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   1815篇
  免费   111篇
  国内免费   6篇
  1932篇
  2024年   6篇
  2023年   23篇
  2022年   49篇
  2021年   73篇
  2020年   72篇
  2019年   113篇
  2018年   101篇
  2017年   58篇
  2016年   70篇
  2015年   77篇
  2014年   120篇
  2013年   153篇
  2012年   172篇
  2011年   149篇
  2010年   76篇
  2009年   64篇
  2008年   82篇
  2007年   94篇
  2006年   57篇
  2005年   58篇
  2004年   34篇
  2003年   55篇
  2002年   45篇
  2001年   10篇
  2000年   12篇
  1999年   11篇
  1998年   11篇
  1997年   9篇
  1996年   6篇
  1995年   5篇
  1994年   5篇
  1993年   3篇
  1992年   7篇
  1991年   4篇
  1990年   5篇
  1989年   4篇
  1988年   7篇
  1987年   4篇
  1986年   4篇
  1984年   4篇
  1983年   4篇
  1979年   2篇
  1972年   2篇
  1971年   1篇
  1970年   1篇
  1966年   1篇
  1962年   1篇
  1961年   2篇
  1960年   1篇
  1959年   1篇
排序方式: 共有1932条查询结果,搜索用时 10 毫秒
61.
62.
Due to its wide industrial application, chromium (Cr) is known to be a critical environmental pollutant. Contamination of water and agricultural soil by Cr inhibits crop productivity and their physiological and biochemical processes. The objective of the current work was to investigate the effects of appropriate reducing agents such as EDTA, iron sulfate (Fe2+), and zerovalent nano iron (Fe0 nanoparticles) on growth and physiology of sunflower plants under Cr(VI) stress. Results showed that the Cr uptake increased by increasing the amount of EDTA, leading to a significant reduction in morphological and physiological parameters except for MDA and H2O2 contents. Treatment with Fe0 nanoparticles and Fe2+ reduced Cr concentration in root and shoot, increased root and shoot dry weight, plastid pigments (chlorophyll and carotenoids) and proline contents; however, the level of MDA and H2O2 decreased significantly. All parameters were affected by Fe2+ during the first week of sampling; however, Fe0 nanoparticles affected all traits until the end of the third sampling stage. A statistically significant and positive correlation was found between root Cr concentration and MDA and H2O2 seedlings treated with EDTA, Fe2+, and Fe0 grown under Cr stress. From the result of this study, it can be concluded that sunflower has the potential for accumulation of Cr as a heavy metal, and treatment with Fe0 nanoparticles to prevent Cr uptake is more effective than other employed treatments.  相似文献   
63.
Brain tumors are the most common form of solid tumors in children and is presently a serious therapeutic challenge worldwide. Traditional treatment with chemotherapy and radiotherapy was shown to be unsuccessful in targeting brain tumor cancer stem cells (CSCs), leading to recurrent, treatment-resistant secondary malignancies. Oncolytic virotherapy (OV) is an effective antitumor therapeutic strategy which offers a novel, targeted approach for eradicating pediatric brain tumor CSCs by utilizing mechanisms of cell killing that differ from conventional therapies. A number of studies and some clinical trials have therefore investigated the effects of combined therapy of radiations or chemotherapies with oncolytic viruses which provide new insights regarding the effectiveness and improvement of treatment responses for brain cancer patients. This review summarizes the current knowledge of the therapeutic potency of OVs-induced CSCs targeting in the treatment of brain tumors for a better understanding and hence a better management of this disease.  相似文献   
64.
Rapid adaptation to global change can counter vulnerability of species to population declines and extinction. Theoretically, under such circumstances both genetic variation and phenotypic plasticity can maintain population fitness, but empirical support for this is currently limited. Here, we aim to characterize the role of environmental and genetic diversity, and their prior evolutionary history (via haplogroup profiles) in shaping patterns of life history traits during biological invasion. Data were derived from both genetic and life history traits including a morphological analysis of 29 native and invasive populations of topmouth gudgeon Pseudorasbora parva coupled with climatic variables from each location. General additive models were constructed to explain distribution of somatic growth rate (SGR) data across native and invasive ranges, with model selection performed using Akaike's information criteria. Genetic and environmental drivers that structured the life history of populations in their native range were less influential in their invasive populations. For some vertebrates at least, fitness‐related trait shifts do not seem to be dependent on the level of genetic diversity or haplogroup makeup of the initial introduced propagule, nor of the availability of local environmental conditions being similar to those experienced in their native range. As long as local conditions are not beyond the species physiological threshold, its local establishment and invasive potential are likely to be determined by local drivers, such as density‐dependent effects linked to resource availability or to local biotic resistance.  相似文献   
65.
The mesic Caspian (Hyrcanian) forest and ecotone communities provide a marked contrast to the arid and semiarid landscapes associated with most of the territory of Iran. To date, the ecological characteristics of these habitats, threatened and of conservation importance, have been little studied. Accordingly, ecological profiles of some important plant species of these communities have been assessed along two altitudinal gradients (300–2300 m a.s.l.). Vegetation and soils were sampled every 100 m in elevation, with the data subsequently analyzed using TWINSPAN and corrected frequency (CF) analyses. Relationships between soil variables (subdivided into three classes, the lowest, the middle and the upper third of all values) and herbaceous and shrub species (presence/absence data) were analyzed by the polythetic divisive classification method. 379 plant species and eleven soil variables – N, P, K, CaCO3, EC, pH, organic matter, C/N ratio and percentage of sand, clay and silt – were considered. The ecological profile method was used to evaluate the affinity and significance of associations between the probability of species’ occurrence and topsoil characteristics found by the polythetic method. Five vegetation groups were identified: two groups, with Acer campestre and Quercus macranthera in the tree layer and Veronica mazanderanae and Phuopsis stylosa as herbs, were restricted to forest-steppe ecotones and the upper mountain areas. Three groups, with Acer velutinum, Ruscus hyrcanus, Carpinus betulus, Danae racemosa, Fagus orientalis and Aruncus vulgaris as indicator species, occurred in the forest itself. Of the 42 plant species assessed as being of particular importance, 13 had significant relationships with eight soil factors. Thus, certain species, including endemic plant species of restricted distribution and conservation importance, can be used as indicators of particular soil conditions in the Hyrcanian forest area.  相似文献   
66.
Ligand-induced down-regulation controls the signaling potency of the epidermal growth factor receptor (EGFR/ErbB1). Overexpression studies have identified Cbl-mediated ubiquitinylation of EGFR as a mechanism of ligand-induced EGFR down-regulation. However, the role of endogenous Cbl in EGFR down-regulation and the precise step in the endocytic pathway regulated by Cbl remain unclear. Using Cbl-/- mouse embryonic fibroblast cell lines, we demonstrate that endogenous Cbl is essential for ligand-induced ubiquitinylation and efficient degradation of EGFR. Further analyses using Chinese hamster ovary cells with a temperature-sensitive defect in ubiquitinylation confirm a crucial role of the ubiquitin machinery in Cbl-mediated EGFR degradation. However, internalization into early endosomes did not require Cbl function or an intact ubiquitin pathway. Confocal immunolocalization studies indicated that Cbl-dependent ubiquitinylation plays a critical role at the early endosome to late endosome/lysosome sorting step of EGFR down-regulation. These findings establish Cbl as the major endogenous ubiquitin ligase responsible for EGFR degradation, and show that the critical role of Cbl-mediated ubiquitinylation is at the level of endosomal sorting, rather than at the level of internalization.  相似文献   
67.
The ER‐bound kinase/endoribonuclease (RNase), inositol‐requiring enzyme‐1 (IRE1), regulates the phylogenetically most conserved arm of the unfolded protein response (UPR). However, the complex biology and pathology regulated by mammalian IRE1 cannot be fully explained by IRE1’s one known, specific RNA target, X box‐binding protein‐1 (XBP1) or the RNA substrates of IRE1‐dependent RNA degradation (RIDD) activity. Investigating other specific substrates of IRE1 kinase and RNase activities may illuminate how it performs these diverse functions in mammalian cells. We report that macrophage IRE1 plays an unprecedented role in regulating phosphatidylinositide‐derived signaling lipid metabolites and has profound impact on the downstream signaling mediated by the mammalian target of rapamycin (mTOR). This cross‐talk between UPR and mTOR pathways occurs through the unconventional maturation of microRNA (miR) 2137 by IRE1’s RNase activity. Furthermore, phosphatidylinositol (3,4,5) phosphate (PI(3,4,5)P3) 5‐phosphatase‐2 (INPPL1) is a direct target of miR‐2137, which controls PI(3,4,5)P3 levels in macrophages. The modulation of cellular PI(3,4,5)P3/PIP2 ratio and anabolic mTOR signaling by the IRE1‐induced miR‐2137 demonstrates how the ER can provide a critical input into cell growth decisions.  相似文献   
68.
Molecular Biology Reports - Celiac disease (CeD) and inflammatory bowel disease (IBD) are accompanied by impaired immune responses. To study the immune regulation of these diseases, we evaluated...  相似文献   
69.
Bulk flow constitutes a substantial part of the slow transport of soluble proteins in axons. Though the underlying mechanism is unclear, evidences indicate that intermittent, kinesin-based movement of large protein-aggregates aids this process. Choline acetyltransferase (ChAT), a soluble enzyme catalyzing acetylcholine synthesis, propagates toward the synapse at an intermediate, slow rate. The presynaptic enrichment of ChAT requires heterotrimeric kinesin-2, comprising KLP64D, KLP68D and DmKAP, in Drosophila. Here, we show that the bulk flow of a recombinant Green Fluorescent Protein-tagged ChAT (GFP::ChAT), in Drosophila axons, lacks particulate features. It occurs for a brief period during the larval stages. In addition, both the endogenous ChAT and GFP::ChAT directly bind to the KLP64D tail, which is essential for the GFP::ChAT entry and anterograde flow in axon. These evidences suggest that a direct interaction with motor proteins could regulate the bulk flow of soluble proteins, and thus establish their asymmetric distribution.  相似文献   
70.
The current study was conducted to assess the relationship between testicular cells in spermatogenesis, through which the production of healthy and mature sperm is essential. However, it seems necessary to obtain more information about the three-dimensional pattern of the testis cells arrangement, which is directly related to the function of the testis after induction of diabetes. Twelve adult mice (28-30 g) were assigned into two experimental groups: (1) control and (2) diabetic (40 mg/kg STZ). The epididymal sperm collected from the tail of the epididymis and testes samples were taken for stereology, immunocytochemistry and RNA extraction. Our data showed that diabetes could notably decrease the number of testicular cells, together with a reduction of total sperm count. In addition, the results from the second-order stereology indicated the significant changes in the spatial arrangement of Sertoli cells and spermatogonial cells in the diabetic groups, in comparison with the control (P < .05). Moreover, the immunohistochemistry results showed a significant reduction in Sex-determining Region Y (SRY) box 9 gene (SOX9), vimentin, occludin, and connexin-43 positive cells in the diabetic groups compared with the control (P < .05). Furthermore, our data showed that the expression of steroidogenic acute regulatory protein steroidogenic acute regulatory protein (StAR) and peripheral benzodiazepine receptor peripheral benzodiazepine receptor (PBR) was significantly reduced in the diabetic groups, in comparison with the control (P < .05). These findings suggest that structural and functional changes of testis cells after induction of diabetes cause the alterations in the spatial arrangement of Sertoli and spermatogonial cells, ultimately influencing the normal spermatogenesis in mice.  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