全文获取类型
收费全文 | 35558篇 |
免费 | 3260篇 |
国内免费 | 5234篇 |
专业分类
44052篇 |
出版年
2024年 | 122篇 |
2023年 | 505篇 |
2022年 | 1175篇 |
2021年 | 1881篇 |
2020年 | 1365篇 |
2019年 | 1743篇 |
2018年 | 1588篇 |
2017年 | 1230篇 |
2016年 | 1663篇 |
2015年 | 2417篇 |
2014年 | 2912篇 |
2013年 | 3009篇 |
2012年 | 3628篇 |
2011年 | 3290篇 |
2010年 | 2110篇 |
2009年 | 1876篇 |
2008年 | 2103篇 |
2007年 | 1889篇 |
2006年 | 1651篇 |
2005年 | 1351篇 |
2004年 | 1105篇 |
2003年 | 1037篇 |
2002年 | 871篇 |
2001年 | 547篇 |
2000年 | 474篇 |
1999年 | 434篇 |
1998年 | 283篇 |
1997年 | 259篇 |
1996年 | 230篇 |
1995年 | 183篇 |
1994年 | 178篇 |
1993年 | 123篇 |
1992年 | 132篇 |
1991年 | 104篇 |
1990年 | 78篇 |
1989年 | 76篇 |
1988年 | 61篇 |
1987年 | 40篇 |
1986年 | 51篇 |
1985年 | 60篇 |
1984年 | 23篇 |
1983年 | 29篇 |
1982年 | 29篇 |
1981年 | 23篇 |
1980年 | 10篇 |
1979年 | 20篇 |
1978年 | 9篇 |
1974年 | 8篇 |
1973年 | 11篇 |
1971年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
251.
rmlB and rmlC genes are essential for growth of mycobacteria 总被引:4,自引:0,他引:4
252.
Feng Xiei ;Wei Liu ;Fen Feng ;Xin Li ;Li Yang ;Deguan Lv ;Xuping Qin ;Lanfang Li ;Linxi Chen 《Acta biochimica et biophysica Sinica》2014,(8):699-708
This study is designed to investigate whether APJ receptor acts as a sensor in static pressure-induced cardiomyocyte hypertrophy and to investigate the mechanism of PI3K- autophagy pathway. The left ventricular hypertrophy rat model was established by coarctation of abdominal aorta. H9c2 rat cardiomyocytes were cultured in the presence of static pressure which was given by a custom-made pressure in- cubator. The results revealed that the expression of apelin/ APJ system, PI3K, Akt and their phosphorylation were sig- nificantly increased in the operation group. Static pressure up-regulated the APJ expression, PI3K phosphorylation, Akt phosphorylation, LC3-Ⅱ/Ⅰ and beclin-1 expression in cardio- myocytes. APJ shRNA pGPU6/Neo-rat-399, PI3K inhibitor LY294002, Akt inhibitor 1701-1 blocked the up-regulation of APJ, PI3K phosphorylation, Akt phosphorylation, LC3-H/I and beclin-1 expression, respectively. Moreover, static pres- sure increased the diameter, volume, protein content of cells, and these could be reversed when the cells were treated with pGPU6/Neo-rat-399, LY294002, and autop- hagy inhibitor 3-methyladenine, respectively. These results suggested that static pressure up-regulates APJ expression to promote cardiomyocyte hypertrophy by a PI3K-autop- hagy pathway. 相似文献
253.
不同鸟类生境网络复合与优化——以苏锡常地区白鹭、鸳鸯、雉鸡为例 总被引:2,自引:0,他引:2
通过复合优化不同焦点物种的生境网络表征多数物种生境网络实现物种多样性保护,具有重要理论价值和实践意义。以苏锡常地区为研究区域,以白鹭、鸳鸯和雉鸡3种鸟类为焦点物种,通过采用生境斑块约束条件模型与最小成本路径法相结合的方法识别出生境网络;从满足集合覆盖问题视角出发,通过采用网络构成要素复合的方法进行网络复合,其中廊道与廊道的复合通过采用评价生态系统服务价值的方法实现;通过采用观测数据为生境斑块和迁移廊道增补依据的方法进行复合生境网络优化。结果表明:生境网络经过复合,廊道总面积、总长度及生态系统服务价值在分别减少了16%、68%和10%的情况下,保持了斑块间的连接度、兼顾到经济效益和生态效益最大化、覆盖了占总数86%以上的75处观测点;复合生境网络经过优化,覆盖了2010年及以后所有84处观测点,廊道总面积、总长度及生态系统服务价值分别增加了19%、21%和27%;说明生境网络复合优化定量分析方法和思路具有可行性,为网络复合研究提供了借鉴。 相似文献
254.
采用氯磺酸-吡啶法化学合成硫酸酯化银耳多糖,对硫酸酯化反应的吡啶与氯磺酸的体积比进行优化,氯化钡-明胶比浊法测定硫酸酯化银耳多糖的取代度,并对不同取代度的硫酸酯化银耳多糖进行体外抗氧化实验。结果表明:硫酸酯化银耳多糖取代度随吡啶与氯磺酸体积比的提高而增加;当吡啶与氯磺酸体积比为4∶1,反应温度60℃,反应时间3 h时,产物的取代度为1.32,在0.8~2.0 mg/mL浓度范围内对Fenton反应产生的羟基自由基体外清除率相对于银耳多糖具有明显优势。 相似文献
255.
Karuppiah Valliappan Wei Sun Zhiyong Li 《Applied microbiology and biotechnology》2014,98(17):7365-7377
Actinobacteria are ubiquitous in the marine environment, playing an important ecological role in the recycling of refractory biomaterials and producing novel natural products with pharmic applications. Actinobacteria have been detected or isolated from the marine creatures such as sponges, corals, mollusks, ascidians, seaweeds, and seagrass. Marine organism-associated actinobacterial 16S rRNA gene sequences, i.e., 3,003 sequences, deposited in the NCBI database clearly revealed enormous numbers of actinobacteria associated with marine organisms. For example, RDP classification of these sequences showed that 112 and 62 actinobacterial genera were associated with the sponges and corals, respectively. In most cases, it is expected that these actinobacteria protect the host against pathogens by producing bioactive compounds. Natural products investigation and functional gene screening of the actinobacteria associated with the marine organisms revealed that they can synthesize numerous natural products including polyketides, isoprenoids, phenazines, peptides, indolocarbazoles, sterols, and others. These compounds showed anticancer, antimicrobial, antiparasitic, neurological, antioxidant, and anti-HIV activities. Therefore, marine organism-associated actinobacteria represent an important resource for marine drugs. It is an upcoming field of research to search for novel actinobacteria and pharmaceutical natural products from actinobacteria associated with the marine organisms. In this review, we attempt to summarize the present knowledge on the diversity and natural products production of actinobacteria associated with the marine organisms, based on the publications from 1991 to 2013. 相似文献
256.
257.
258.
X Chen L Tao J Shekiro A Mohaghaghi S Decker W Wang H Smith S Park M Tucker 《Biotechnology for biofuels》2012,5(1):60
ABSTRACT: BACKGROUND: Historically, acid pretreatment technology for the production of bio-ethanol from corn stover has required severe conditions to overcome biomass recalcitrance. However, the high usage of acid and steam at severe pretreatment conditions hinders the economic feasibility of the ethanol production from biomass. In addition, the amount of acetate and furfural produced during harsh pretreatment is in the range that strongly inhibits cell growth and impedes ethanol fermentation. The current work addresses these issues through pretreatment with lower acid concentrations and temperatures incorporated with deacetylation and mechanical refining. RESULTS: The results showed that deacetylation with 0.1 M NaOH before acid pretreatment improved the monomeric xylose yield in pretreatment by up to 20 % while keeping the furfural yield under 2 %. Deacetylation also improved the glucose yield by 10 % and the xylose yield by 20 % during low solids enzymatic hydrolysis. Mechanical refining using a PFI mill further improved sugar yields during both low- and high-solids enzymatic hydrolysis. Mechanical refining also allowed enzyme loadings to be reduced while maintaining high yields. Deacetylation and mechanical refining are shown to assist in achieving 90 % cellulose yield in high-solids (20 %) enzymatic hydrolysis. When fermentations were performed under pH control to evaluate the effect of deacetylation and mechanical refining on the ethanol yields, glucose and xylose utilizations over 90 % and ethanol yields over 90 % were achieved. Overall ethanol yields were calculated based on experimental results for the base case and modified cases. One modified case that integrated deacetylation, mechanical refining, and washing was estimated to produce 88 gallons of ethanol per ton of biomass. CONCLUSION: The current work developed a novel bio-ethanol process that features pretreatment with lower acid concentrations and temperatures incorporated with deacetylation and mechanical refining. The new process shows improved overall ethanol yields compared to traditional dilute acid pretreatment. The experimental results from this work support the techno-economic analysis and calculation of Minimum Ethanol Selling Price (MESP) detailed in our companion paper. 相似文献
259.
260.