全文获取类型
收费全文 | 22394篇 |
免费 | 1987篇 |
国内免费 | 2310篇 |
专业分类
26691篇 |
出版年
2024年 | 41篇 |
2023年 | 209篇 |
2022年 | 444篇 |
2021年 | 711篇 |
2020年 | 600篇 |
2019年 | 689篇 |
2018年 | 689篇 |
2017年 | 582篇 |
2016年 | 784篇 |
2015年 | 1265篇 |
2014年 | 1469篇 |
2013年 | 1615篇 |
2012年 | 2052篇 |
2011年 | 1774篇 |
2010年 | 1262篇 |
2009年 | 1159篇 |
2008年 | 1443篇 |
2007年 | 1310篇 |
2006年 | 1231篇 |
2005年 | 1136篇 |
2004年 | 1047篇 |
2003年 | 956篇 |
2002年 | 796篇 |
2001年 | 556篇 |
2000年 | 464篇 |
1999年 | 427篇 |
1998年 | 278篇 |
1997年 | 212篇 |
1996年 | 194篇 |
1995年 | 171篇 |
1994年 | 149篇 |
1993年 | 110篇 |
1992年 | 138篇 |
1991年 | 109篇 |
1990年 | 91篇 |
1989年 | 89篇 |
1988年 | 59篇 |
1987年 | 70篇 |
1986年 | 53篇 |
1985年 | 28篇 |
1984年 | 18篇 |
1983年 | 35篇 |
1982年 | 16篇 |
1981年 | 17篇 |
1979年 | 17篇 |
1978年 | 14篇 |
1977年 | 13篇 |
1975年 | 10篇 |
1972年 | 12篇 |
1970年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
941.
942.
943.
944.
SPHK1 (sphingosine kinase 1), a regulator of sphingolipid metabolites, plays a causal role in the development of hepatocellular carcinoma (HCC) through augmenting HCC invasion and metastasis. However, the mechanism by which SPHK1 signaling promotes invasion and metastasis in HCC remains to be clarified. Here, we reported that SPHK1 induced the epithelial-mesenchymal transition (EMT) by accelerating CDH1/E-cadherin lysosomal degradation and facilitating the invasion and metastasis of HepG2 cells. Initially, we found that SPHK1 promoted cell migration and invasion and induced the EMT process through decreasing the expression of CDH1, which is an epithelial marker. Furthermore, SPHK1 accelerated the lysosomal degradation of CDH1 to induce EMT, which depended on TRAF2 (TNF receptor associated factor 2)-mediated macroautophagy/autophagy activation. In addition, the inhibition of autophagy recovered CDH1 expression and reduced cell migration and invasion through delaying the degradation of CDH1 in SPHK1-overexpressing cells. Moreover, the overexpression of SPHK1 produced intracellular sphingosine-1-phosphate (S1P). In response to S1P stimulation, TRAF2 bound to BECN1/Beclin 1 and catalyzed the lysine 63-linked ubiquitination of BECN1 for triggering autophagy. The deletion of the RING domain of TRAF2 inhibited autophagy and the interaction of BECN1 and TRAF2. Our findings define a novel mechanism responsible for the regulation of the EMT via SPHK1-TRAF2-BECN1-CDH1 signal cascades in HCC cells. Our work indicates that the blockage of SPHK1 activity to attenuate autophagy may be a promising strategy for the prevention and treatment of HCC. 相似文献
945.
Few species in the genus Grateloupia have been investigated in detail with respect to the development of the auxiliary cell ampullae before or after diploidization. In this study, we document the vegetative and reproductive structures of two new species of Grateloupia, G. taiwanensis S.‐M. Lin et H.‐Y. Liang sp. nov. and G. orientalis S.‐M. Lin et H.‐Y. Liang sp. nov., plus a third species, G. ramosissima Okamura, from Taiwan. Two distinct patterns are reported for the development of the auxiliary cell ampullae: (1) ampullae consisting of three orders of unbranched filaments that branch after diploidization of the auxiliary cell and form a pericarp together with the surrounding secondary medullary filaments (G. taiwanensis type), and (2) ampullae composed of only two orders of unbranched filaments in which only a few cells are incorporated into a basal fusion cell after diploization of the auxiliary cell and the pericarp consists almost entirely of secondary medullary filaments (G. orientalis type). G. orientalis is positioned in a large clade based on rbcL gene sequence analysis that includes the type species of Grateloupia C. Agardh 1822 , G. filicina. G. taiwanensis clusters with a clade that includes the generitype of Phyllymenia J. Agardh 1848 , Ph. belangeri from South Africa; that of Prionitis J. Agardh 1851 , Pr. lanceolata from Pacific North America; and that of Pachymeniopsis Y. Yamada ex Kawab. 1954, Pa. lanceolata from Japan. A reexamination of the type species of the genera Grateloupia, Phyllymenia, Prionitis, and Pachymeniopsis is required to clarify the generic and interspecific relationships among the species presently placed in Grateloupia. 相似文献
946.
This study investigated the respiratory properties and the role of the mitochondria isolated from one phosphoenolpyruvate carboxykinase (PCK)-CAM plant, Hoya carnosa, in malate metabolism during CAM phase III. The mitochondria showed high malate dehydrogenase (mMDH) and aspartate amino transferase (mAST), and a significant amount of malic enzyme (mME) activities. H. carnosa readily oxidized malate via mME and mMDH in the presence of some cofactors such as thiamine pyrophosphate (TPP), coenzyme A (CoA) or NAD(+). A high respiration rate of malate oxidation was observed at pH 7.2 with NAD(+) and glutamate (Glu). Providing AST and Glu simultaneously into the respiratory medium strongly increased the rates of malate oxidation, and this oxidation was gradually inhibited by an inhibitor of alpha-ketoglutarate (alpha-KG) carrier, pyridoxal-5'-phosphate (PLP). The mitochondria readily oxidized aspartate (Asp) or alpha-KG individually with low rates, while they oxidized Asp and alpha-KG simultaneously with high rates, and this simultaneous oxidation was also inhibited by PLP. By measuring the capacity of the mitochondrial shuttle, it was found that the OAA produced via mMDH seemed not to be transported outside the mitochondria, but mAST interconverted OAA and Glu to Asp and alpha-KG, respectively, and exported them out via a malate-aspartate (malate-Asp) shuttle. The data in this research suggest that during phase III of PCK-CAM, H. carnosa mitochondria oxidized malate via both mME and the mMDH systems depending on metabolic requirements. However, malate metabolism by the mMDH system did not operate via a malate-OAA shuttle similarly to Ananas comosus mitochondria, but it operated via a malate-Asp shuttle similarly to Kalancho? daigremontiana mitochondria. 相似文献
947.
The mechanism by which lipopolysaccharide (LPS) or phorbol 12-myristate 13-acetate (PMA) induces production of proinflammatory
cytokines in murine macrophages, and the role of phosphatidylinositol 3-kinase (PI3-kinase) have not been well investigated.
Activation of nuclear factor κB (NF-κB) is initiated by the phosphorylation of the inhibitory subunit, IκB, which targets
IκB for degradation and leads to the release of active NF-κB. In this study we demonstrate that 2- (4-morpholinyl)-8-phenylchromone
(LY294002), which inhibits PI3-kinase, specifically inhibited degradation of IκBα in RAW264.7 cells stimulated with interferon-γ
(IFN-γ) plus LPS or IFN-γ plus PMA. To elucidate the importance of this activity in RAW264.7 cells, we examined tumor necrosis
factor-α (TNF-α) and interleukin IL)-6 production in the activated cells. Pretreatment of the cells with LY294002 resulted
in the inhibition of TNF-α and IL-6 production in RAW264.7 cells stimulated with IFN-γ plus LPS or IFN-γ plus PMA. Furthermore,
LY294002 inhibited the production of nitric oxide NO) in RAW264.7 cells stimulated with IFN-γ plus LPS or IFN-γ plus PMA.
LY294002 also inhibited inducible NO synthase (iNOS) mRNA expression in the activated RAW264.7 cells. In conclusion, the present
results suggest that PI3-kinase is involved in the signal transduction pathway responsible for LPS- or PMA-mediated TNF-α
and IL-6 production, and that LY294002 inhibits NO generation through blocking the degradation of IκBα in activated RAW264.7
cells.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
948.
长花柱型滇丁香小孢子发生及雄配子体发育 总被引:2,自引:0,他引:2
利用常规石蜡切片法和细胞学压片法,对异型花柱植物滇丁香的长花柱型植株的小孢子发生、雄配子体发育及花粉萌发进行观察。结果表明:(1)长花柱型滇丁香具5枚花药,花药4室。(2)花药壁由1层表皮、1层花药内壁、2层中层和1~3层绒毡层组成;花药壁发育方式为基本型,绒毡层类型为腺质绒毡层。(3)小孢子母细胞减数分裂的胞质分裂为同时型,四分体排列方式为四面体型,偶有左右对称型;不同药室间小孢子母细胞减数分裂不同步。(4)成熟花粉粒为二细胞型。(5)小孢子发生和雄配子体发育过程正常,表明长花柱型滇丁香属于发育正常的两性花。(6)授粉4h后,长花柱型滇丁香的花粉在长、短两种花柱柱头上的萌发率分别达(91.8±1.6)%和(93.2±1.1)%,且两者间无显著性差异(t=1.585,df=8,p=0.152),表明长花柱型滇丁香的成熟花粉粒在长、短两种花柱柱头上的萌发均正常。 相似文献
949.
四川宜宾七种中药提取物体外抗菌活性研究 总被引:1,自引:0,他引:1
用肉汤二倍稀释法和琼脂平板培养计数法,研究四川宜宾七种中药的提取物对大肠杆菌、沙门氏菌和巴氏杆菌的体外抑制作用.结果表明:黄柏提取物的抗菌活性最强,最小抑菌浓度(Minimum inhibitory concentration,MIC)为15.6~125 mg·mL-1,最小杀菌浓度(Minimum bactericidal concentration,MBC)为31.25~250 mg·mL-1;栀子提取物抗菌活性次之,MIC 为62.5~125 mg·mL-1,MBC 为125~>250 mg·mL-1;佛手提取物有较稳定的抗菌活性,MIC均为125 mg·mL-1,MBC为125 mg·mL-1和250 mg·mL-1;姜黄、杜仲、何首乌、细毡毛忍冬的提取物抗菌活性相对较差,大部分 MIC≥250 mg·mL-1.说明四川宜宾的黄柏和栀子的提取物抗菌活性较强,佛手提取物抗菌活性较稳定,有进一步研究的价值. 相似文献