全文获取类型
收费全文 | 24513篇 |
免费 | 769篇 |
国内免费 | 1199篇 |
出版年
2023年 | 156篇 |
2022年 | 224篇 |
2021年 | 301篇 |
2020年 | 304篇 |
2019年 | 756篇 |
2018年 | 717篇 |
2017年 | 472篇 |
2016年 | 331篇 |
2015年 | 353篇 |
2014年 | 692篇 |
2013年 | 868篇 |
2012年 | 560篇 |
2011年 | 547篇 |
2010年 | 417篇 |
2009年 | 488篇 |
2008年 | 559篇 |
2007年 | 552篇 |
2006年 | 439篇 |
2005年 | 400篇 |
2004年 | 348篇 |
2003年 | 272篇 |
2002年 | 314篇 |
2001年 | 251篇 |
2000年 | 270篇 |
1999年 | 213篇 |
1998年 | 215篇 |
1997年 | 190篇 |
1996年 | 161篇 |
1995年 | 148篇 |
1994年 | 170篇 |
1993年 | 166篇 |
1992年 | 133篇 |
1991年 | 122篇 |
1990年 | 112篇 |
1989年 | 75篇 |
1988年 | 88篇 |
1987年 | 79篇 |
1985年 | 942篇 |
1984年 | 2030篇 |
1983年 | 1588篇 |
1982年 | 1632篇 |
1981年 | 1328篇 |
1980年 | 1214篇 |
1979年 | 1121篇 |
1978年 | 993篇 |
1977年 | 848篇 |
1976年 | 683篇 |
1975年 | 616篇 |
1974年 | 513篇 |
1973年 | 394篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Classification and ordination of plant communities along an altitudinal gradient on the Presidential Range,New Hampshire,USA 总被引:1,自引:0,他引:1
An analysis of vegetation along an altitudinal gradient on the Presidential Range, New Hampshire, USA, using the Braun–Blanquet approach followed by multivariate data analysis is presented. Twelve main plant communities have been distinguished. Floristic information is presented in twelve tables and one appendix. The relationships of the communities to complex environmental gradients are analyzed using Correspondence Analysis. Floristic composition and community structure are controlled primarily by the altitudinal gradient (temperature, precipitation), and by mesotopographic conditions (snow accumulation, exposure and cryoturbation, slope position, and soil moisture). 相似文献
2.
An earlier report (1) of an adverse effect of high doses of oxytocin on human memory included results of studies on women receiving oxytocin as part of the treatment to induce 2nd trimester therapeutic abortion. These women served as their own controls. We have now been able to study a group of women who have been treated in all ways like the original group, with the exception that they did not receive oxytocin. The results from this external control corroborate the finding that oxytocin affected memory. 相似文献
3.
4.
As part of systematic studies of the genus Porphyrain New Zealand, constituent sugar analyses of cell wall polysaccharidesin situ in dry thalli were found to yield data that weretaxonomically informative. Variation in constituent sugar levels betweenspecieswas sufficient in some cases to be useful in species differentiation. Thereproductive state of thallus regions had a significant impact on the levels ofconstituent sugars, whereas storage of dried thalli for eight months had noeffect. Three epiphytic taxa currently classified as species ofPorphyra appear to be incorrectly placed within the genus,as their constituent sugars and the levels of these sugars differed markedlyfrom those of all other species examined. 相似文献
5.
Complementarity of regulation for the two glutamine synthetases from Bacillus caldolyticus, an extreme thermophile 总被引:1,自引:0,他引:1
F C Wedler D S Shreve K E Fisher D J Merkler 《Archives of biochemistry and biophysics》1981,211(1):276-287
Chromatophores from Rhodopseudomonas capsulata cells grown semiaerobically in the dark oxidize NADH, succinate, and dichlorophenolindophenol. In the presence of N3? these activities are inhibited, but light induces oxidation of dichlorophenolindophenol with O2 as a terminal electron acceptor. Cyanide also inhibits electron transport but much higher concentrations are required to inhibit the photooxidation than the dark oxidation. The photooxidation was studied in a mutant strain of Rhodopseudomonas capsulata (YIV) which cannot grow anaerobically in the light, but similarly to the wild type, grows in the presence of oxygen. Chromatophores from YIV mutant catalyze photophosphorylation and dark oxidation activities with the same properties as those of the wild type. However, the rate of photooxidation in the mutant is only one-third that of the wild type. Based on the differential inhibitor sensitivity and on the mutation it is suggested that the photooxidase is different from the two respiratory oxidases and that this photooxidation activity might be essential for growth of the cells under anaerobic conditions in the light. 相似文献
6.
Gregory L. Dilworth 《Archives of biochemistry and biophysics》1982,219(1):30-38
The nicotinic acid hydroxylase from Clostridium barkeri is a selenoenzyme, as evidenced by the copurification of selenium with enzyme activity. This conclusion is supported by data showing a 23-fold increase in nicotinic acid hydroxylase activity when C. barkeri was cultured in media supplemented with selenium. A labile, selenium-containing compound was released from the native protein by treatment with either chaotropic agents and heat or by heating alone. A stable selenium compound was formed when the enzyme was alkylated prior to denaturation. This compound had the same chromatographic properties as dialykyl selenide in a number of systems. The formation of dialkyl selenide upon alkylation is not consistent with the selenium moiety being selenocysteine. Thus, nicotinic acid hydroxylase represents a new type of selenoenzyme. 相似文献
7.
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
8.
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
9.
Field observations of two sympatric pitheciine species reveal that the positional repertoire of the white-faced saki, Pithecia pithecia, is dominated by leaping behaviors, whereas the bearded saki, Chiropotes satanas, is predominantly quadrupedal. Examination and comparison of the postcranial skeletal morphologies and limb proportions of these species display numerous features associated with their respective locomotor behaviors. These observations accord with associations found in other primate and mammalian groups and with predictions based on theoretical and experimental biomechanics. Preliminary observations of the skeletal morphology of Cacajao calvus demonstrate a marked similarity to that of Chiropotes. The fossil platyrrhine Cebupithecia sarmientoi displays greater similarity to Pithecia, suggesting that its positional repertoire also included significant leaping and clinging behaviors. 相似文献
10.
"Pertussis toxin induces tachycardia and impairs the increase in blood pressure produced by alpha 2-adrenergic agonists" 总被引:4,自引:0,他引:4
Administration of purified pertussis toxin to rats induced persistent tachycardia, (observed in conscious rats but not after pithing); as little as 0.05 microgram/100 g produced a significant effect. Pertussis toxin-treatment did not affected the pressor response produced in the pithed rats by the alpha 2-adrenergic agonist methoxamine but markedly diminished the pressor effect of the alpha 2-adrenergic agonists clonidine and azepexole. A role of adenylate cyclase inhibition in the action of postsynaptic vascular alpha 2-adrenergic receptors is suggested. 相似文献