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   52414篇
  免费   4920篇
  国内免费   2231篇
  59565篇
  2025年   114篇
  2024年   627篇
  2023年   749篇
  2022年   1558篇
  2021年   2375篇
  2020年   1794篇
  2019年   2129篇
  2018年   2154篇
  2017年   1773篇
  2016年   2285篇
  2015年   2634篇
  2014年   3418篇
  2013年   3628篇
  2012年   4172篇
  2011年   3895篇
  2010年   2763篇
  2009年   2451篇
  2008年   2783篇
  2007年   2502篇
  2006年   2206篇
  2005年   1828篇
  2004年   1701篇
  2003年   1578篇
  2002年   1316篇
  2001年   1082篇
  2000年   912篇
  1999年   654篇
  1998年   399篇
  1997年   333篇
  1996年   317篇
  1995年   327篇
  1994年   296篇
  1993年   239篇
  1992年   297篇
  1991年   266篇
  1990年   213篇
  1989年   193篇
  1988年   130篇
  1987年   178篇
  1986年   145篇
  1985年   135篇
  1984年   102篇
  1983年   97篇
  1982年   91篇
  1981年   81篇
  1980年   52篇
  1979年   63篇
  1978年   66篇
  1973年   63篇
  1972年   52篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
用免疫细胞化学方法,观察研究了马桑内酯(CL)对培养的海马神经元内γ-氨基丁酸(GABA)和谷氨酸(Glu)神经元的影响.结果表明:CL作用后,GABA免疫反应阳性神经元数目减少,反应强度减弱;Glu免疫反应阳性神经元数目变化不明显,但反应增强.推测:CL可能引起海马神经元兴奋性增高是使动物模型致痫的基础,其机理可能与阻断GABA的合成途径有关.  相似文献   
2.
The specific activity of NAD+ kinase (ATP:NAD+ 2-phosphotransferase, EC 2.7.1.23) from Neurospora crassa shows sharp peaks when the organism enters a new developmental stage of the asexual life cycle: the peaks are observed during hydration and germination of conidia, at the transition from exponential to stationary growth and at the photostimulated conidiation. As stimulation of NAD+ kinase activity by light in conidiating mycelium is not sensitive to translation inhibitors, the activiation of pre-existing molecules, rather than induction of protein synthesis de novo may be supposed. Enzyme electrophoresis revealed the presence of four forms of NAD+ kinase having different apparent molecular weights (I=333,000; II=306,000; III=229,000 and IV=203,000). Manifestation of the activity of individual forms of NAD+ kinase is developmentally controlled: form III is most abundant during vegetative growth, forms I and II prevail in conidia. At the conidial germination the increase of NAD+ kinase activity is associated with the activation of form III, whereas during photostimulation of conidiation form II is the most activated one. Therefore, certain molecular forms of the enzyme may be regarded as biochemical markers for different developmental stages of N. crassa.  相似文献   
3.
AIMS: Studies were performed to demonstrate the function of the putative signal peptide of Vip3A proteins in Escherichia coli. METHODS AND RESULTS: The full-length vip3A-S184 gene was isolated from a soil-isolated Bacillus thuringiensis, and the vip3AdeltaN was constructed by deleting 81 nucleotides at the 5'-terminus of vip3A-S184. Both were transformed and expressed in E. coli. About 19.2% of Vip3A-S184 proteins secreted soluble proteins and others formed inclusion bodies in the periplasmic space. In contrast, the Vip3AdeltaN was insoluble and formed inclusion bodies in the cytoplasm. Bioassay indicated that Vip3A-S184 showed different toxicity against Spodoptera exigua, Helicoverpa armigera and S. litura, but Vip3AdeltaN showed no toxicity to either of them because of the deletion of the first 27 amino acids at the N-terminus. CONCLUSIONS: The results suggest that the deleted N-terminal sequences were essential for the secretion of Vip3A-S184 protein in E. coli and might be required for toxicity. SIGNIFICANCE AND IMPACT OF THE STUDY: The function of the putative signal peptide of Vip3A protein in E. coli was investigated. These would be helpful to make clear the unknown secretion pathway of Vip3A protein in B. thuringiensis and determine the receptor-binding domain or toxic fragment of Vip3A-S184 protein.  相似文献   
4.
5.
A comparative study of the sorption capacity of low-esterified pectin from the seagrass Zostera marina and drugs used for hemosorption and enterosorption was made. Low-esterified pectin was less efficient in binding in vitro lead, cadmium, and copper compared to chelating and thiol-containing compounds; but it was much more efficient than activated carbon, polyphepan, microcrystalline cellulose, and enterodez.  相似文献   
6.
A series of large chromosomal deletions in Streptomyces hygroscopicus 10-22 were aligned on the physical map of the wild-type strain and the mutants were assessed for their ability to produce the aminocyclitol antibiotic 5102-I (jinggangmycin). Twenty-eight mutants were blocked for jinggangmycin production and all of them were found to lack a 300 kb AseI-F fragment of the wild-type chromosome. An ordered cosmid library of the 300 kb AseI-F fragment was made and one of the cosmids conferred jinggangmycin productivity to Streptomyces lividans ZX1. Three of the overlapping cosmids (18G7, 5H3 and 9A2) also hybridized to the valA gene of the validamycin pathway from S. hygroscopicus 5008 as a probe. This gene resembles acbC from Actinoplanes sp. 50/110, which encodes a C7-cyclitol synthase that catalyses the transformation of sedoheptulose 7-phosphate into 2-5-epi-valiolone for acarbose biosynthesis. The valA/acbC-homolog (orf1) of S. hygroscopicus 10-22 was shown to be essential for jinggangmycin biosynthesis as an engineered mutant with a specific in-frame deletion removing a 609 bp sequence internal to orf1 completely abolished jinggangmycin production and the corresponding knock-out mutant (JXH4) could be complemented for jinggangmycin production by the introduction of an orf1-containing construct. Concurrently, the identities of the genes common to S. hygroscopicus strains 10-22 and 5008 prompted a comparison of the chemical structures of jinggangmycin and validamycin, which led to a clear demonstration that they are identical.The first two authors contributed equally to this study.  相似文献   
7.
A novel compound CF66I produced by Burkholeria cepacia was investigated for its antifungal effects against Fusarium solani by three different fluorescent dyes. Dual staining with propidium iodide (PI) and fluorescein diacetate (FDA) demonstrated high doses of CF66I (120.0 μg ml−1) killed the fungi by acting primarily on the cell membrane. However, at fungistatic concentration (20.0 μg ml−1) of this compound, microscopic observations revealed swelling hyphae with abnormal chitin deposition, as determined by Calcofluor white (CFW) staining, which was indicative of the alterations in cell wall structure. In addition, inhibition of intracellular esterases activity was observed. These results led us to conclude that low doses of CF66I probably inhibited the fungal growth by interfering with the cell metabolic pathways.  相似文献   
8.
9.
The effect of the β-amyloid peptide Aβ25–35 and fullerene C60 on the activity of the cytoplasmic enzymes lactate dehydrogenase (LDH) and glutathione peroxidase (GLP), and membrane-bound phosphofructokinase (PFK) and Na+,K+-ATPase in human erythrocytes has been studied. When used in combination, the cytotoxins decrease the activity of LDH and PFK in a nonadditive manner; in this case, Aβ25–35 protects PFK against the inhibitory effect of C60. The activity of LDH, GLP, and PFK decreases within the first 2–20 min of incubation of erythrocytes with Aβ25–35 in the absence of glucose. The addition of glucose sharply decreases the inhibitory action of Aβ25–35 on LDH and GLP but does not affect the fourfold decrease in activity of PFK; the activity of membrane-bound Na+,K+-ATPase does not depend on the presence of glucose. Possible mechanisms of interaction of Aβ25–35 and fullerene C60 with the erythrocyte membrane and enzymes are discussed.  相似文献   
10.
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