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   70988篇
  免费   18363篇
  国内免费   21篇
  2023年   164篇
  2022年   210篇
  2021年   1133篇
  2020年   2611篇
  2019年   4225篇
  2018年   4638篇
  2017年   4820篇
  2016年   5352篇
  2015年   6100篇
  2014年   6168篇
  2013年   6869篇
  2012年   5661篇
  2011年   5229篇
  2010年   5288篇
  2009年   3896篇
  2008年   3706篇
  2007年   3217篇
  2006年   2818篇
  2005年   2672篇
  2004年   2477篇
  2003年   2140篇
  2002年   1866篇
  2001年   1498篇
  2000年   1370篇
  1999年   1011篇
  1998年   391篇
  1997年   330篇
  1996年   238篇
  1995年   224篇
  1994年   219篇
  1993年   182篇
  1992年   334篇
  1991年   303篇
  1990年   270篇
  1989年   231篇
  1988年   175篇
  1987年   165篇
  1986年   132篇
  1985年   105篇
  1984年   81篇
  1983年   85篇
  1982年   64篇
  1981年   53篇
  1980年   56篇
  1979年   70篇
  1978年   52篇
  1977年   50篇
  1976年   43篇
  1975年   43篇
  1974年   66篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
82.
Beyond its role as an electron acceptor in aerobic respiration, oxygen is also a key effector of many developmental events. The oxygen‐sensing machinery and the very fabric of cell identity and function have been shown to be deeply intertwined. Here we take a first look at how oxygen might lie at the crossroads of at least two of the major molecular pathways that shape pancreatic development. Based on recent evidence and a thorough review of the literature, we present a theoretical model whereby evolving oxygen tensions might choreograph to a large extent the sequence of molecular events resulting in the development of the organ. In particular, we propose that lower oxygenation prior to the expansion of the vasculature may favour HIF (hypoxia inducible factor)‐mediated activation of Notch and repression of Wnt/β‐catenin signalling, limiting endocrine cell differentiation. With the development of vasculature and improved oxygen delivery to the developing organ, HIF‐mediated support for Notch signalling may decline while the β‐catenin‐directed Wnt signalling is favoured, which would support endocrine cell differentiation and perhaps exocrine cell proliferation/differentiation.  相似文献   
83.
84.

Background  

Restriction enzymes are one of the everyday tools used in molecular biology. The continuously expanding panel of known restriction enzymes (several thousands) renders their optimal use virtually impossible without computerized assistance. Several manufacturers propose on-line sites that assist scientists in their restriction enzyme work, however, none of these sites meet all the actual needs of laboratory workers, and they do not take into account the enzymes actually present in one's own laboratory.  相似文献   
85.
Alternaria alternata is of major significance as a food and feed contaminant and is able to produce a range of mycotoxins that may elicit adverse effects in both animals and humans. We describe the isolation and characterization of five microsatellite markers for studying A. alternata. Marker polymorphism was screened in 64 isolates of A. alternata. The number of alleles per locus ranged from eight to 24, and allelic diversity ranged from 0.425 to 0.882. These markers will be useful in the study of relationships and population genetics amongst isolates of A. alternata.  相似文献   
86.
87.
Although behavioral studies have been conducted at zoos and aquaria for years, documentation concerning the effectiveness of environmental enrichment has dealt primarily with terrestrial animals and marine mammals. Few enrichment studies have been conducted on reptiles. For this study, behavioral observations were made on four captive sea turtles (three loggerhead, Caretta caretta, and one blind green, Chelonia mydas) with enrichment present and absent. Enrichment devices were modified for the special needs of the blind turtle. Behaviors were classified as Resting, Pattern Swimming, Random Swimming, Focused Behavior, Aggression, Hiding, Orientation, and Noncategorized Behavior. It was hypothesized that, when enrichment was present, a decrease in Resting and stereotypic Pattern Swimming would be seen along with an increase in Random Swimming and Focused Behavior. It was found that, when no enrichment devices were present, 77% of the turtles' time was spent in Resting and Pattern Swimming. When enrichment devices were provided, 88% of their time was spent in Random Swimming and Focused Behavior with only 8% spent in Pattern Swimming and Resting. Statistically, there were significant increases in Random Swimming (three of the four turtles) and Focused Behavior (4/4) and significant decreases in Resting (3/4) and Pattern Swimming (3/4) when enrichment devices were present. These results suggest that environmental enrichment is as effective with marine reptiles as has been found with other animals and should be encouraged for all captive sea turtles. Zoo Biol 26:407–416, 2007. © 2007 Wiley‐Liss, Inc.  相似文献   
88.
Pycnogenol® (PYC), a patented combination of bioflavonoids extracted from the bark of French maritime pine (Pinus maritima), scavenges free radicals and promotes cellular health. The protective capacity of PYC against ethanol toxicity of neurons has not previously been explored. The present study demonstrates that in postnatal day 9 (P9) rat cerebellar granule cells the antioxidants vitamin E (VE) and PYC (1) dose dependently block cell death following 400, 800, and 1600 mg/dL ethanol exposure (2) inhibit the ethanol‐induced activation of caspase‐3 in the same model system; and (3) reduce neuronal membrane disruption as assayed by phosphatidylserine translocation to the cell surface. These results suggest that both PYC and VE have the potential to act as therapeutic agents, antagonizing the induction of neuronal cell death by ethanol exposure. © 2004 Wiley Periodicals, Inc. J Neurobiol 59: 261–271, 2004  相似文献   
89.
A total of 15 polymorphic tetranucleotide microsatellite loci in the red panda, Ailurus fulgens, were characterized in this study. Based on evaluations of 33 red pandas, the number of observed alleles for each locus ranged from seven to 17 and the expected and observed heterozygosities were 0.412–0.897 and 0.121–0.909, respectively. The mean polymorphic information content was 0.721. These markers would greatly strengthen the utilization of microsatellite tools in genetic variation studies in red panda populations.  相似文献   
90.
We present a rapid and high-throughput human serum N-glycan preparation technology using 96-well plate-based procedures. The released N-glycans from polyvinylidene fluoride (PVDF) membrane filter plate are subsequently loaded to porous graphitic carbon (PGC) containing a 96-well plate to remove salts and other contaminants without sacrificing accuracy or reproducibility. This robust glycan preparation technology is applied to ovarian cancer diagnosis using 5 μl of patient serum.  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