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   30193篇
  免费   2011篇
  国内免费   2285篇
  2023年   515篇
  2022年   519篇
  2021年   756篇
  2020年   894篇
  2019年   1285篇
  2018年   1089篇
  2017年   1009篇
  2016年   1013篇
  2015年   877篇
  2014年   1579篇
  2013年   2728篇
  2012年   1042篇
  2011年   1651篇
  2010年   1197篇
  2009年   1628篇
  2008年   1668篇
  2007年   1565篇
  2006年   1542篇
  2005年   1290篇
  2004年   1170篇
  2003年   1052篇
  2002年   909篇
  2001年   590篇
  2000年   484篇
  1999年   509篇
  1998年   459篇
  1997年   379篇
  1996年   356篇
  1995年   442篇
  1994年   417篇
  1993年   375篇
  1992年   378篇
  1991年   286篇
  1990年   280篇
  1989年   238篇
  1988年   221篇
  1987年   226篇
  1986年   175篇
  1985年   244篇
  1984年   286篇
  1983年   219篇
  1982年   258篇
  1981年   173篇
  1980年   137篇
  1979年   117篇
  1978年   61篇
  1977年   53篇
  1976年   46篇
  1975年   22篇
  1973年   23篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
11.
Summary The development ofGasteria verrucosa ovules and seeds seems to follow a pattern of growth in which the majority of carbohydrates is first used in the sporophytic tissue (nucellus, integuments, and arillus) around the gametophyte-derived cells. After fertilization the carbohydrates are used for further development of the arillus and seed coat. During the next stage carbohydrates are directed to develop the endosperm, followed by carbohydrate investment in the developing embryo and in storage products. This utilization pattern is deducted from a localization study on sucrose synthase and invertase. These two enzymes break down imported sucrose and are in that perspective used as markers for carbohydrate transport since diffusion is expected to be induced towards cells and tissues with high sucrose-hydrolyzing activities.  相似文献   
12.
Hemisuccinates, hemiphthalates, acetylsalicylates, cinnamates, andp-methoxycinnamates of lupeol, betulin, and 3-O-acetylbetulin were synthesized via interaction with corresponding acid anhydrides or acid chlorides. A number of betulin esters in position 3 and 28 were shown to exhibit a pronounced hepatoprotective effect similar to that of betulin and silibor. These experimental data were in a good agreement with the computer prediction of their biological activity. Betulin 3,28-bishemiphthalate was more effective than carsil in models of experimental hepatitis caused by carbon tetrachloride, tetracycline, and ethanol.  相似文献   
13.
The objective of the present study was to compare the pattern of motor activity in unmedicated schizophrenia patients and healthy subjects, and to examine whether the pattern was affected by treatment with typical and atypical antipsychotics. Twenty unmedicated schizophrenic patients wore a wrist actigraph for five days. The actigraph recorded activity levels in one-minute epochs. Patients' pattern of activity was compared with that of healthy subjects. Patients were randomly assigned to treatment with low-dose haloperidol or risperidone. The impact of treatment on the pattern of activity was examined. Compared to controls, untreated patients showed a diminished mean activity count during morning, early and late night periods. Treatment with haloperidol or risperidone at effective doses showed a significant effect on activity level, being more prominent with haloperidol. The results suggest that unmedicated schizophrenic patients exhibit abnormally low levels of motor activity as measured with an objective activity meter. This persists after antipsychotic treatment even though symptoms improve. Future studies should clarify whether motor disturbances are a primary effect of the illness, or related to the illness-related lifestyle.  相似文献   
14.
15.
Silkworm hemolymph contains unique proteins that exhibit anti-apoptotic activity in mammalian cells. Among them, 30 K protein, which is one of the major anti-apoptotic molecules in silkworm hemolymph, has been well investigated. However, little is known about the biological functions of storage protein 1 (SP1), another main protein in silkworm hemolymph. In this study, the anti-apoptotic and anti-oxidative activities of SP1 were analyzed. A stable cell line expressing SP1 was constructed, which showed strong anti-apoptotic effect induced by staurosporine treatment. In addition, the cell line exhibited resistance to oxidative stress caused by hydrogen peroxide. For practical applications of SP1, recombinant SP1 was produced in Escherichia coli, and the supplementation of recombinant SP1 into culture medium exhibited anti-apoptotic and anti-oxidative activities. In addition, SP1 was found to be a cell-penetrating protein and localized in the cytosol as well as on the plasma membrane. The findings showed that SP1 itself is not an anti-oxidant; rather, it mediates intracellular anti-oxidative activity. In conclusion, the cellular resistance of SP1 to apoptosis and oxidative stress will provide a new strategy that could be utilized in the bio-industry for the production of biologics as well as for the development of anti-aging cosmetics.  相似文献   
16.
Comparative phytochemical analyses of hydroalcoholic (50% EtOH) extracts from roots of S. miltiorrhiza (SM) and S. przewalskii (SP) were performed using two complementary LC–MS systems: the first system HPLC-DAD-MSn an ion trap mass spectrometer and the second system consisted high resolution MS/MS Orbitrap mass spectrometer. The individual compounds were identified using a previously published approach via comparison of the exact molecular masses, mass spectra and retention times to those of standard compounds, online available databases and literature data. Moreover, the determination of antioxidative activities of extracts by DPPH and FRAP methods was carried out. Analysis allowed to identify 39 chemical compounds in extracts from both species. Extract from root of SP differs from SM in the presence of several metabolites such as: przewalskinic acid and their derivatives, przewaquinone C, przewaquinonate A, glycosides of rosmarinic acid, methyltanshinonate, whereas tanshinones, salvianolic acids and lithospermic acids occurred in both species. Moreover, it was shown that hydroalcoholic extract from roots of SM exerted stronger antioxidant properties in a FRAP test (max. 323.92 μM Fe2+/L) and in DPPH test (max. 78.64 nM TE) in comparison with SP extract.  相似文献   
17.
18.
A series of 11 novel amides of salinomycin were synthesized for the first time. All the obtained compounds were found to show potent antiproliferative activity against human cancer cell lines including the drug-resistant cancer cells. Four new salinomycin derivatives revealed good antibacterial activity against clinical isolates of methicillin-resistant Staphylococcus epidermidis (MRSE).  相似文献   
19.
Identification of an interleukin-1 beta binding protein in human plasma   总被引:5,自引:0,他引:5  
J.A. Eastgate  J.A. Symons  G.W. Duff   《FEBS letters》1990,260(2):217-219
A covalent cross-linking technique was used to bind iodinated interleukin-1 (IL1) alpha and beta to plasma proteins. One specific IL1 beta binding protein was observed, that when cross-linked to 125I-ILl beta migrated to approximately 60 kDa on SDS-PAGE. The protein did not bind IL1 alpha. The 43 -kDa protein was partially purified using a wheat germ agglutinin affinity column. The isolated factor again specifically bound IL1 beta, and appeared to consist of single chain glycoprotein. The protein was heat stable and had a rapid association time with IL1 beta. This protein may be an important carrier molecule for IL1 beta in vivo.  相似文献   
20.
Over the past decade, dramatic declines in frog populations have been noticed worldwide. To examine this decline, monitoring frogs is becoming increasingly important. Compared to traditional field survey methods, recent advances in acoustic sensor technology have greatly extended spatial and temporal scales for monitoring animal populations. In this paper, we examine the problem of monitoring frog populations by analysing acoustic sensor data, where the population is reflected by community calling activity and species richness. Specifically, a novel acoustic event detection (AED) algorithm is first proposed to filter out those recordings without frog calls. Then, multi-label learning is used to classify each individual recording with six acoustic features: linear predictive coding coefficients, Mel-frequency cepstral coefficients, linear-frequency cepstral coefficients, acoustic complexity index, acoustic diversity index, and acoustic evenness index. Next, frog community calling activity and species richness are estimated by accumulating the results of AED and multi-label learning, respectively. Finally, ordinary least squares regression (OLS) is conducted to reveal the relationship between frog populations (frog calling activity and species richness) and weather variables (maximum temperature and rainfall). Experimental results demonstrate that our proposed intelligent system can significantly facilitate the effort to estimate frog community calling activity and species richness with comparable accuracies. The statistical results of OLS indicate that rainfall pattern has a lagged impact on frog community calling activity (significant in the first day after rainy day) and species richness (significant in the fourth day after rainy day). Temperature is shown to affect species richness but is less likely to change calling activity.  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