全文获取类型
收费全文 | 3243篇 |
免费 | 136篇 |
国内免费 | 294篇 |
专业分类
系统科学 | 262篇 |
丛书文集 | 74篇 |
教育与普及 | 44篇 |
理论与方法论 | 60篇 |
现状及发展 | 117篇 |
综合类 | 3087篇 |
自然研究 | 29篇 |
出版年
2024年 | 18篇 |
2023年 | 42篇 |
2022年 | 79篇 |
2021年 | 87篇 |
2020年 | 99篇 |
2019年 | 71篇 |
2018年 | 79篇 |
2017年 | 85篇 |
2016年 | 78篇 |
2015年 | 118篇 |
2014年 | 166篇 |
2013年 | 156篇 |
2012年 | 219篇 |
2011年 | 189篇 |
2010年 | 138篇 |
2009年 | 212篇 |
2008年 | 159篇 |
2007年 | 204篇 |
2006年 | 154篇 |
2005年 | 161篇 |
2004年 | 144篇 |
2003年 | 121篇 |
2002年 | 115篇 |
2001年 | 88篇 |
2000年 | 71篇 |
1999年 | 93篇 |
1998年 | 83篇 |
1997年 | 65篇 |
1996年 | 62篇 |
1995年 | 44篇 |
1994年 | 51篇 |
1993年 | 38篇 |
1992年 | 35篇 |
1991年 | 33篇 |
1990年 | 29篇 |
1989年 | 30篇 |
1988年 | 23篇 |
1987年 | 16篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 1篇 |
排序方式: 共有3673条查询结果,搜索用时 0 毫秒
781.
王道永 《成都理工大学学报(自然科学版)》1994,(3)
依据龙门山中段中生代盆地的基底性质、沉积构造组合和构造变形,论证了川西拗陷是在印支期发育于扬子板块活动大陆边缘的弧背(前陆)盆地,前陆盆地的构造变形是经印支期构造变动和喜马拉雅期叠加改造形成的推覆构造;并讨论了龙门山中段中生代前陆盆地的构造演化历史。 相似文献
782.
偶氮苯类化合物分子在一定波长的光照射下会产生顺式反式的结构变化,利用东南大学分子逊实验室提出的薄膜实时演变分析技术和基于此技术所研制的系统对这一现象进行研究。我们发现甩膜法制备的异丁烯酸(2-对硝基偶氮苯)乙酯P12薄膜在光照下的折射率的变化是由于偶氮苯分子的光致异构化造成的,用360nm光照使偶氮苯分子产生由反式到顺式的结构变化,再用450nm光照使偶氮苯分子产生由顺式到反式的结构变化。在薄膜实 相似文献
783.
张瑞生 《温州大学学报(自然科学版)》2005,26(2):55-58
介绍了偏振模色散成为光通信领域研究热点的技术背景;重点介绍了偏振模色散的成因、偏振模色散的定义及偏振模色散的偏振主态模型;详细讨论了偏振模色散沿光纤的演化理论. 相似文献
784.
选择与批判:中国社会剧变中进化论的利用 总被引:1,自引:0,他引:1
在上一个世纪之交的时候,进化论思想曾经被人有不同政治倾向的进步思想家所利用。本文考察了中国近代思想爱对达尔文进化论的取舍和发挥,从而使其成为变法维新,批判尊孔复古思潮和改造国民性武器的过程,分析了进化论在近代中国文化使命转换的根本原因。 相似文献
785.
J. Hofsteenge A. Vicentini O. Zelenko 《Cellular and molecular life sciences : CMLS》1998,54(8):804-810
The structural and enzymatic properties of RNase 4 are reviewed. This RNase shows a much higher interspecies similarity (approximately 90%) than the other members of the RNase A superfamily. The enzyme is ubiquitous, with the highest amounts present in liver and lung. Its unique uridine specificity results from alterations in and around the pyrimidine-binding site. In particular, the shortened C-terminus and the side chains of Phe-42, Asp-80 and Arg-101 appear to be involved. RNase 4 binds tightly to the intracellular RNase inhibitor, with a K d of 4 × 10−15 M. 相似文献
786.
The distribution of the various cell wall and cell envelope (S-layer) polymers among the main lineages of the domain Archaea (Archaebacteria) and the chemical composition and primary structure of polymers forming rigid cell wall sacculi is described. Differences between bacteria and archaea in their sensitivity to antibiotics which inhibit cell wall synthesis in bacteria are discussed. 相似文献
787.
The angiogenins 总被引:8,自引:0,他引:8
D. J. Strydom 《Cellular and molecular life sciences : CMLS》1998,54(8):811-824
The angiogenic and other biological functions of the angiogenins, members of the pancreatic RNase superfamily of proteins, are reviewed in the context of their primary and tertiary structures. The ribonucleolytic activity and interactions with the placental ribonuclease inhibitor have seen much study in the last few years. The mechanism of the angiogenic activity of angiogenin has recently been postulated as involving multiple interactions with other proteins through specific regions on the molecular surface of angiogenin. These molecular partners include heparin, plasminogen, elastase, angiostatin, actin and most importantly a 170-kilodalton receptor on subconfluent endothelial cells. The existence of the latter receptor was established in conjunction with a mitogenic activity of angiogenin on subconfluent cells. The levels of angiogenin in various physiological and disease states are summarized, including various studies on pregnancy and angiogenin. Correlations are seen between states of enhanced angiogenesis and angiogenin levels. An overview of the relationship of angiogenin and the other RNases of the superfamily showed that their genes all are in relative close proximity on human chromosome 14. Examination of the many expressed sequence tags published in the public databanks, for angiogenin and the other RNases, revealed that angiogenin and RNase-4 (the most evolutionarily conserved RNase), share various identical 5′-untranslated regions on their sets of messenger RNAs, suggesting that their genes are in very close proximity on chromosome 14 and that they are products of differential splicing. This in turn suggests that, in both humans and mice, expression of these two proteins is under identical control, with obvious implications for their biological activities. The evolutionary history of the angiogenins is examined briefly on the basis of the protein sequences of the human, rabbit, pig, two bovine and four mouse angiogenins, and two mouse angiogenin pseudogene sequences. The discrepancy between the conventional requirement for conservatism in structure to allow multimolecule interactions, and the actual fast-changing sequence of the angiogenins, in concert with the wide-ranging activity even in birds, of human angiogenin, is discussed. 相似文献
788.
Xue QG Itoh N Schey KL Li YL Cooper RK La Peyre JF 《Cellular and molecular life sciences : CMLS》2007,64(1):82-95
A new lysozyme (cv-lysozyme 2) with a MALDI molecular mass of 12 984.6 Da was purified from crystalline styles and digestive
glands of eastern oysters (Crassostrea virginica) and its cDNA sequenced. Quantitative real time RT-PCR detected cv-lysozyme 2 gene expression primarily in digestive gland
tissues, and in situ hybridization located cv-lysozyme 2 gene expression in basophil cells of digestive tubules. Cv-lysozyme 2 showed high amino
acid sequence similarity to other bivalve mollusk lysozymes, including cv-lysozyme 1, a lysozyme recently purified from C. virginica plasma. Differences between cv-lysozyme 2 and cv-lysozyme 1 molecular characteristics, enzymatic properties, antibacterial
activities, distribution in the oyster body and site of gene expression indicate that the main role of cv-lysozyme 2 is in
digestion. While showing that a bivalve mollusk employs different lysozymes for different functions, findings in this study
suggest adaptive evolution of i type lysozymes for nutrition.
Received 30 August 2006; received after revision 14 October 2006; accepted 6 November 2006 相似文献
789.
研究了一个新的可积非线性演化方程,基于其Lax对和谱问题的规范变换,构造出该方程的一个达布变换,进而利用此达布变换,得到该方程的精确解,包括有理解、孤子解与周期解. 相似文献
790.
通过对莫里青断陷构造、沉积特征及油气成藏规律系统分析,总结出西北缘走滑断裂带演化特征及其对油气勘探的意义。研究结果表明:西北缘断裂成盆期为一条高角度的走滑张性断裂,新近系受挤压应力作用发生反转,形成了内部发育众多具走滑性质的次级逆冲断层、挤压褶皱、花状等构造的西北缘走滑逆冲断裂带;宏观来看西北缘断裂经历了早期走滑拉张和晚期走滑挤压2个演化阶段;西北缘走滑断裂对油气勘探具重要意义:深大断裂通过提供大坡度构造环境、有效物源及保存条件为湖泊水下扇有效储集体形成创造了有利条件,且水下扇中扇辫状水道微相上的油气产能较高;断裂带内部形成断褶皱等构造圈闭、具有挤压性质的逆冲断层封堵油气等因素为油气聚集提供了有效圈闭条件;各条件有效配置使得研究区西北缘走滑断裂带成为较有利的油气富集区带。 相似文献