全文获取类型
收费全文 | 10281篇 |
免费 | 25篇 |
国内免费 | 37篇 |
专业分类
系统科学 | 31篇 |
丛书文集 | 48篇 |
教育与普及 | 30篇 |
理论与方法论 | 24篇 |
现状及发展 | 3980篇 |
研究方法 | 532篇 |
综合类 | 5519篇 |
自然研究 | 179篇 |
出版年
2012年 | 156篇 |
2011年 | 341篇 |
2010年 | 61篇 |
2009年 | 54篇 |
2008年 | 172篇 |
2007年 | 211篇 |
2006年 | 195篇 |
2005年 | 219篇 |
2004年 | 214篇 |
2003年 | 193篇 |
2002年 | 209篇 |
2001年 | 430篇 |
2000年 | 439篇 |
1999年 | 294篇 |
1996年 | 52篇 |
1994年 | 272篇 |
1992年 | 255篇 |
1991年 | 203篇 |
1990年 | 231篇 |
1989年 | 194篇 |
1988年 | 185篇 |
1987年 | 191篇 |
1986年 | 201篇 |
1985年 | 266篇 |
1984年 | 203篇 |
1983年 | 164篇 |
1982年 | 137篇 |
1981年 | 142篇 |
1980年 | 147篇 |
1979年 | 326篇 |
1978年 | 263篇 |
1977年 | 216篇 |
1976年 | 191篇 |
1975年 | 196篇 |
1974年 | 208篇 |
1973年 | 180篇 |
1972年 | 207篇 |
1971年 | 246篇 |
1970年 | 304篇 |
1969年 | 229篇 |
1968年 | 226篇 |
1967年 | 194篇 |
1966年 | 223篇 |
1965年 | 149篇 |
1959年 | 75篇 |
1958年 | 123篇 |
1957年 | 81篇 |
1956年 | 58篇 |
1954年 | 62篇 |
1948年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
941.
Reactivation effects by glycerol and ethylene glycol of inactivated ALA synthetase of R. spheroids were observed. Accompanying the reactivation of the inactivated enzyme, Km value for PLP decreased to levels similar to those of the freshly prepared enzyme. 相似文献
942.
Administration of large dose of neostigmine caused very quickly marked myopathic changes at the motor end-plate region. With continued injections, however, some recovery of the structural features did occur suggesting the reconstructive changes in the affected regions. 相似文献
943.
Specific activity of glutamine synthetase in Squalus acanthias (spiny dogfish) central nervous system regions was highest in the cerebellum and lowest in the spinal cord. The levels of activity may relate to the excitability of each region by regulating the glutamate pool. 相似文献
944.
Mutations in a gene encoding a new oxygen-regulated photoreceptor protein cause dominant retinitis pigmentosa. 总被引:12,自引:0,他引:12
The autosomal dominant retinitis pigmentosa (RP) locus, designated RP1, has been mapped through linkage studies to a 4-cM interval at 8q11-13. Here we describe a new photoreceptor-specific gene that maps in this interval and whose expression is modulated by retinal oxygen levels in vivo. This gene consists of at least 4 exons that encode a predicted protein of 2,156 amino acids. A nonsense mutation at codon 677 of this gene is present in approximately 3% of cases of dominant RP in North America. We also detected two deletion mutations that cause frameshifts and introduce premature termination codons in three other families with dominant RP. Our data suggest that mutations in this gene cause dominant RP, and that the encoded protein has an important but unknown role in photoreceptor biology. 相似文献
945.
946.
本文研究以琼脂糖为基的固体支撑双层脂膜(sBLM)对味觉物质(乙酸、盐酸喹咛、蔗糖)的响应.这三种味觉物质显著地改变了膜电位,盐酸喹咛、蔗糖对膜电阻有一定影响,脂膜对乙酸和盐酸喹咛也有良好的线性响应.检测灵敏度分别为12μmol/L和1.3μmol/L,其响应特性、检测灵敏度都达到或优于现有的其它检测方法.用C60修饰卵磷脂双层脂膜,加大了膜强度,用它检测上述三种物质,得到了相同结果. 相似文献
947.
Nucleosome mobilization catalysed by the yeast SWI/SNF complex. 总被引:18,自引:0,他引:18
948.
光学头定位控制由于快速性的要求使得系统产生振荡,影响定位精度.本文应用输入成形方法抑制光头在定位时的振动.通过建立光学头机电系统的数学模型,对于无参数摄动和有参数摄动两种情形分别应用零振荡()和零振荡加微分()两种输入成形控制器进行振动抑制,计算机数值仿真结果表明,该方法对于抑制光学头定位过程中的振动具有较好的效果. 相似文献
949.
Distinct and evolutionarily conserved signal-transduction cascades mediate the survival or death of cells during development. The c-Jun amino-terminal kinases (JNKs) of the mitogen-activated protein kinase superfamily are involved in apoptotic signalling in various cultured cells. However, the role of the JNK pathway in development is less well understood. In Drosophila, Decapentaplegic (Dpp; a homologue of transforming growth factor-beta) and Wingless (Wg; a Wnt homologue) proteins are secretory morphogens that act cooperatively to induce formation of the proximodistal axis of appendages. Here we show that either decreased Dpp signalling in the distal wing cells or increased Dpp signalling in the proximal wing cells causes apoptosis. Inappropriate levels of Dpp signalling lead to aberrant morphogenesis in the respective wing zones, and these apoptotic zones are also determined by the strength of the Wg signal. Our results indicate that distortion of the positional information determined by Dpp and Wg signalling gradients leads to activation of the JNK apoptotic pathway, and the consequent induction of cell death thereby maintains normal morphogenesis. 相似文献
950.
Detection of a cystic fibrosis modifier locus for meconium ileus on human chromosome 19q13. 总被引:14,自引:0,他引:14