全文获取类型
收费全文 | 5274篇 |
免费 | 467篇 |
国内免费 | 7篇 |
专业分类
5748篇 |
出版年
2021年 | 44篇 |
2018年 | 36篇 |
2017年 | 39篇 |
2016年 | 80篇 |
2015年 | 139篇 |
2014年 | 149篇 |
2013年 | 215篇 |
2012年 | 253篇 |
2011年 | 254篇 |
2010年 | 151篇 |
2009年 | 173篇 |
2008年 | 222篇 |
2007年 | 220篇 |
2006年 | 250篇 |
2005年 | 219篇 |
2004年 | 186篇 |
2003年 | 222篇 |
2002年 | 223篇 |
2001年 | 73篇 |
2000年 | 74篇 |
1999年 | 84篇 |
1998年 | 83篇 |
1997年 | 69篇 |
1996年 | 65篇 |
1995年 | 53篇 |
1994年 | 57篇 |
1993年 | 58篇 |
1992年 | 49篇 |
1991年 | 55篇 |
1990年 | 63篇 |
1989年 | 52篇 |
1987年 | 40篇 |
1986年 | 38篇 |
1985年 | 47篇 |
1984年 | 71篇 |
1983年 | 67篇 |
1982年 | 58篇 |
1981年 | 65篇 |
1980年 | 57篇 |
1979年 | 61篇 |
1978年 | 65篇 |
1977年 | 51篇 |
1976年 | 53篇 |
1974年 | 40篇 |
1973年 | 58篇 |
1972年 | 45篇 |
1971年 | 36篇 |
1970年 | 40篇 |
1969年 | 43篇 |
1967年 | 35篇 |
排序方式: 共有5748条查询结果,搜索用时 0 毫秒
61.
A mutant strain of Escherichia coli K12 requiring lipoic acid, W1485 lip 2 (ATCC 25645), was used to develop a turbidimetric assay for lipoic acid and a polarographic assay based on the oxidation of pyruvate by suspensions of lipoic acid-deficient organisms. The turbidimetric assay was more sensitive with a working range equivalent to 0.2–2.0 ng of dl-α-lipoic acid compared with 5–50 ng for the polarographic method. The mutant responded equally to racemic mixtures of α-lipoic acid, β-lipoic acid and dihydrolipoic acid but gave little response to lipoamide, and other derivatives without prior hydrolysis; 8-methyllipoic acid was a competitive inhibitor of the response to lipoic acid. A high specificity of the mutant for the natural stereoisomer was indicated by the fact that (+)-α-lipoic acid had twice the activity of the racemic mixture. Escherichia coli K12 contained less than 0.05 ng of free (+)-α-lipoic acid per mg dry weight but, depending on the growth substrate, the equivalent of between 13 and 47 ng of (+)-α-lipoic acid per mg dry weight after acid extraction. There was a strong correlation between the lipoic acid content and the sum of the specific activities for the pyruvate and α-ketoglutarate dehydrogenase complexes. Experiments with washed suspensions of Escherichia coli showed only small increases in lipoic acid content (18%) when incubated with pyruvate, cysteine and methionine. When supplied with exogenous lipoic acid the mutant, W1485 lip 2, accumulated very little more than was demanded by its metabolism. The lipoic acid contents of several organisms were measured and correlated with their metabolism. 相似文献
62.
63.
Populations of the cladoceran Daphniopsis studeri Rühe in freshwater and brackish lakes of eastern Antarctica have been thought to consist solely of females that reproduce
parthenogenetically by the production of ameiotic subitaneous eggs. This note reports the presence of male D. studeri and the production of ephippial (sexual) eggs in a number of lakes of the Vestfold and Larsemann Hills, which indicate the
possibility of sexual reproduction within these populations.
Received: 12 May 1997 / Accepted: 8 September 1997 相似文献
64.
65.
Hiltrud White Herbert Lebertz Iordanes Thanos Helmut Simon 《FEMS microbiology letters》1987,43(2):173-176
Whole cells of Clostridium thermoaceticum, crude extracts of such cells as well as the supernatant of 100 000 × g centrifugations catalyse the reduction of carbon monoxide to methanol in the presence of viologens or cobalt sepulchrate. Without such a mediator methanol could not be detected. The reaction shows a marked optimum at pH 5. The incubation of [5-14C]methyltetrahydrofolate led only to the formation of 14C-labeled ethanol; the radioactivity in methanol was negligible. The reaction seems to be catalysed by carbon monoxide dehydrogenase. 相似文献
66.
Hepatitis Delta Virus RNA Editing Is Highly Specific for the Amber/W Site and Is Suppressed by Hepatitis Delta Antigen 总被引:7,自引:1,他引:7 下载免费PDF全文
Andrew G. Polson Herbert L. Ley III Brenda L. Bass John L. Casey 《Molecular and cellular biology》1998,18(4):1919-1926
RNA editing at adenosine 1012 (amber/W site) in the antigenomic RNA of hepatitis delta virus (HDV) allows two essential forms of the viral protein, hepatitis delta antigen (HDAg), to be synthesized from a single open reading frame. Editing at the amber/W site is thought to be catalyzed by one of the cellular enzymes known as adenosine deaminases that act on RNA (ADARs). In vitro, the enzymes ADAR1 and ADAR2 deaminate adenosines within many different sequences of base-paired RNA. Since promiscuous deamination could compromise the viability of HDV, we wondered if additional deamination events occurred within the highly base paired HDV RNA. By sequencing cDNAs derived from HDV RNA from transfected Huh-7 cells, we determined that the RNA was not extensively modified at other adenosines. Approximately 0.16 to 0.32 adenosines were modified per antigenome during 6 to 13 days posttransfection. Interestingly, all observed non-amber/W adenosine modifications, which occurred mostly at positions that are highly conserved among naturally occurring HDV isolates, were found in RNAs that were also modified at the amber/W site. Such coordinate modification likely limits potential deleterious effects of promiscuous editing. Neither viral replication nor HDAg was required for the highly specific editing observed in cells. However, HDAg was found to suppress editing at the amber/W site when expressed at levels similar to those found during HDV replication. These data suggest HDAg may regulate amber/W site editing during virus replication. 相似文献
67.
68.
Water oxidation in photosynthesis takes place in photosystem II (PSII). This photosystem is built around a reaction center (RC) where sunlight-induced charge separation occurs. This RC consists of various polypeptides that bind only a few chromophores or pigments, next to several other cofactors. It can handle far more photons than the ones absorbed by its own pigments and therefore, additional excitations are provided by the surrounding light-harvesting complexes or antennae. The RC is located in the PSII core that also contains the inner light-harvesting complexes CP43 and CP47, harboring 13 and 16 chlorophyll pigments, respectively. The core is surrounded by outer light-harvesting complexes (Lhcs), together forming the so-called supercomplexes, at least in plants. These PSII supercomplexes are complemented by some “extra” Lhcs, but their exact location in the thylakoid membrane is unknown. The whole system consists of many subunits and appears to be modular, i.e., both its composition and organization depend on environmental conditions, especially on the quality and intensity of the light. In this review, we will provide a short overview of the relation between the structure and organization of pigment-protein complexes in PSII, ranging from individual complexes to entire membranes and experimental and theoretical results on excitation energy transfer and charge separation. It will become clear that time-resolved fluorescence data can provide invaluable information about the organization and functioning of thylakoid membranes. At the end, an overview will be given of unanswered questions that should be addressed in the near future. 相似文献
69.
K.-Peter Stahmann Herbert N. Arst Jr Henning Althöfer José Luis Revuelta Nicole Monschau Christina Schlüpen Cornelia Gätgens reas Wiesenburg Thomas Schlösser 《Environmental microbiology》2001,3(9):545-550
Riboflavin (vitamin B2 ), essential in tiny amounts as a precursor for oxidoreductase coenzymes, is a yellow pigment. Although it causes cytotoxicity via photoinduced damage of macromolecules, several microorganisms are striking overproducers. A question, unanswered for decades, is whether riboflavin overproducers can benefit from this property. Here, we report an ultraviolet (UV) protective effect of riboflavin. The spores of Ashbya gossypii , a riboflavin-overproducing fungus, are more sensitive to UV than those of Aspergillus nidulans . The addition of riboflavin to suspensions improves the UV resistance of both spore types. Interestingly, we show that regulation of sporulation and riboflavin overproduction in A. gossypii are linked. In batch culture, both were elevated when growth ceased. At constant growth rates, obtained in a chemostat culture, neither was elevated. Supplementation of cultures by cAMP, a known stress signal, negatively affected sporulation as well as riboflavin overproduction, establishing a second, independent argument for the linkage. 相似文献
70.