全文获取类型
收费全文 | 6783篇 |
免费 | 692篇 |
国内免费 | 823篇 |
出版年
2024年 | 33篇 |
2023年 | 113篇 |
2022年 | 224篇 |
2021年 | 388篇 |
2020年 | 311篇 |
2019年 | 345篇 |
2018年 | 305篇 |
2017年 | 224篇 |
2016年 | 330篇 |
2015年 | 473篇 |
2014年 | 527篇 |
2013年 | 528篇 |
2012年 | 613篇 |
2011年 | 561篇 |
2010年 | 364篇 |
2009年 | 295篇 |
2008年 | 372篇 |
2007年 | 385篇 |
2006年 | 323篇 |
2005年 | 265篇 |
2004年 | 218篇 |
2003年 | 216篇 |
2002年 | 194篇 |
2001年 | 102篇 |
2000年 | 74篇 |
1999年 | 75篇 |
1998年 | 71篇 |
1997年 | 42篇 |
1996年 | 36篇 |
1995年 | 22篇 |
1994年 | 23篇 |
1993年 | 30篇 |
1992年 | 26篇 |
1991年 | 23篇 |
1990年 | 13篇 |
1989年 | 24篇 |
1988年 | 15篇 |
1987年 | 13篇 |
1986年 | 13篇 |
1985年 | 11篇 |
1984年 | 9篇 |
1983年 | 9篇 |
1982年 | 7篇 |
1981年 | 7篇 |
1980年 | 4篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1974年 | 5篇 |
1970年 | 4篇 |
1968年 | 4篇 |
排序方式: 共有8298条查询结果,搜索用时 15 毫秒
21.
22.
23.
Kai Larsen 《Nordic Journal of Botany》1996,16(6):636-636
24.
25.
By means of an in vivo brain microdialysis, the effect of different concentrations of physostigmine on the acetylcholine level in the dialysate of rat frontal cortex was studied. Perfusion of the various degrees of physostigmine (eserine) concentration (10 nM−10 μM) into the cortex through the dialysis membrane increased the basal acetylcholine level in a dose-dependent manner. In the presence of 10 nM, 0.1 μM and 10 μM physostigmine in the perfusate, systemic treatment with scopolamine (0.5 mg/kg, i.p.) increased 200, 270 and 510%, respectively, the relative acetylcholine level in the dialysates in comparison with the corresponding basal levels, while in the absence of physostigmine the treatment increased it only 40%. From these results, it appears that perfusion of physostigmine at a variety of concentrations, changes not only the basal level of acetylcholine induced by the inhibition of acetylcholinesterase but also the relative acetylcholine output induced by systemic treatment with scopolamine. 相似文献
26.
Akira Kai Takaaki Arashida
Kenichi Hatanaka
Toshihiro AkaikeKei Matsuzaki
Tohru Mimura
Yutaro Kaneko 《Carbohydrate polymers》1994,23(4):235-239In order to elucidate the biosynthetic process of cellulose and curdlan, 13C-labeled polysaccharides were biosynthesized by Acetobacter xylinum (IFO 13693) and Agrobacterium sp. (ATCC 31749), from culture media containing
-(1-13C)glucose,
-(2-13C)glucose,
-(4-13C)glucose, or
-(6-13C)glucose as the carbon source, and their structures were determined by 13C NMR spectroscopy. The labeling was mainly found in the original position, indicating direct polymerization of introduced glucoses. In addition, the transfer of labeling from C-2 to C-1, C-3 and C-5, from C-4 to C-1, C-2 and C-3, and from C-6 to C-1 was found in celluloses. In curdlan, the transfer of labeling from C-1 to C-3, from C-2 to C-1 and C-3, from C-4 to C-1, C-2 and C-3, and from C-6 to C-1 and C-3 was observed. From analysis of this labeling, the biosynthetic process of cellulose and curdlan was explained as involving six routes. The percentages of each route via which cellulose or curdlan is biosynthesized were estimated for upper (C-1 to C-3) and lower portions (C-4 to C-6) of glucosidic units in the polysaccharides. It is noted that very few polysaccharides are formed via the Embden-Meyerhof pathway. The lower half (C-4 to C-6) structure of introduced glucoses is well preserved in the polysaccharides. 相似文献
27.
Repeated polyketide synthase modules involved in the biosynthesis of a heptaene macrolide by Streptomyces sp. FR-008 总被引:4,自引:2,他引:2
Zhihao Hu Kai Bao Xiufen Zhou Qi Zhou David A. Hopwood Tobias Kieser Zixln Deng 《Molecular microbiology》1994,14(1):163-172
Genes for biosynthesis of a Streptomyces sp. FR-008 heptaene macrolide antibiotic with antifungal and mosquito larvicidal activity were cloned in Escherichia coli using heterologous DNA probes. The cloned genes were implicated in heptaene biosynthiesis by gene replacement. The FR-008 antibiotic contains a 38-membered, poiyketide-derived macrolide ring. Southern hybridization using probes encoding domains of the type i modular erythromycin polyketide synthase (PKS) showed that the Streptomyces sp. FR-008 PKS gene cluster contains repeated sequences spanning c. 105 kb of contiguous DNA; assuming c. 5 kb for each PKS module, this is in striking agreement with the expectation for the 21-step condensation process required for synthesis of the FR-008 carbon chain. The methods developed for transformation and gene replacement in Streptomyces sp. FR-008 make it possible to genetically manipulate polyene macrolide production, and may later lead to the biosynthesis of novel polyene macrolides. 相似文献
28.
S. Takai M. Kasama K. Yamada N. Kai N. Hirayama H. Namiki T. Taniyama 《Human genetics》1994,93(1):13-15
The human FcRI gene encodes for a highaffinity Fc receptor that plays pivotal roles in the immune response. We have used fluorescence in situ hybridization analysis to localize the FcRI gene to human chromosome 1. The human FcRI (CD64) gene has been assigned to human chromosome 1q21.2-q21.3 using R-banded human (pro)metaphase chromosomes. 相似文献
29.
The genes encoding apolipoproteins (apos) A-I, B, C-III and E as well as that encoding the angiotensin converting enyzme (ACE) have been proposed as candidate genes for coronary heart disease (CHD). We determined the common polymorphisms of the apo genes, previously found to influence serum lipid levels at the population level, and the insertion/deletion polymorphism of the ACE gene, recently reported to reflect the risk of myocardial infarction, in 82 very young (mean, 41 years) North Karelian Finns with symptomatic CHD and 50 controls of similar age. Patients with familial hypercholesterolemia had been excluded from this material. None of the polymorphisms examined, including the apo A-I promoter MspI, apo C-III SstI and apo B XbaI restriction fragment polymorphisms, a common variation of apo E (2, 3 and 4 alleles) and an ACE insertion/deletion (I/D) polymorphism, was significantly associated with the risk of premature CHD. Patients with CHD had a higher mean serum LDL cholesterol/HDL cholesterol ratio than controls (3.15±1.30 vs 2.72±0.98, P < 0.05), but no significant associations between the common apo gene polymorphisms and serum lipid levels were disclosed in either group. It is possible that other genetic loci than those proposed to be associated with accelerated atherosclerosis may be more important as risk factors of symptomatic CHD at the age of 40 years. 相似文献
30.
小麦体细胞胚发生的超微结构研究 总被引:24,自引:0,他引:24
对小麦(Triticum aestivum L.)幼胚培养中体细胞胚发生过程的超微结构变化进行了研究,结果如下:(1)原体细胞中大液泡消失,存在大量小液泡,细胞质的电子密度加强,核大,核仁明显,出现多核仁;(2)细胞器数量和种类,如质体、核糖体和线粒体增加;(3)细胞壁加厚,胞间连丝逐渐消失,细胞器数量丰富,胚性细胞中积累淀粉;(4)细胞壁加厚的胚性细胞中存在核仁液泡、自体吞噬泡和分泌泡;(5)多细胞原胚、球形胚和梨形胚被一层外壁包围,但胚体内细胞间广泛存在胞间连丝;(6)成熟胚生长点部位的细胞内质体中出现膜系结构,已向叶绿体转变,类囊体已形成 相似文献