首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   15736篇
  免费   1603篇
  国内免费   2375篇
  2024年   37篇
  2023年   259篇
  2022年   408篇
  2021年   844篇
  2020年   628篇
  2019年   799篇
  2018年   705篇
  2017年   520篇
  2016年   659篇
  2015年   1107篇
  2014年   1256篇
  2013年   1313篇
  2012年   1600篇
  2011年   1414篇
  2010年   1009篇
  2009年   922篇
  2008年   1030篇
  2007年   905篇
  2006年   801篇
  2005年   656篇
  2004年   568篇
  2003年   535篇
  2002年   444篇
  2001年   218篇
  2000年   189篇
  1999年   170篇
  1998年   147篇
  1997年   91篇
  1996年   83篇
  1995年   59篇
  1994年   81篇
  1993年   39篇
  1992年   44篇
  1991年   25篇
  1990年   20篇
  1989年   16篇
  1988年   8篇
  1987年   24篇
  1986年   10篇
  1985年   11篇
  1984年   8篇
  1983年   10篇
  1982年   6篇
  1981年   7篇
  1979年   5篇
  1976年   3篇
  1973年   3篇
  1972年   4篇
  1966年   2篇
  1965年   2篇
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
101.
Zea mays (Z. mays) is one of the main cereal crops in the world, and it′s by-products have exhibited medicinal properties to explore. This article intends to review the chemical compositions and pharmacological activities of by-products of Z. mays (corn silks, roots, bract, stems, bran, and leaves) which support the therapeutic potential in the treatment of different diseases, with emphasis on the natural occurring compounds and detailed pharmacological developments. Based on this review, 231 natural compounds are presented. Among them, flavonoids, terpenes, phenylpropanoids, and alkaloids are the most frequently reported. The by-products of Z. mays possess diuretic effects, hepatoprotective, anti-diabetic, antioxidant, neuroprotective, anti-inflammatory, anti-cancer, plant protection activity, and other activities. This article reviewed the phytochemistry and pharmacological activities of Z. mays for comprehensive quality control and the safety and effectiveness to enhance future application.  相似文献   
102.
Three previously undescribed diterpenoids, helioscopnoids A–C, and eight known compounds were isolated from the whole plants of Euphorbia helioscopia. Their structures were established by extensive analysis of spectra and data comparison with previous literatures. Among them, compound 4 was identified as 24,24-dimethoxy-25,26,27-trinoreuphan-3β-ol with revised configurations of C-13, C-14, and C-17 (13R*, 14R*, 17R*). Cytotoxicity assays revealed that all compounds exhibited varying levels of cytotoxicity against H1975 cells, with compound 9 displaying the most potent activity, as indicated by cell viability rates of 18.13 % and 20.76 % at concentrations of 20 μM and 5 μM, respectively. This study expands the understanding of E. helioscopia terpenoids’ structural diversity and biological activities, contributing to the exploration of potential therapeutic applications.  相似文献   
103.
Hu  Xiaoyu  Tang  Ruibo  Zhao  Caijun  Mu  Ruiying  Wang  Ying  Cao  Yongguo  Zhang  Naisheng  Fu  Yunhe 《Probiotics and antimicrobial proteins》2023,15(1):74-81
Probiotics and Antimicrobial Proteins - Mastitis, common inflammation of the mammary gland, caused by various factors, is a challenge for the dairy industry. Escherichia coli (E. coli), a...  相似文献   
104.
Microtubule dynamics and organization are important for plant cell morphogenesis and development. The microtubule-based motor protein kinesins are mainly responsible for the transport of some organelles and vesicles, although several have also been shown to regulate microtubule organization. The ARMADILLO REPEAT KINESIN (ARK) family is a plant-specific motor protein subfamily that consists of three members (ARK1, ARK2, and ARK3) in Arabidopsis thaliana. ARK2 has been shown to participate in root epidermal cell morphogenesis. However, whether and how ARK2 associates with microtubules needs further elucidation. Here, we demonstrated that ARK2 co-localizes with microtubules and facilitates microtubule bundling in vitro and in vivo. Pharmacological assays and microtubule dynamics analyses indicated that ARK2 stabilizes cortical microtubules. Live-cell imaging revealed that ARK2 moves along cortical microtubules in a processive mode and localizes both at the plus-end and the sidewall of microtubules. ARK2 therefore tracks and stabilizes the growing plus-ends of microtubules, which facilitates the formation of parallel microtubule bundles.  相似文献   
105.
106.
Gomphomastacinae is a grasshopper subfamily in Eumastacidae, with a morphology and distribution distinct from other subfamilies. The alpine genera of Gomphomastacinae that inhabit the Qinghai–Tibet Plateau in China show unique characteristics adapted to high-altitude life. However, their phylogenetic position and biogeographic history remain controversial. Thus, to determine the diversification history of these alpine genera and the origin of the subfamily, we obtained mitochondrial genome sequences from all seven Gomphomastacinae genera distributed in China. The reconstructed phylogeny was well supported and confirmed the phylogenetic position of Gomphomastacinae within Eumastacidae. Time calibration revealed a deep-time origin of the subfamily dating back to the Cretaceous period, and the diversification among alpine genera was also an ancient pre-Miocene event (30–50 Ma). Based on phylogeny and time estimates, the most likely biogeographic scenario is that Gomphomastacinae originated from an ancestral lineage that lived in East Gondwana and dispersed to Central and Western Asia through India. Subsequently, the alpine genera likely diverged along with the uplift of the Qinghai–Tibet Plateau and survived drastic climate change by in situ adaptation to high-altitude dwellings.  相似文献   
107.
九种蹄盖蕨科植物配子体的酯酶同工酶分析   总被引:5,自引:1,他引:4  
林孝辉  傅桂荣 《植物研究》1996,16(1):100-107
本文培养了蹄盖蕨科9个种即中华蹄盖蕨(Athyriumsinense)带岭蹄盖蕨(A.dalingense)、多齿蹄盖蕨(A.multidentatum)(包括青柄和紫柄2种带岭蹄蕨类型)、东北蛾眉蕨(Lunathyriumpycnosorum)、朝鲜介蕨(Dryoathyriumcoreanum)、山冷蕨(Cystopterissudetica)、假冷蕨(Pseudocystopterisspinulosa)、欧洲羽节蕨(Gymnocarpiumdryopteris)和黑鳞短肠蕨(Alantodiacrenata)的配子体为材料,进行酯酶同工酶的分析,以表明它们的种间差异,其中多齿蹄盖蕨的青柄和紫柄两种类型的酶谱存在明显差异,应考虑紫柄为多齿蹄盖蕨的变型。  相似文献   
108.
A new type of liquid chromatographic (LC) dual thin-layer amperometric detector for the simultaneous measurement of trace levels of dopamine and serotonin in microdialysates is described. The concentrations of these analytes in rat dialysates are usually in the sub-nanomolar concentration range (typically, 0.10–5.00 pg in 5-μl dialysates). With this dual electrode, a glass-lined microbore column provides excellent sensitivity, selectivity, and separation. In addition, a three- to five-fold improvement in anodic current or cathodic responses over conventional dual electrodes in microbore LC can be achieved. Due to the irreversible electrochemical properties of some interference peaks, this dual electrode provides reliable measurement of dopamine based on the cathodic signal. The detection limit (signal-to-noise RATIO=3) of this assay is 0.02 pg per injection for dopamine or serotonin. This new dual electrode allows the simultaneous measurements of basal dopamine and serotonin in rat striatum dialysates without the use of re-uptake inhibitors in perfusion medium.  相似文献   
109.
外源DNA或染色质在非洲爪蟾卵提取物中可以诱导细胞核样结构的重建。重建核除不具有核仁样结构外,在其它形态结构上与真核细胞核十分相似。前人的工作表明在重建核中具有核仁前体结构。但可能是由于缺少活性核仁组织者的缘故,这些核仁前体不能相互融合形成新生核仁。那么活性核仁组织者在重建核中是否能发挥其功能呢?为了研究这一问题,我们提取纯化了四膜虫的大核与大核的周边核仁。进一步去除大核的核被膜,并将去除核被膜的大核与大核核仁分别加入非洲爪赡卵非细胞体系中。通过电镜超薄切片观察,我们发现无论是与大核染色质相连的周边核仁还是分离纯化的核仁结构在非洲爪赡卵非细胞体系中都不能保持其原有结构特征,而是发生了典型核重建变化,并且在诱导形成的重建核中也看不到核仁样结构。这些结果说明具有活性的核仁组织者在加入非洲爪蟾卵提取物后既不能继续保持其原有的RNA转录功能也不能诱导新的核仁的出现。  相似文献   
110.
We compared the pattern of 3-phosphorylated phosphoinositides produced by confluent 3T3-L1 preadipose cells upon exposure to growth factors that either induce differentiation (insulin, insulin-like growth factor-1) or do not (platelet-derived growth factor). Following addition of insulin or insulin-like growth factor-1, PI(3,4,5)P3 rapidly rose, on average, to levels tenfold over basal. PI(3,4)P2 either did not change (after insulin) or slightly increased (1.5 fold). Time course studies with insulin demonstrated that the rise in PI(3,4,5)P3 peaked by 1 minute, and levels then remained steady over 30 minutes. Dose-response experiments showed that insulin at a concentration of 1 nM was sufficient for the PI(3,4,5)P3 response. Insulin failed to increase PI(3,4)P2 at any of the time points or at any of the doses used. In contrast, after addition of platelet-derived growth factor, both PI(3,4)P2 and PI(3,4,5)P3 rose concurrently and to comparable extents. These data suggest that one possible mechanism contributing to insulin/insulin-like growth factor-1-induced 3T3-L1 preadipose cell differentiation is a distinct pattern of 3-phosphorylated phosphoinositide accumulation, defined by a prominent increase in PI(3,4,5)P3 with no (in the case of insulin), or a minimal (in the case of IGF-1), rise in PI(3,4)P2.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号