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   2319篇
  免费   263篇
  2022年   16篇
  2021年   26篇
  2020年   21篇
  2019年   20篇
  2018年   30篇
  2017年   27篇
  2016年   63篇
  2015年   109篇
  2014年   110篇
  2013年   127篇
  2012年   151篇
  2011年   144篇
  2010年   116篇
  2009年   67篇
  2008年   109篇
  2007年   118篇
  2006年   115篇
  2005年   89篇
  2004年   102篇
  2003年   72篇
  2002年   71篇
  2001年   55篇
  2000年   81篇
  1999年   61篇
  1998年   26篇
  1997年   21篇
  1996年   21篇
  1995年   15篇
  1994年   16篇
  1993年   25篇
  1992年   32篇
  1991年   34篇
  1990年   35篇
  1989年   26篇
  1988年   26篇
  1987年   19篇
  1986年   20篇
  1985年   19篇
  1984年   24篇
  1983年   25篇
  1982年   19篇
  1981年   19篇
  1979年   16篇
  1978年   14篇
  1977年   20篇
  1974年   16篇
  1973年   21篇
  1972年   16篇
  1969年   21篇
  1968年   12篇
排序方式: 共有2582条查询结果,搜索用时 515 毫秒
931.
932.
Literature data for vehicle life cycle water consumption are limited and contradictory; there are no published estimates of vehicle life cycle water withdrawal. To place future discussions of sustainable mobility on a firmer technical basis, we report the results of a cradle‐to‐grave assessment of water withdrawal and water consumption for the gasoline internal combustion engine vehicle (ICEV) and battery electric vehicle (BEV) variants of the 2012 Ford Focus. U.S. average life cycle water withdrawal and consumption of 531 and 131 cubic meters (m3), respectively, for a lifetime driving distance of 160,000 miles are estimated for the Focus ICEV using E10 gasoline. Employing our upper bound of water use in oil refinery operations and corn and ethanol production increases the life cycle withdrawal and consumption to 1,570 and 761 m3, respectively. The U.S. average life cycle water withdrawal for the Focus BEV is 3,770 m3 (7 times that for the ICEV, reflecting the large volume of cooling water required during electricity generation), whereas the water consumption is 170 m3 (comparable to that for the ICEV). Vehicle use is the most significant phase of the life cycle with fuel production, accounting for 49% of water withdrawal and 82% of water consumption for the ICEV. For the BEV, fuel (electricity) production accounts for 92% of life cycle water withdrawal and 85% of consumption. The results highlight the importance of renewable and sustainable fuels and increased vehicle energy efficiency in providing sustainable mobility.  相似文献   
933.
Honeybees show a remarkable behavioral plasticity at the transition from nursing inside the hive to foraging for nectar and/or pollen outside. This plasticity is important for age‐related division of labor in honeybee colonies. The behavioral transition is associated with significant volume and synaptic changes in the mushroom bodies (MBs), brain centers for sensory integration, learning, and memory. We tested whether precocious sensory exposure to light leads to changes in the density of synaptic complexes [microglomeruli (MG)] in the MBs. The results show that exposure to light pulses over 3 days induces a significant decrease in the MG density in visual subregions (collar) of the MB. Earlier studies had shown that foragers have increased levels of juvenile hormone (JH) co‐occurring with a decrease of vitellogenin (Vg). Previous work further established that RNAi‐mediated knockdown of vg and ultraspiracle (usp) induced an upregulation of JH levels, which can lead to precocious foraging. By disturbing both Vg and JH pathways using gene knockdown of vg and usp, we tested whether the changes in the hormonal system directly affect MG densities. Our study shows that MG numbers remained unchanged when Vg and JH pathways were perturbed, suggesting no direct hormonal influences on MG densities. However, mass spectrometry detection of JH revealed that precocious light exposure triggered an increase in JH levels in the hemolymph (HL) of young bees. This suggests a dual effect following light exposure via direct effects on MG reorganization in the MB calyx and a possible positive feedback on HL JH levels. © 2014 Wiley Periodicals, Inc. Develop Neurobiol 74: 1141–1153, 2014  相似文献   
934.
935.
Mapping global cropland and field size   总被引:8,自引:0,他引:8       下载免费PDF全文
Steffen Fritz  Linda See  Ian McCallum  Liangzhi You  Andriy Bun  Elena Moltchanova  Martina Duerauer  Fransizka Albrecht  Christian Schill  Christoph Perger  Petr Havlik  Aline Mosnier  Philip Thornton  Ulrike Wood‐Sichra  Mario Herrero  Inbal Becker‐Reshef  Chris Justice  Matthew Hansen  Peng Gong  Sheta Abdel Aziz  Anna Cipriani  Renato Cumani  Giuliano Cecchi  Giulia Conchedda  Stefanus Ferreira  Adriana Gomez  Myriam Haffani  Francois Kayitakire  Jaiteh Malanding  Rick Mueller  Terence Newby  Andre Nonguierma  Adeaga Olusegun  Simone Ortner  D. Ram Rajak  Jansle Rocha  Dmitry Schepaschenko  Maria Schepaschenko  Alexey Terekhov  Alex Tiangwa  Christelle Vancutsem  Elodie Vintrou  Wu Wenbin  Marijn van der Velde  Antonia Dunwoody  Florian Kraxner  Michael Obersteiner 《Global Change Biology》2015,21(5):1980-1992
A new 1 km global IIASA‐IFPRI cropland percentage map for the baseline year 2005 has been developed which integrates a number of individual cropland maps at global to regional to national scales. The individual map products include existing global land cover maps such as GlobCover 2005 and MODIS v.5, regional maps such as AFRICOVER and national maps from mapping agencies and other organizations. The different products are ranked at the national level using crowdsourced data from Geo‐Wiki to create a map that reflects the likelihood of cropland. Calibration with national and subnational crop statistics was then undertaken to distribute the cropland within each country and subnational unit. The new IIASA‐IFPRI cropland product has been validated using very high‐resolution satellite imagery via Geo‐Wiki and has an overall accuracy of 82.4%. It has also been compared with the EarthStat cropland product and shows a lower root mean square error on an independent data set collected from Geo‐Wiki. The first ever global field size map was produced at the same resolution as the IIASA‐IFPRI cropland map based on interpolation of field size data collected via a Geo‐Wiki crowdsourcing campaign. A validation exercise of the global field size map revealed satisfactory agreement with control data, particularly given the relatively modest size of the field size data set used to create the map. Both are critical inputs to global agricultural monitoring in the frame of GEOGLAM and will serve the global land modelling and integrated assessment community, in particular for improving land use models that require baseline cropland information. These products are freely available for downloading from the http://cropland.geo-wiki.org website.  相似文献   
936.
AN9 is a glutathione S-transferase from petunia (Petunia hybrida) required for efficient anthocyanin export from the site of synthesis in the cytoplasm into permanent storage in the vacuole. For many xenobiotics it is well established that a covalent glutathione (GSH) tag mediates recognition of molecules destined for vacuolar sequestration by a tonoplast-localized ATP-binding cassette pump. Here we inquired whether AN9 catalyzes the formation of GSH conjugates with flavonoid substrates. Using high-performance liquid chromatography analysis of reaction mixtures containing enzyme, GSH, and flavonoids, including anthocyanins, we could detect neither conjugates nor a decrease in the free thiol concentration. These results suggest that no conjugate is formed in vitro. However, AN9 was shown to bind flavonoids using three assays: inhibition of the glutathione S-transferase activity of AN9 toward the common substrate 1-chloro 2,4-dinitrobenzene, equilibrium dialysis, and tryptophan quenching. We conclude that AN9 is a flavonoid-binding protein, and propose that in vivo it serves as a cytoplasmic flavonoid carrier protein.  相似文献   
937.
938.
939.
T2Rs function as bitter taste receptors   总被引:49,自引:0,他引:49  
Chandrashekar J  Mueller KL  Hoon MA  Adler E  Feng L  Guo W  Zuker CS  Ryba NJ 《Cell》2000,100(6):703-711
Bitter taste perception provides animals with critical protection against ingestion of poisonous compounds. In the accompanying paper, we report the characterization of a large family of putative mammalian taste receptors (T2Rs). Here we use a heterologous expression system to show that specific T2Rs function as bitter taste receptors. A mouse T2R (mT2R-5) responds to the bitter tastant cycloheximide, and a human and a mouse receptor (hT2R-4 and mT2R-8) responded to denatonium and 6-n-propyl-2-thiouracil. Mice strains deficient in their ability to detect cycloheximide have amino acid substitutions in the mT2R-5 gene; these changes render the receptor significantly less responsive to cycloheximide. We also expressed mT2R-5 in insect cells and demonstrate specific tastant-dependent activation of gustducin, a G protein implicated in bitter signaling. Since a single taste receptor cell expresses a large repertoire of T2Rs, these findings provide a plausible explanation for the uniform bitter taste that is evoked by many structurally unrelated toxic compounds.  相似文献   
940.
The repulsive guidance molecule (RGM) is a membrane-bound protein that has diverse functions in the developing central nervous system. Identification of neogenin as a receptor for RGM provided evidence of its cell death-inducing activity in the absence of RGM. Here, we show that the serine/threonine kinase death-associated protein kinase (DAPK) is involved in the signal transduction of neogenin. Neogenin interacts with DAPK and reduces DAPK autophosphorylation on Ser308 in vitro. Neogenin-induced cell death is abolished in the presence of RGM or by blocking DAPK. Although neogenin overexpression or RGM downregulation in the chick neural tube in vivo induces apoptosis, coexpression of the dominant-negative mutant or small-interference RNA of DAPK attenuates this proapoptotic activity. Thus, RGM/neogenin regulates cell fate by controlling the DAPK 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