全文获取类型
收费全文 | 15785篇 |
免费 | 1382篇 |
国内免费 | 1595篇 |
专业分类
18762篇 |
出版年
2024年 | 38篇 |
2023年 | 222篇 |
2022年 | 580篇 |
2021年 | 868篇 |
2020年 | 655篇 |
2019年 | 747篇 |
2018年 | 716篇 |
2017年 | 523篇 |
2016年 | 673篇 |
2015年 | 999篇 |
2014年 | 1153篇 |
2013年 | 1277篇 |
2012年 | 1531篇 |
2011年 | 1286篇 |
2010年 | 791篇 |
2009年 | 758篇 |
2008年 | 819篇 |
2007年 | 690篇 |
2006年 | 608篇 |
2005年 | 552篇 |
2004年 | 396篇 |
2003年 | 397篇 |
2002年 | 357篇 |
2001年 | 243篇 |
2000年 | 208篇 |
1999年 | 238篇 |
1998年 | 135篇 |
1997年 | 144篇 |
1996年 | 137篇 |
1995年 | 120篇 |
1994年 | 116篇 |
1993年 | 88篇 |
1992年 | 96篇 |
1991年 | 74篇 |
1990年 | 82篇 |
1989年 | 61篇 |
1988年 | 58篇 |
1987年 | 43篇 |
1986年 | 50篇 |
1985年 | 50篇 |
1984年 | 33篇 |
1983年 | 21篇 |
1982年 | 22篇 |
1981年 | 11篇 |
1980年 | 10篇 |
1979年 | 16篇 |
1978年 | 6篇 |
1977年 | 12篇 |
1976年 | 9篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Vitamin C is essential for many enzymatic reactions and also acts as a free radical scavenger. Specific non-overlapping transport proteins mediate the transport of the oxidized form of vitamin C, dehydroascorbic acid, and the reduced form, L-ascorbic acid, across biological membranes. Dehydroascorbic acid uptake is via the facilitated-diffusion glucose transporters, GLUT 1, 3 and 4, but under physiological conditions these transporters are unlikely to play a major role in the uptake of vitamin C due to the high concentrations of glucose that will effectively block influx. L-ascorbic acid enters cells via Na+-dependent systems, and two isoforms of these transporters (SVCT1 and SVCT2) have recently been cloned from humans and rats. Transport by both isoforms is stereospecific, with a pH optimum of approximately 7.5 and a Na+:ascorbic acid stoichiometry of 2:1. SVCT2 may exhibit a higher affinity for ascorbic acid than SVCT1 but with a lower maximum velocity. SVCT1 and SVCT2 are predicted to have 12 transmembrane domains, but they share no structural homology with other Na+ co-transporters. Potential sites for phosphorylation by protein kinase C exist on the cytoplasmic surface of both proteins, with an additional protein kinase A site in SVCT1. The two isoforms also differ in their tissue distribution: SVCT1 is present in epithelial tissues, whereas SVCT2 is present in most tissues with the exception of lung and skeletal muscle. 相似文献
82.
83.
The present paper reports the chromosome numbers and karyotypes of 5 species
in Ranunculus from Jiangxi. The result is shown in Table 1-2. The chromosome numbers of
R. ternatus Thunb. (2n=4x=32; 2n=2x=16=8m+2sm+6st) , R. polii Franch.
(2n = 2x = 16 = 8m+2sm+6st) and R. sieboldii Miq. (2n = 8x-1 = 63 = 15m+18sm+22st+8t) are
first reported. The essential points are as follows:
(1) The karyotypes of R. ternatus Thunb. and R. polii Franch. are rather similar, which
shows a close relationship between the two species.
(2) Polyploid complexes are common in Ranunculus.
(3) According to the taxonomical system of Wang Wen-cai, the karyotypes of the two
species investigated in Sect. Auricomus belong to “2A” of Stebbins; that of the only species in
Sect. Hecatonia belong to “2B'; the karyotypes of the two species investigated in Sect.
Ranunculus belong to “3A” or “3B”. The relationships among the three sections from thekaryotype are basically consistent with those based on morphology. 相似文献
84.
85.
86.
87.
88.
Lin Xu Yong Zhang Zheng Shan Deng Liang Zhao Xiu Li Wei Ge Hong Wei 《Antonie van Leeuwenhoek》2013,103(3):559-565
During a study of the diversity and phylogeny of rhizobia isolated from root nodules of Oxytropis ochrocephala grown in the northwest of China, four strains were classified in the genus Rhizobium on the basis of their 16S rRNA gene sequences. These strains have identical 16S rRNA gene sequences, which showed a mean similarity of 94.4 % with the most closely related species, Rhizobium oryzae. Analysis of recA and glnA sequences showed that these strains have less than 88.1 and 88.7 % similarity with the defined species of Rhizobium, respectively. The genetic diversity revealed by ERIC-PCR fingerprinting indicated that the isolates correspond to different strains. Strain CCNWQLS01T contains Q-10 as the predominant ubiquinone. The major fatty acids were identified as feature 8 (C18: 1ω7c and/or C18: 1ω6c; 67.2 %). Therefore, a novel species Rhizobium qilianshanense sp. nov. is proposed, and CCNWQLS01T (= ACCC 05747T = JCM 18337T) is designated as the type strain. 相似文献
89.
SIRT1 expression is refractory to hypoxia and inflammatory cytokines in nucleus pulposus cells: Novel regulation by HIF‐1α and NF‐κB signaling 下载免费PDF全文
90.