全文获取类型
收费全文 | 395969篇 |
免费 | 45400篇 |
国内免费 | 360篇 |
专业分类
441729篇 |
出版年
2016年 | 3567篇 |
2015年 | 5347篇 |
2014年 | 6240篇 |
2013年 | 9011篇 |
2012年 | 9977篇 |
2011年 | 10059篇 |
2010年 | 6741篇 |
2009年 | 6353篇 |
2008年 | 8951篇 |
2007年 | 9453篇 |
2006年 | 8980篇 |
2005年 | 8735篇 |
2004年 | 8646篇 |
2003年 | 8233篇 |
2002年 | 8215篇 |
2001年 | 18215篇 |
2000年 | 18778篇 |
1999年 | 14910篇 |
1998年 | 4937篇 |
1997年 | 5215篇 |
1996年 | 4876篇 |
1995年 | 4651篇 |
1994年 | 4578篇 |
1993年 | 4582篇 |
1992年 | 12088篇 |
1991年 | 11673篇 |
1990年 | 11337篇 |
1989年 | 10994篇 |
1988年 | 10372篇 |
1987年 | 9885篇 |
1986年 | 9395篇 |
1985年 | 9483篇 |
1984年 | 7836篇 |
1983年 | 6734篇 |
1982年 | 5362篇 |
1981年 | 4998篇 |
1980年 | 4540篇 |
1979年 | 7649篇 |
1978年 | 6174篇 |
1977年 | 5670篇 |
1976年 | 5348篇 |
1975年 | 6012篇 |
1974年 | 6637篇 |
1973年 | 6586篇 |
1972年 | 6105篇 |
1971年 | 5538篇 |
1970年 | 4758篇 |
1969年 | 4721篇 |
1968年 | 4301篇 |
1967年 | 3621篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
Bodil Kjær Yean-Sung Jung Lian Yu John H. Golbeck Henrik Vibe Scheller 《Photosynthesis research》1994,41(1):105-114
The photosynthetic reaction center complex from the green sulfur bacteriumChlorobium vibrioforme has been isolated under anaerobic conditions. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals polypeptides with apparent molecular masses of 80, 40, 30, 18, 15, and 9 kDa. The 80- and 18-kDa polypeptides are identified as the reaction center polypeptide and the secondary donor cytochromec
551 encoded by thepscA andpscC genes, respectively. N-terminal amino acid sequences identify the 40-kDa polypeptide as the bacteriochlorophylla-protein of the baseplate (the Fenna-Matthews-Olson protein) and the 30-kDa polypeptide as the putative 2[4Fe-4S] protein encoded bypscB. Electron paramagnetic resonance (EPR) analysis shows the presence of an iron-sulfur cluster which is irreversibly photoreduced at 9K. Photoaccumulation at higher temperature shows the presence of an additional photoreduced cluster. The EPR spectra of the two iron-sulfur clusters resemble those of FA and FB of Photosystem I, but also show significantly differentg-values, lineshapes, and temperature and power dependencies. We suggest that the two centers are designated Center I (with calculatedg-values of 2.085, 1.898, 1.841), and Center II (with calculatedg-values of 2.083, 1.941, 1.878). The data suggest that Centers I and II are bound to thepscB polypeptide. 相似文献
82.
Construction of a human cytochrome c gene and its functional expression in Saccharomyces cerevisiae 总被引:1,自引:0,他引:1
Y Tanaka T Ashikari Y Shibano T Amachi H Yoshizumi H Matsubara 《Journal of biochemistry》1988,103(6):954-961
The nucleotide sequences of a partial cDNA and three pseudogenes of human cytochrome c were determined. The complete nucleotide sequences which encode human cytochrome c were constructed on the basis of one of the pseudogenes by in vitro mutagenesis. The constructed human cytochrome c was functionally expressed in Saccharomyces cerevisiae. The recombinant human cytochrome c was purified and characterized. 相似文献
83.
Characterization of phosphotyrosyl-protein phosphatase activity associated with calcineurin 总被引:3,自引:0,他引:3
J Chernoff M A Sells H C Li 《Biochemical and biophysical research communications》1984,121(1):141-148
Calcineurin purified from bovine brain is shown to possess phosphotyrosyl -protein phosphatase activity towards proteins phosphorylated by the epidermal growth factor receptor/kinase. The phosphatase activity is augmented by Ca2+/calmodulin or divalent cation (Ni2+ greater than Mn2+ greater than Mg2+ greater than Co2+). In the simultaneous presence of all three effectors, the enzymatic activity is synergistically increased. Ca2+/calmodulin activates the Mg2+-supported activity by decreasing the Km value for phosphotyrosyl -casein from 2.2 to 0.6 microM, and increasing the Vmax from 0.4 to 4.6 nmol/min/mg. These results represent the first demonstration that calcineurin can dephosphorylate phosphotyrosyl -proteins and suggest a novel mechanism of activation of this enzyme. 相似文献
84.
ABSTRACT. An actograph is described for recording the behaviour of small insects, by using a standing-wave radar system in an enclosed metal cabinet. Experiments show that the system produces an output frequency dependent only on the speed of movement of the subject. By filtering the signal electrically into components below and above 5 Hz, walking and flying of adult Musca domestica could be separately recorded. Each filter is followed by a level-comparator with hysteresis. Activity causing signal to cross the hysteresis-band is recorded as an event on a digital counter, and also integrated or summed to be displayed as a stepped-integral or bar-chart of total distance covered by the subject in each counting period. The energy-density of the radar can be kept very low, so that possible influence on the subject is minimal. The detector is remote from the box containing the subject, giving great flexibility in container design, so that the apparatus is ideally suited to the study of environmental factors. 相似文献
85.
86.
L-myo-Inositol-1-phosphate synthase has been found to have at least a 5-fold preference for the beta-anomer of its natural substrate D-Glc-6-P. The alpha-anomer appears to be an inhibitor of the reaction and may be converted to product as well. As well as showing an enzymatic preference for the equatorial C-1 hydroxyl of D-Glc-6-P, our results suggest that it is the pyranose form of D-Glc-6-P that binds to the enzyme and that ring-opening is an enzymatic step. We have also found D-2-dGlc-6-P, D-2-F-2-dGlc-6-P, and D-Man-6-P each to be both competitive inhibitors and substrates that are converted to inositol phosphates by the synthase. D-Allose-6-P is a weak inhibitor of the enzyme, but not a substrate. D-Gal-6-P is neither substrate nor inhibitor. Thus the specificity of the synthase with respect to single position epimers of D-Glc-6-P increases in the order C1 less than C2 much less than C3 less than C4. 相似文献
87.
Essential fatty acid-deficient rats were supplemented with 300 mg per day of pure fatty acid esters: oleate (O), linoleate (L), arachidonate (A), and columbinate (C) for 10 days. During this period, the rats in groups L, A, and C all showed a decrease in their initially high trans-epidermal water loss, a classical essential fatty acid-deficiency symptom, to a level seen in non-deficient rats (group N). The trans-epidermal water loss in rats of group O was unaffected by the supplementation. Fatty acid composition of two epidermal sphingolipids, acylglucosylceramide and acylceramide, from the skin were determined. The results indicate that re-establishment of a low trans-epidermal water loss was associated with incorporation of linolenate into the two epidermal sphingolipids. Supplementation with columbinate resulted in relatively high amounts of this fatty acid in the investigated epidermal sphingolipids. Analysis of pooled skin specimens from a previous study in which weanling rats were fed a fat-free diet and supplemented orally with pure alpha-linolenate for 13 weeks (Hansen, H.S. and Jensen, B. (1983) Lipids 18, 682-690) revealed very little polyunsaturated fatty acid in the two sphingolipids. These rats showed increased evaporation which was comparable to that of essential fatty acid-deficient rats. We interpret these results as strong evidence for a very specific and essential function of linoleic acid in maintaining the integrity of the epidermal water permeability barrier. This function of linoleate is independent of its role as precursor for arachidonate and icosanoids. 相似文献
88.
THE TIMING OF DIVISION IN CHLAMYDOMONAS 总被引:3,自引:2,他引:1
89.
Cryptococcosis is an often fatal opportunistic fungal infection. Despite efforts to elucidate the role of immunity in host defense against the disease, much remains to be learned. The purpose of this brief review is to provide the reader with an overview of the history of research concerned with host immunity in cryptococcosis. Both humoral and cell-mediated studies are included. An effort has been made to present the reader with a comprehensive list of references in the hope of encouraging additional reading and research in this important area. 相似文献
90.