全文获取类型
收费全文 | 43116篇 |
免费 | 3868篇 |
国内免费 | 31篇 |
专业分类
47015篇 |
出版年
2023年 | 166篇 |
2022年 | 309篇 |
2021年 | 599篇 |
2020年 | 395篇 |
2019年 | 489篇 |
2018年 | 641篇 |
2017年 | 555篇 |
2016年 | 1044篇 |
2015年 | 1758篇 |
2014年 | 1880篇 |
2013年 | 2456篇 |
2012年 | 2979篇 |
2011年 | 3057篇 |
2010年 | 1946篇 |
2009年 | 1657篇 |
2008年 | 2517篇 |
2007年 | 2508篇 |
2006年 | 2271篇 |
2005年 | 2339篇 |
2004年 | 2291篇 |
2003年 | 2181篇 |
2002年 | 2130篇 |
2001年 | 507篇 |
2000年 | 406篇 |
1999年 | 510篇 |
1998年 | 606篇 |
1997年 | 462篇 |
1996年 | 411篇 |
1995年 | 393篇 |
1994年 | 355篇 |
1993年 | 345篇 |
1992年 | 386篇 |
1991年 | 328篇 |
1990年 | 271篇 |
1989年 | 272篇 |
1988年 | 274篇 |
1987年 | 251篇 |
1986年 | 234篇 |
1985年 | 302篇 |
1984年 | 322篇 |
1983年 | 257篇 |
1982年 | 351篇 |
1981年 | 275篇 |
1980年 | 255篇 |
1979年 | 191篇 |
1978年 | 222篇 |
1977年 | 209篇 |
1976年 | 178篇 |
1974年 | 188篇 |
1973年 | 194篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
Kiyoko M. Gotanda Lari C. Delaire Joost A. M. Raeymaekers Felipe Pérez-Jvostov Felipe Dargent Paul Bentzen Marilyn E. Scott Gregor F. Fussmann Andrew P. Hendry 《Oecologia》2013,172(1):155-166
Studies of phenotypic variation in nature often consider only a single potential selective agent. In such cases, it remains an open question as to whether variation attributed to that single measured agent might be influenced by some other unmeasured agent. Previous research has shown that phenotypic variation in the Trinidadian guppy (Poecilia reticulata) is strongly influenced by predation regime, and we here ask whether parasitism might represent an additional important selective agent shaping this variation. We performed a field survey of 26 natural guppy populations of known predation regime in northern Trinidad. We quantified levels of parasitism of guppies by the monogenean ecotoparasite, Gyrodactylus, and examined whether this parasite was associated with guppy body size or male colour. Spatial variation in Gyrodactylus parasitism was consistent between years, and parasite prevalence was generally, but not always, higher at high-predation sites than at low-predation sites. Consistent with previous work, predation regime was related to guppy size and some aspects of male colour, whereas parasitism showed few and only minor associations with the same traits. Moreover, a consideration of parasitism did not alter any interpretations regarding associations between guppy traits and predation regimes. These results suggest that parasitism, at least as quantified in the present study, does not play a major role in shaping variation in guppy body size or colour. Nevertheless, considerable variation in these traits, even within a predation regime, suggests the likely importance of other selective agents beyond just predation regime. 相似文献
952.
Vitaly A Selivanov Pedro Vizán Faustino Mollinedo Teresa WM Fan Paul WN Lee Marta Cascante 《BMC systems biology》2010,4(1):135
Background
Metabolic flux profiling based on the analysis of distribution of stable isotope tracer in metabolites is an important method widely used in cancer research to understand the regulation of cell metabolism and elaborate new therapeutic strategies. Recently, we developed software Isodyn, which extends the methodology of kinetic modeling to the analysis of isotopic isomer distribution for the evaluation of cellular metabolic flux profile under relevant conditions. This tool can be applied to reveal the metabolic effect of proapoptotic drug edelfosine in leukemia Jurkat cell line, uncovering the mechanisms of induction of apoptosis in cancer cells. 相似文献953.
Jack A. Grebb Jean-Antoine Girault† Michelle Ehrlich‡ Paul Greengard 《Journal of neurochemistry》1990,55(1):204-207
DARPP-32 (dopamine- and cyclic AMP-regulated phosphoprotein, Mr = 32,000, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) is a neuronal phosphoprotein that is enriched in neurons which possess dopamine D1 receptors, particularly striatonigral neurons. In rat brain slices, the phosphorylation state of DARPP-32 is regulated by dopamine, acting through the dopamine D1 receptor and the adenylyl cyclase system. This study reports that chronic blockade (21 days) of either dopamine D1 receptors by SCH-23390 or dopamine D2 receptors by raclopride does not affect the concentrations of DARPP-32 in specific rat brain regions (striatum, thalamus, hippocampus, frontal cerebral cortical pole). Northern blot analysis indicates that the steady-state level of DARPP-32 mRNA in striatum is also unchanged by these treatments. 相似文献
954.
Denny PW Shams-Eldin H Price HP Smith DF Schwarz RT 《The Journal of biological chemistry》2006,281(38):28200-28209
Sphingolipids are ubiquitous and essential components of eukaryotic membranes, particularly the plasma membrane. The biosynthetic pathway for the formation of these lipid species is conserved up to the formation of sphinganine. However, a divergence is apparent in the synthesis of complex sphingolipids. In animal cells, ceramide is a substrate for sphingomyelin (SM) production via the enzyme SM synthase. In contrast, fungi utilize phytoceramide in the synthesis of inositol phosphorylceramide (IPC) catalyzed by IPC synthase. Because of the absence of a mammalian equivalent, this essential enzyme represents an attractive target for anti-fungal compounds. In common with the fungi, the kinetoplastid protozoa (and higher plants) synthesize IPC rather than SM. However, orthologues of the gene believed to encode the fungal IPC synthase (AUR1) are not readily identified in the complete genome data bases of these species. By utilizing bioinformatic and functional genetic approaches, we have isolated a functional orthologue of AUR1 in the kinetoplastids, causative agents of a range of important human diseases. Expression of this gene in a mammalian cell line led to the synthesis of an IPC-like species, strongly indicating that IPC synthase activity is reconstituted. Furthermore, the gene product can be specifically inhibited by an anti-fungal-targeting IPC synthase. We propose that the kinetoplastid AUR1 functional orthologue encodes an enzyme that defines a new class of protozoan sphingolipid synthase. The identification and characterization of the protozoan IPC synthase, an enzyme with no mammalian equivalent, will raise the possibility of developing anti-protozoal drugs with minimal toxic side affects. 相似文献
955.
Paul D. Pratt Paul T. Madeira Gevork Arakelian Matthew Purcell Min B. Rayamajhi Ted D. Center 《Biocontrol Science and Technology》2013,23(5):602-606
The Australian psyllid Boreioglycaspis melaleucae is a biological control agent of Melaleuca quinquenervia in Florida (USA) but was observed attacking M. quinquenervia trees in southern California (USA). Genotyping revealed the California population matched three of eight Australian haplotypes and all three Florida haplotypes. It remains unclear if the California psyllid population arrived directly from Australia or via Florida. 相似文献
956.
957.
958.
Black swallowtail females laid more eggs on plant models treated with contact stimulants and volatiles from carrot leaves than on models treated only with contact stimulants. The volatiles enhanced landing rates and females alighted more frequently on artificial leaves treated with host volatiles than on adjacent control leaves. Volatiles from cabbage, a nonhost, inhibited landing rates on artificial leaves treated with carrot contact stimulants. Examination of antennae revealed two major types of sensilla, believed to be olfactory in function. Electroantennogram preparations responded more strongly to carrot volatiles than to cabbage volatiles and several shared responses at particular retention times to carrot volatile components eluting from a gas chromatograph. Our results are consistent with a long-standing hypothesis that behavioral responses to essential oil components characteristic of the larval food plants have facilitated host shifts in the genus Papilio. 相似文献
959.
Singh S D'mello V van Bergen en Henegouwen P Birge RB 《Biochemical and biophysical research communications》2007,364(3):540-548
Integrin receptors are heterodimeric transmembrane receptors with critical functions in cell adhesion and migration, cell cycle progression, differentiation, apoptosis, and phagocytosis of apoptotic cells. Integrins are activated by intracellular signaling that alter the binding affinity for extracellular ligands, so-called inside to outside signaling. A common element for integrin activation involves binding of the cytoskeletal protein talin, via its FERM domain, to a highly conserved NPxY motif in the β chain cytoplasmic tails, which is involved in long-range conformation changes to the extracellular domain that impinges on ligand affinity. When the human beta-5 (β5) integrin cDNA was expressed in αv positive, β5 and β3 negative hamster CS-1 cells, it promoted NPxY-dependent adhesion to VTN-coated surfaces, phosphorylation of FAK, and concomitantly, β5 integrin-EGFP protein was recruited into talin and paxillin-containing focal adhesions. Expression of a NPxY destabilizing β5 mutant (Y750A) abrogated adhesion and β5-Y750A-EGFP was excluded from focal adhesions at the tips of stress fibers. Surprisingly, expression of β5 Y750A integrin had a potent gain-of-function effect on apoptotic cell phagocytosis, and further, a β5-Y750A-EGFP fusion integrin readily bound MFG-E8-coated 10 μm diameter microspheres developed as apoptotic cell mimetics. The critical sequences in β5 integrin were mapped to a YEMAS motif just proximal to the NPxY motif. Our studies suggest that the phagocytic function of β5 integrin is regulated by an unconventional NPxY-talin-independent activation signal and argue for the existence of molecular switches in the β5 cytoplasmic tail for adhesion and phagocytosis. 相似文献
960.