全文获取类型
收费全文 | 4324篇 |
免费 | 387篇 |
国内免费 | 1篇 |
出版年
2023年 | 23篇 |
2022年 | 18篇 |
2021年 | 66篇 |
2020年 | 36篇 |
2019年 | 52篇 |
2018年 | 89篇 |
2017年 | 62篇 |
2016年 | 108篇 |
2015年 | 174篇 |
2014年 | 218篇 |
2013年 | 219篇 |
2012年 | 317篇 |
2011年 | 280篇 |
2010年 | 195篇 |
2009年 | 166篇 |
2008年 | 236篇 |
2007年 | 253篇 |
2006年 | 229篇 |
2005年 | 233篇 |
2004年 | 187篇 |
2003年 | 216篇 |
2002年 | 191篇 |
2001年 | 54篇 |
2000年 | 42篇 |
1999年 | 40篇 |
1998年 | 54篇 |
1997年 | 38篇 |
1996年 | 38篇 |
1995年 | 31篇 |
1994年 | 27篇 |
1993年 | 31篇 |
1992年 | 27篇 |
1991年 | 39篇 |
1990年 | 52篇 |
1989年 | 29篇 |
1988年 | 36篇 |
1987年 | 31篇 |
1986年 | 21篇 |
1985年 | 22篇 |
1984年 | 31篇 |
1983年 | 24篇 |
1982年 | 24篇 |
1981年 | 28篇 |
1980年 | 19篇 |
1978年 | 20篇 |
1977年 | 19篇 |
1976年 | 15篇 |
1974年 | 19篇 |
1968年 | 15篇 |
1967年 | 17篇 |
排序方式: 共有4712条查询结果,搜索用时 671 毫秒
61.
Shells of probable former living communities ofDreissena polymorpha were found within sediments of the shallow polytrophic to hypertrophic hard water Lake Breitling (Havel-Lake system, Germany).
Corresponding sediments have been deposited between approximately 1940 and 1970 and reflect increasing eutrophication and
heavy metal pollution of the Lake during this period (Schettler, 1992). Single shells from various sediment depths were analysed
by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) along a line on the outer part of the shell. The
response of these freshwater mussels to increasing heavy metal pollution is clearly reflected in the distribution of Pb, Cu,
Cd and Zn within their valves. In general, Cd, Cu, Pb and Zn contents are lower, and the distribution more even, in the outer
parts of the deepest (oldest) shells compared to shells from higher in the cored sediments. Notably higher contents of Cu,
Pb and Zn were recorded from the central (umbonal) part of the more recent shells, but this behaviour is not recorded for
Cd. Metabolic changes brought on by worsening environmental conditions are proposed to explain this phenomena. Acidity produced
during anaerobic metabolism can be neutralised by dissolution of the carbonate part of the shell. Copper, Zn and Pb, which
show an affinity for the organic component of the shell, may thus accumulate by repeated dissolution and reprecipitation of
the shell during the lifetime of an individual organism. Cadmium, which is bound mainly in the aragonite of the shells, is
released during the dissolution of carbonate and is not concentrated in the umbonal area of the shell. 相似文献
62.
Summary Amino acid starvation of Escherichia coli relA mutants may be used as a method for efficient plasmid DNA amplification. Here we demonstrate DNA degradation which occurs at elevated temperatures (42–43°C) after plasmid amplification in amino acid-starved bacteria. These results may explain the previously described low efficiency of plasmid DNA amplification at elevated temperatures. 相似文献
63.
Anaerobic metabolism of 2-hydroxybenzoic acid (salicylic acid) by a denitrifying bacterium 总被引:6,自引:0,他引:6
The anaerobic metabolism of 2-hydroxybenzoic acid (salicylic acid) was studied in a denitrifying bacterium. Cells grown with
2-hydroxybenzoate were simultaneously adapted to degrade benzoate. Extract of these cells formed benzoate or benzoyl-CoA when
incubated under reducing conditions with salicylate, MgATP, and coenzyme A, suggesting a degradation of 2-hydroxybenzoate
via benzoate or benzoyl-CoA. This suggestion was supported by enzyme activity measurements. In extracts of 2-hydroxybenzoate-grown
cells, the following enzyme activities were detected: two CoA ligases, one specific for 2-hydroxybenzoate, the other for benzoate,
and two different enzyme activities catalyzing the reductive transformation of 2-hydroxybenzoyl-CoA. These findings suggest
a degradation of salicylic acid by two new enzymes, 2-hydroxybenzoate-CoA ligase (AMP-forming) and 2-hydroxybenzoyl-CoA reductase
(dehydroxylating), catalyzing (1) 2-hydroxybenzoate + MgATP + CoASH → 2-hydroxybenzoyl-CoA + MgAMP + PPi (2) 2-hydroxybenzoyl-CoA + 2[H] → benzoyl-CoA + H2O Benzoyl-CoA was dearomatized by reduction of the ring. This represents another case in which benzoyl-CoA is a central intermediate
in anaerobic aromatic metabolism.
Received: 1 February 1996 / Accepted: 24 February 1996 相似文献
64.
Abstract: Derivatives of the lac promoter (tac, pac, rac) belong to the strongest bacterial promoters which are frequently used for the induced overexpression of foreign genes in Escherichia coli . However, their use in fermentation processes is strongly restricted because of the high cost of the inducer iso-propyl-β-D-thiogalactopyranoside (IPTG). The aim of this work was to investigate the possibility of using lac-derived promoters in high cell density processes resulting in a high yield of the induced recombinant protein if glucose is the main carbon and energy source. Lactose is tested as inducer of the main antigenic coat protein (VP1) of the foot and mouth disease (FMD) virus in a T7-RNA polymerase expression system. It was shown that lactose is able to induce the expression of the recombinant gene to an amount of the VP1 protein corresponding to 20% of the total cell protein. 相似文献
65.
PET1402, a nuclear gene required for proteolytic processing of cytochrome oxidase subunit 2 in yeast
Mathias Bauer Meinhardt Behrens Karlheinz Esser Georg Michaelis Elke Pratje 《Molecular & general genetics : MGG》1994,245(3):272-278
The nuclear mutation pet ts1402 prevents proteolytic processing of the precursor of cytochrome oxidase subunit 2 (cox2) in Saccharomyces cerevisiae. The structural gene PET1402 was isolated by genetic complementation of the temperature-sensitive mutation. DNA sequence analysis identified a 1206-bp open reading frame, which is located 215 by upstream of the PET122 gene. The DNA sequence of PET1402 predicts a hydrophobic, integral membrane protein with four transmembrane segments and a typical mitochondrial targeting sequence. Weak sequence similarity was found to two bacterial proteins of unknown function. Haploid cells containing a null allelle of PET1402 are respiratory deficient. 相似文献
66.
Merete Grung Georg Kreimer Michael Calenberg Michael Melkonian Synnøve Liaaen-Jensen 《Planta》1994,193(1):38-43
Isolated intact eyespot apparatuses, the photoreceptive organelles involved in blue-light-mediated photoresponses of flagellate green algae, were analyzed regarding their carotenoid composition. Carotenoids from the eyespot apparatuses of Spermatozopsis similis were identified by high-performance liquid chromatography, visible-light absorption spectra, mass spectroscopy and by 1H-nuclear magnetic resonance spectroscopy (carotenes), and compared with those of whole-cell extracts. Both extracts contained ,-carotene, ,-carotene (formerly -carotene), lycopene, lutein, zeaxanthin, violaxanthin and all-E-and 9-Z-neoxanthin. The relative carotenoid compositions, however, differed significantly. A twofold relative increase in the total carotene level was evident in the fraction enriched in eyespot apparatuses. This was mainly due to an increase in the monocyclic ,-carotene and the aliphatic lycopene, whereas the relative content of ,-carotene remained unchanged. On the other hand a relative decrease in the total xanthophyll content, especially of lutein and the epoxidic carotenoid neoxanthin, was observed in the eyespot apparatuses compared with the whole-cell extracts. The decrease of the latter resulted almost solely from a reduction of the 9-Z-rather than the all-E-isomer. The bulk of the carotenes is thought to be localized in the highly organized eyespot lipid globules, which act as a combined quarter-wave interference reflector and absorption screen for the photoreceptor in green algae. The enrichment of ,-carotene and lycopene in the eyespot apparatuses, extending the range of visible light absorption to longer wavelengths, represents an adaptation of the screen to the retinal-based photoreceptor of flagellate green algae and is one of the prerequisites for maximal directional sensitivity of the eyespot apparatus.Abbreviations
1H-NMR
nuclear magnetic resonance
- IUPAC
International Union of Pure and Applied Chemistry
- VIS
visible absorption spectra
This work was supported by the Deutsche Forschungsgemeinschaft (G.K. and M.M.). M.G. was supported by a fellowship from the Norwegian Research Council of Science and Humanities. 相似文献
67.
Modification of sialic acids by 9-O-acetylation is detected in human leucocytes using the lectin property of influenza C virus 总被引:1,自引:0,他引:1
Zimmer Gert; Suguri Toshiaki; Reuter Gerd; Yu Robert K.; Schauer Roland; Herrler Georg 《Glycobiology》1994,4(3):343-349
Influenza C virus spike glycoprotein HEF specifically recognizesglycoconjugates containing 9-O-acetyl-N-acetylneuraminic acid.The same protein also contains an esterase activity. Takingadvantage of these two properties, influenza C virus was usedas a very sensitive probe for the detection of traces of 9-O-acetyl-N-acetylneuraminicacid in human leucocytes. The binding of influenza C virus toleucocyte glycoproteins and gangliosides separated by sodiumdodecyl sulphatepolyacrylamide gel electrophoresis andthin-layer chromatography, respectively, was assayed using achromogenic esterase substrate. In this way, glycoproteins ofB-lymphocytes and T-lymphocytes were found to contain 9-O-acetylatedsialic acids. Of the various 9-O-acetylated gangliosides detected,one had the characteristics of 9-O-acetylated GD3. The identificationof 9-O-acetylated sialic acids on distinct glycoproteins andglycolipids should be helpful in assigning a physiological roleto this sugar. O-acetylation gangliosides influenza C virus lymphocytes sialic acids 相似文献
68.
Multiple Effects of Dithiothreitol on Nonphotochemical Fluorescence Quenching in Intact Chloroplasts (Influence on Violaxanthin De-epoxidase and Ascorbate Peroxidase Activity) 总被引:2,自引:1,他引:1
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Neubauer C 《Plant physiology》1993,103(2):575-583
Reversible nonphotochemical fluorescence quenching depends on thylakoid lumen acidification and violaxanthin de-epoxidation and is correlated with photoprotection of photosynthesis. The O2-dependent electron flow in the coupled Mehler-ascorbate peroxidase reaction (MP-reaction) mediates the electron flow necessary for lumen acidification and violaxanthin de-epoxidation in isolated, intact chloroplasts. Inhibition of violaxanthin de-epoxidation by dithiothreitol (DTT) was correlated with suppression of fluorescence quenching. In addition, DTT was also found to suppress fluorescence quenching due to inhibition of ascorbate peroxidase activity, a main enzyme of the MP-reaction, even in the presence of zeaxanthin. In intact, non-CO2-fixing chloroplasts, violaxanthin and antheraxanthin de-epoxidation and the ascorbate peroxidase activity show different sensitivities to increasing DTT concentrations. Violaxanthin de-epoxidase activity, measured as the sum of zeaxanthin and antheraxanthin formed, was inhibited with an inhibitor concentration for 50% inhibition (I50) of 0.35 mM DTT. In contrast, inhibition of the O2-dependent electron flow and corresponding lumen acidification occurred with higher I50 values of 2.5 and 3 mM DTT, respectively, and was attributed to inhibition of ascorbate peroxidase activity (I50 = 2 mM DTT). Accordingly, the DTT-induced inhibition of the nigericin-sensitive nonphotochemical fluorescence quenching was correlated linearly with the decreasing concentrations of zeaxanthin and antheraxanthin and was almost unaffected by DTT inhibition of the MP-reaction and correlated [delta]pH. The nigericin-insensitive, photoinhibitory kind of nonphotochemical fluorescence quenching up to 1 mM was mainly correlated with inhibition of violaxanthin de-epoxidation. At higher DTT concentrations, it was attributed to inhibition of both violaxanthin de-epoxidation and MP-reaction. The results show that DTT has multiple, but distinguishable, effects on nonphotochemical fluorescence quenching in isolated chloroplasts, necessitating careful interpretation. 相似文献
69.
70.
Thomas Schiewe Björn Gutschmann Lara Santolin Saskia Waldburger Peter Neubauer Roland Hass Sebastian L. Riedel 《Biotechnology and bioengineering》2023,120(10):2880-2889
An efficient monitoring and control strategy is the basis for a reliable production process. Conventional optical density (OD) measurements involve superpositions of light absorption and scattering, and the results are only given in arbitrary units. In contrast, photon density wave (PDW) spectroscopy is a dilution-free method that allows independent quantification of both effects with defined units. For the first time, PDW spectroscopy was evaluated as a novel optical process analytical technology tool for real-time monitoring of biomass formation in Escherichia coli high-cell-density fed-batch cultivations. Inline PDW measurements were compared to a commercially available inline turbidity probe and with offline measurements of OD and cell dry weight (CDW). An accurate correlation of the reduced PDW scattering coefficient µs′ with CDW was observed in the range of 5–69 g L−1 (R2 = 0.98). The growth rates calculated based on µs′ were comparable to the rates determined with all reference methods. Furthermore, quantification of the reduced PDW scattering coefficient µs′ as a function of the absorption coefficient µa allowed direct detection of unintended process trends caused by overfeeding and subsequent acetate accumulation. Inline PDW spectroscopy can contribute to more robust bioprocess monitoring and consequently improved process performance. 相似文献