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   45755篇
  免费   1887篇
  国内免费   2120篇
  2023年   477篇
  2022年   738篇
  2021年   786篇
  2020年   939篇
  2019年   1120篇
  2018年   1143篇
  2017年   991篇
  2016年   1023篇
  2015年   1001篇
  2014年   2085篇
  2013年   3500篇
  2012年   1497篇
  2011年   2203篇
  2010年   1649篇
  2009年   2127篇
  2008年   2308篇
  2007年   2301篇
  2006年   1935篇
  2005年   1828篇
  2004年   1475篇
  2003年   1428篇
  2002年   1175篇
  2001年   934篇
  2000年   823篇
  1999年   771篇
  1998年   787篇
  1997年   753篇
  1996年   720篇
  1995年   682篇
  1994年   702篇
  1993年   635篇
  1992年   601篇
  1991年   532篇
  1990年   483篇
  1989年   470篇
  1988年   419篇
  1987年   435篇
  1986年   303篇
  1985年   668篇
  1984年   951篇
  1983年   654篇
  1982年   730篇
  1981年   575篇
  1980年   508篇
  1979年   447篇
  1978年   279篇
  1977年   274篇
  1976年   227篇
  1974年   186篇
  1973年   183篇
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
51.
Fourier transform infrared (FTIR) spectroscopic imaging is an emerging microscopy modality for clinical histopathologic diagnoses as well as for biomedical research. Spectral data recorded in this modality are indicative of the underlying, spatially resolved biochemical composition but need computerized algorithms to digitally recognize and transform this information to a diagnostic tool to identify cancer or other physiologic conditions. Statistical pattern recognition forms the backbone of these recognition protocols and can be used for highly accurate results. Aided by biochemical correlations with normal and diseased states and the power of modern computer-aided pattern recognition, this approach is capable of combating many standing questions of traditional histology-based diagnosis models. For example, a simple diagnostic test can be developed to determine cell types in tissue. As a more advanced application, IR spectral data can be integrated with patient information to predict risk of cancer, providing a potential road to precision medicine and personalized care in cancer treatment. The IR imaging approach can be implemented to complement conventional diagnoses, as the samples remain unperturbed and are not destroyed. Despite high potential and utility of this approach, clinical implementation has not yet been achieved due to practical hurdles like speed of data acquisition and lack of optimized computational procedures for extracting clinically actionable information rapidly. The latter problem has been addressed by developing highly efficient ways to process IR imaging data but remains one that has considerable scope for progress. Here, we summarize the major issues and provide practical considerations in implementing a modified Bayesian classification protocol for digital molecular pathology. We hope to familiarize readers with analysis methods in IR imaging data and enable researchers to develop methods that can lead to the use of this promising technique for digital diagnosis of cancer.  相似文献   
52.
The cellular energy and biomass demands of cancer drive a complex dynamic between uptake of extracellular FAs and their de novo synthesis. Given that oxidation of de novo synthesized FAs for energy would result in net-energy loss, there is an implication that FAs from these two sources must have distinct metabolic fates; however, hitherto, all FAs have been considered part of a common pool. To probe potential metabolic partitioning of cellular FAs, cancer cells were supplemented with stable isotope-labeled FAs. Structural analysis of the resulting glycerophospholipids revealed that labeled FAs from uptake were largely incorporated to canonical (sn-) positions on the glycerol backbone. Surprisingly, labeled FA uptake also disrupted canonical isomer patterns of the unlabeled lipidome and induced repartitioning of n-3 and n-6 PUFAs into glycerophospholipid classes. These structural changes support the existence of differences in the metabolic fates of FAs derived from uptake or de novo sources and demonstrate unique signaling and remodeling behaviors usually hidden from conventional lipidomics.  相似文献   
53.
Abstract The growth, biomass δ13C values, and ability to accumulate titratable acidity at night were compared in eight environmental treatments for Cremnophila linguifolia, Sedum greggii, and their F1 hybrid. In the phytotron, differences in treatment daylength, day/night temperature and water availability were all found to have effects on total plant dry weight, nocturnal accumulation of titratable acidity and biomass δ13C value of at least some of the genotypes. However, there were differences between the genotypes both in the magnitude and direction of response of the phenotypic properties to the treatment variables. The phytotron δ13C values ranged from -12.9 to -19.2‰ for C. linguifolia, from -22.2 to -33.4‰ for S. greggii, and from -19.2 to -24.9‰ for the hybrid. After with-holding water for 76 h both C. linguifolia and the hybrid had midday Ψleaf values of -0.23 MPa; however, S. greggii had a value of -1.05 MPa. In contrast to past observations of other species, the daily watered plants of C. linguifolia had less negative δ13C values than did the plants watered only weekly.  相似文献   
54.
55.
SUMO化是一种重要的蛋白质翻译后修饰,对植物正常生长发育不可或缺。到目前为止已筛选到上千个可能的SUMO底物,但由于SUMO化修饰水平普遍很低,其生物学功能研究相对较少。该文详细描述了检测蛋白SUMO化修饰的常用方法,包括体外和体内SUMO化实验,以及SUMO化修饰位点的检测方法,旨在为深入研究植物蛋白SUMO化修饰提供技术支持。  相似文献   
56.
57.
The use of miniaturized video cameras to study the at‐sea behavior of flying seabirds has increased in recent years. These cameras allow researchers to record several behaviors that were not previously possible to observe. However, video recorders produce large amounts of data and videos can often be time‐consuming to analyze. We present a new technique using open‐source software to extract bank angles from bird‐borne video footage. Bank angle is a key facet of dynamic soaring, which allows albatrosses and petrels to efficiently search vast areas of ocean for food. Miniaturized video cameras were deployed on 28 Wandering Albatrosses (Diomedea exulans) on Marion Island (one of the two Prince Edward Islands) from 2016 to 2018. The OpenCV library for the Python programming language was used to extract the angle of the horizon relative to the bird’s body (= bank angle) from footage when the birds were flying using a series of steps focused on edge detection. The extracted angles were not significantly different from angles measured manually by three independent observers, thus being a valid method to measure bank angles. Image quality, high wind speeds, and sunlight all influenced the accuracy of angle estimates, but post‐processing eliminated most of these errors. Birds flew most often with cross‐winds (58%) and tailwinds (39%), resulting in skewed distributions of bank angles when birds turned into the wind more often. Higher wind speeds resulted in extreme bank angles (maximum observed was 94°). We present a novel method for measuring postural data from seabirds that can be used to describe the fine‐scale movements of the dynamic‐soaring cycle. Birds appeared to alter their bank angle in response to varying wind conditions to counter wind drift associated with the prevailing westerly winds in the Southern Ocean. These data, in combination with fine‐scale positional data, may lead to new insights into dynamic‐soaring flight.  相似文献   
58.
Fish finders have already been widely available in the fishing market for a number of years.However,the sizes of these fishfinders are too big and their prices are expensive to suit for the research of robotic fish or mini-submarine.The goal of thisresearch is to propose a low-cost fish detector and classifier which suits for underwater robot or submarine as a proximity sensor.With some pre-condition in hardware and algorithms,the experimental results show that the proposed design has good per-formance,with a detection rate of 100 % and a classification rate of 94 %.Both the existing type of fish and the group behaviorcan be revealed by statistical interpretations such as hovering passion and sparse swimming mode.  相似文献   
59.
Alcohol-extractable, hydrophobic zein proteins contaminate starch granule surfaces and can be removed by enzymatic digestion with thermolysin. The goal of this research was to find practical alternatives to thermolysin that might be used during the corn wet-milling process. All of the commercial thermostable alkaline proteases studied (SP 709, Neutrase, and Spezyme FAN) removed the zein proteins from various types of cornstarch, as demonstrated by the lack of protein bands below 30 kDa under the reducing conditions of SDS-PAGE gel. Each enzyme removed the zein proteins as effectively as thermolysin removed them. However, the removal of the zein protein did not reduce the quantity of free fatty acids associated with the starch. Journal of Industrial Microbiology & Biotechnology (2000) 24, 71–74. Received 27 May 1999/ Accepted in revised form 01 October 1999  相似文献   
60.
Semiempirical and ab initio theoretical methods have been used to investigate molecular structures of the chalcogen-substituted carboxylic acid isomers RC(=O)XH (chalcogenol acid) and RC(=X)OH (chalcogenon acid). A recent experimental report suggests that the chalcogenon isomers, although less stable at room temperature, predominate at low temperature in polar solvents and that there is only a small barrier to isomerization between the isomers. Theoretical calculations have been used to locate minimum energy structures of chalcogen-substituted carboxylic acid isomers and to calculate energy differences between pairs of isomers. Carboxylic acids are well known to dimerize, especially in the gas phase and in non-polar solvents. We have, therefore, also calculated energies of dimerization of the chalcogen-substituted acids by optimizing the geometries of the symmetric dimers. We note that the PM3 level of theory is only qualitatively correct for sulfur- and selenium-containing species but fails even qualitatively for the tellurium-containing compounds. Ab initio results confirm the experimental observations and provide good estimates of both isomerization and dimerization energies. We conclude that for many functional groups with tautomers RC(=X)YH and RC(=Y)XH, the more acidic tautomer is the one with the acid proton on the smaller, more electronegative atom, although in many cases this may not be the more stable tautomer.Electronic Supplementary Material available.  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