 2 method, was found to be significantly higher in transformed cells than in normal cells. These results indicate significant differences in Ca homeostasis between normal and transformed cells. 相似文献
147.
We investigated the firing rate of motor units in the vastus lateralis muscle in five healthy young men (mean = 21.4 yr, SD = 0.9) during a sequence of isometric constant-torque contractions repeated to exhaustion. The contractions were sustained at 20% of the maximal voluntary level, measured at the beginning of the test sequence. Electromyographic (EMG) signals were recorded via quadrifilar fine-wire electrodes and subsequently decomposed into their constituent motor unit action potentials to obtain the motor unit firing times. In addition, we measured the whole muscle mechanical properties during the fatigue task using electrical stimulation. The firing rate of motor units first decreased within the first 10-20% of the endurance time of the contractions and then increased. The firing rate increase was accompanied by recruitment of additional motor units as the force output remained constant. The elicited twitch and tetanic torque responses first increased and then decreased. The two processes modulated in a complementary fashion at the same time. Our data suggest that, when the vastus lateralis muscle is activated to maintain a constant torque output, its motoneuron pool receives a net excitatory drive that first decreases to compensate for the short-lived potentiation of the muscle force twitch and then increases to compensate for the diminution of the force twitch. The underlying inverse relationship between the firing rate and the recruitment threshold that has been reported for nonfatigued contractions is maintained. We, therefore, conclude that the central nervous system control of vastus lateralis motor units remains invariant during fatigue in submaximal isometric isotonic contractions. 相似文献
148.
Pilon-Thomas S Verhaegen M Kuhn L Riker A Mulé JJ 《Cancer immunology, immunotherapy : CII》2006,55(10):1238-1246
Due to the pivotal role that dendritic cells (DC) play in eliciting and maintaining functional anti-tumor T cell responses, these APC have been exploited against tumors. DC express several receptors for the Fc portion of IgG (Fcγ receptors) that mediate the internalization of antigen-IgG complexes and promote efficient MHC class I and II restricted antigen presentation. In this study, the efficacy of vaccination with DC pulsed with apoptotic B16 melanoma cells opsonized with an anti-CD44 IgG (B16-CD44) was explored. Immature bone marrow derived DC grown in vitro with IL-4 and GM-CSF were pulsed with B16-CD44. After 48 h of pulsing, maturation of DC was demonstrated by production of IL-12 and upregulation of CD80 and CD40 expression. To test the efficacy of vaccination with DC+B16-CD44, mice were vaccinated subcutaneously Lymphocytes from mice vaccinated with DC+B16-CD44 produced IFN-γ in response to B16 melanoma lysates as well as an MHC class I restricted B16 melanoma-associated peptide, indicating B16 specific CD8 T cell activation. Upon challenge with viable B16 cells, all mice vaccinated with DC alone developed tumor compared to 40% of mice vaccinated with DC+B16-CD44; 60% of the latter mice remained tumor free for at least 8 months. In addition, established lung tumors and distant metastases were significantly reduced in mice treated with DC+B16-CD44. Lastly, delayed growth of established subcutaneous tumors was induced by combination therapy with anti-CD44 antibodies followed by DC injection. This study demonstrates the efficacy of targeting tumor antigens to DC via Fcγ receptors. 相似文献
149.
150.
Adam NR Wall GW Kimball BA Pinter PJ Lamorte RL Hunsaker DJ Adamsen FJ Thompson T Matthias AD Leavitt SW Webber AN 《Photosynthesis research》2000,66(1-2):65-77
We have examined the photosynthetic acclimation of wheat leaves grown at an elevated CO2 concentration, and ample and limiting N supplies, within a field experiment using free-air CO2 enrichment (FACE). To understand how leaf age and developmental stage affected any acclimation response, measurements were
made on a vertical profile of leaves every week from tillering until maturity. The response of assimilation (A) to internal
CO2 concentration (Ci) was used to estimate the in vivo carboxylation capacity (Vcmax) and maximum rate of ribulose-1,5-bisphosphate limited photosynthesis (A
sat). The total activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), and leaf content of Rubisco and the Light
Harvesting Chlorophyll a/b protein associated with Photosystem II (LHC II), were determined. Elevated CO2 did not alter Vcmax in the flag leaf at either low or high N. In the older shaded leaves lower in the canopy, acclimatory decline in Vcmax and A
sat was observed, and was found to correlate with reduced Rubisco activity and content. The dependency of acclimation on N supply
was different at each developmental stage. With adequate N supply, acclimation to elevated CO2 was also accompanied by an increased LHC II/Rubisco ratio. At low N supply, contents of Rubisco and LHC II were reduced in
all leaves, although an increased LHC II/Rubisco ratio under elevated CO2 was still observed. These results underscore the importance of leaf position, leaf age and crop developmental stage in understanding
the acclimation of photosynthesis to elevated CO2 and nutrient stress.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献