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   15565篇
  免费   1148篇
  国内免费   2539篇
  2024年   45篇
  2023年   294篇
  2022年   380篇
  2021年   494篇
  2020年   463篇
  2019年   605篇
  2018年   557篇
  2017年   554篇
  2016年   614篇
  2015年   638篇
  2014年   840篇
  2013年   1043篇
  2012年   645篇
  2011年   778篇
  2010年   578篇
  2009年   888篇
  2008年   893篇
  2007年   843篇
  2006年   772篇
  2005年   778篇
  2004年   718篇
  2003年   636篇
  2002年   537篇
  2001年   411篇
  2000年   382篇
  1999年   353篇
  1998年   350篇
  1997年   265篇
  1996年   287篇
  1995年   269篇
  1994年   251篇
  1993年   207篇
  1992年   200篇
  1991年   210篇
  1990年   171篇
  1989年   132篇
  1988年   100篇
  1987年   99篇
  1986年   100篇
  1985年   129篇
  1984年   108篇
  1983年   75篇
  1982年   105篇
  1981年   90篇
  1980年   67篇
  1979年   57篇
  1978年   36篇
  1977年   36篇
  1976年   30篇
  1973年   36篇
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
911.
The data obtained during an extensive multidisciplinary investigation of a polluted harbor basin in Hamburg were analyzed in various ways in order to determine which statistical methods are easiest to interpret and permit the most accurate evaluations. The methods analyzed include the species deficit, index of species similarity, and saprobity index, as well as the more complex cluster and gradient analyses, which revealed a variety of relationships among the biotic communities at the various sampling sites and some ecological tolerance limits of various species. The shortcomings of numerical indices for characterizing water bodies are discussed. It is suggested that biostatistical methods are much more valuable for the empirical evaluation of data than for artificially classifying ecosystems.  相似文献   
912.
As the mammalian central nervous system matures, its regenerative ability decreases, leading to incomplete or non‐recovery from the neurodegenerative diseases and central nervous system insults that we are increasingly facing in our aging world population. Current neuroregenerative research is largely directed toward identifying the molecular and cellular players that underlie central nervous system repair, yet it repeatedly ignores the aging context in which many of these diseases appear. Using an optic nerve crush model in a novel biogerontology model, that is, the short‐living African turquoise killifish, the impact of aging on injury‐induced optic nerve repair was investigated. This work reveals an age‐related decline in axonal regeneration in female killifish, with different phases of the repair process being affected depending on the age. Interestingly, as in mammals, both a reduced intrinsic growth potential and a non‐supportive cellular environment seem to lie at the basis of this impairment. Overall, we introduce the killifish visual system and its age‐dependent regenerative ability as a model to identify new targets for neurorepair in non‐regenerating individuals, thereby also considering the effects of aging on neurorepair.  相似文献   
913.
We reported herein an efficient, environmentally friendly synthesis of hydrazine carboxamides (6a–l) in a water-glycerol (6:4) solvent system using ultrasonic irradiation. Ultrasonicated reactions were found to be much faster and more productive than conventional synthesis. The prepared compounds (6a–l) were tested against nine panels of 60 cancer cell lines according to the National Cancer Institute (NCI US) protocol. N-(4-Chlorophenyl)-2-(2-oxoindolin-3-ylidene)hydrazine-1-carboxamide (6b) was discovered to be promising anticancer agents with higher sensitivity against CCRF-CEM, HOP-92, UO-31, RMPI-8226, HL-60(TB), and MDA-MB-468 with percent growth inhibitions (%GIs) of 143.44, 33.46, 33.21, 33.09, 29.81, and 29.55 respectively. Compounds (6a–l) tested showed greater anticancer activity than Imatinib, except for compound 6k. Compounds 6b and 6c were found to be lethal on the CCRF-CEM leukaemia cell line, with %GIs of 143.44 and 108.91, respectively. Furthermore, molecular docking analysis was performed to investigate ligand binding affinity at the active site of epidermal growth factor (EGFR).  相似文献   
914.
Naked mole-rats (NM-R; Heterocephalus glaber) live in multi-generational colonies with a social hierarchy, and show low cancer incidence and long life-spans. Here we asked if an immune component might underlie such extreme physiology. The largest lymphoid organ is the spleen, which plays an essential role in responding to immunological insults and may participate in combating cancer and slowing ageing. We investigated the anatomy, molecular composition and function of the NM-R spleen using RNA-sequencing and histological analysis in healthy NM-Rs. Spleen size in healthy NM-Rs showed considerable inter-individual variability, with some animals displaying enlarged spleens. In all healthy NM-Rs, the spleen is a major site of adult haematopoiesis under normal physiological conditions. However, myeloid-to-lymphoid cell ratio is increased and splenic marginal zone showed markedly altered morphology when compared to other rodents. Healthy NM-Rs with enlarged spleens showed potentially better anti-microbial profiles and were much more likely to have a high rank within the colony. We propose that the anatomical plasticity of the spleen might be regulated by social interaction and gives immunological advantage to increase the lifespan of higher-ranked animals.  相似文献   
915.
铁是影响初级生产力的主要限制性因子之一,其在海洋环境中的分布具有空间异质性。由于铁在海洋中主要以溶解度较低、易沉降的三价态(Fe3+)形式存在,因此溶解铁对海洋生物而言是一种稀缺资源。为了获得生命代谢所需的铁,微生物进化出多种铁摄取的策略来满足需求,其中铁载体(siderophores)是最典型的代表。铁载体作为重要的代谢辅因子,除了铁循环以外,也强烈影响着其他元素的循环。基于铁载体的重要性,深入理解它的合成、转运和调控机制是系统认识海洋铁循环和生命过程的重要环节之一。本文以近20年的研究为重点,总结了铁载体的最新进展,包括其类型、合成/运输系统、获取途径、调控机制以及铁载体的功能与应用,旨在更好地认识铁载体在海洋微生物生态学过程中的作用,加深对海洋铁循环动力学机制的理解。  相似文献   
916.
Streptomyces genes involved in aerial mycelium formation   总被引:1,自引:0,他引:1  
Abstract Cloning of genes as suppressors of the aerial mycelium-defective phenotype of Streptomyces griseus HH1 resulting from A-factor-deficiency has led to the identification of several genes, including amfR, amfAlamfB, amfC , and orf1590 . These genes are involved in aerial mycelium formation independent of secondary metabolic function. Among these, AmfR which belongs to the family of response regulators of two-component regulatory systems and AmfA/AmfB similar to ATP-dependent membrane translocators are analogous to the multicomponent phosphorelay and the Spo0K system, respectively, both of which are required for the initiation of sporulation in Bacillus subtilis . Involvement of a protein serine/threonine kinase in aerial mycelium formation is also suggested, because the Streptomyces coelicolor A3(2) afsK gene encoding a 'eukaryotic'-type protein kinase reverses the aerial mycelium-defective phenotype of strain HH1, independent of secondary metabolic function.  相似文献   
917.
Substance P binding sites in the nucleus tractus solitarius were visualized with receptor autoradiography using Bolton-Hunter [125I]substance P. Substance P binding sites were found to have distinct patterns within the cat nucleus tractus solitarius. The majority of substance P binding sites were present in the medial, intermediate and the peripheral rim of the parvocellular subdivisions. Lower amounts of substance P binding sites were present in the commissural, ventrolateral, interstitial and dorsolateral subdivisions. No substance P binding sites were present in the central region of the parvocellular subdivision or the solitary tract. The localization of substance P binding sites in the nucleus tractus solitarius is very similar to the patterns of substance P immunoreactive fibers previously described for this region. Results of this study add further support for a functional role of substance P in synaptic circuits of the nucleus tractus solitarius.  相似文献   
918.
Alexander disease (AxD) is a rare and fatal neurodegenerative disorder caused by mutations in the gene encoding glial fibrillary acidic protein (GFAP). In this report, a mouse model of AxD (GFAPTg;Gfap+/R236H) was analyzed that contains a heterozygous R236H point mutation in murine Gfap as well as a transgene with a GFAP promoter to overexpress human GFAP. Using label-free quantitative proteomic comparisons of brain tissue from GFAPTg;Gfap+/R236H versus wild-type mice confirmed upregulation of the glutathione metabolism pathway and indicated proteins were elevated in the peroxisome proliferator-activated receptor (PPAR) signaling pathway, which had not been reported previously in AxD. Relative protein-level differences were confirmed by a targeted proteomics assay, including proteins related to astrocytes and oligodendrocytes. Of particular interest was the decreased level of the oligodendrocyte protein, 2-hydroxyacylsphingosine 1-beta-galactosyltransferase (Ugt8), since Ugt8-deficient mice exhibit a phenotype similar to GFAPTg;Gfap+/R236H mice (e.g., tremors, ataxia, hind-limb paralysis). In addition, decreased levels of myelin-associated proteins were found in the GFAPTg;Gfap+/R236H mice, consistent with the role of Ugt8 in myelin synthesis. Fabp7 upregulation in GFAPTg;Gfap+/R236H mice was also selected for further investigation due to its uncharacterized association to AxD, critical function in astrocyte proliferation, and functional ability to inhibit the anti-inflammatory PPAR signaling pathway in models of amyotrophic lateral sclerosis (ALS). Within Gfap+ astrocytes, Fabp7 was markedly increased in the hippocampus, a brain region subjected to extensive pathology and chronic reactive gliosis in GFAPTg;Gfap+/R236H mice. Last, to determine whether the findings in GFAPTg;Gfap+/R236H mice are present in the human condition, AxD patient and control samples were analyzed by Western blot, which indicated that Type I AxD patients have a significant fourfold upregulation of FABP7. However, immunohistochemistry analysis showed that UGT8 accumulates in AxD patient subpial brain regions where abundant amounts of Rosenthal fibers are located, which was not observed in the GFAPTg;Gfap+/R236H mice.  相似文献   
919.
920.
The objective of this present research is to use agricultural residues as a source of energy for heating greenhouses during winter seasons and sequestrating soil carbon dioxide through adding biochar to the soil media. To fulfill the objective of the research work, summer squash was transplanted in a constructed greenhouse and heated using an attached biomass-burning system. The performance of the attached biomass-burning system was experimentally studied under different agricultural residues (corn stalks, cotton stalks and okra stalks), heating fluids (water and oil) and air fan operating periods (10, 15 and 20 min/h). Results indicated that the biomass-burning system allowed increasing temperature and relative humidity inside the greenhouse up to 27.2 and 80 %, respectively. The maximum biomass-burning system efficiency of 81 % was achieved with the use of okra stalks as a source of energy and oil as a heating fluid side by side with adjusting the suction fan operating period at 15 min/h. Adding bio-charcoal to the soil media, enhanced the soil carbon, resulting in a total fresh yield of 3.7 and 2.9 kg/pot with a total number of leaves per plant of 55 and 47 leaves under conditions of with and without charcoal addition, respectively.  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