全文获取类型
收费全文 | 80996篇 |
免费 | 5352篇 |
国内免费 | 12篇 |
出版年
2023年 | 612篇 |
2022年 | 675篇 |
2021年 | 1360篇 |
2020年 | 1153篇 |
2019年 | 1215篇 |
2018年 | 2590篇 |
2017年 | 2323篇 |
2016年 | 2997篇 |
2015年 | 3847篇 |
2014年 | 3924篇 |
2013年 | 5336篇 |
2012年 | 6310篇 |
2011年 | 5849篇 |
2010年 | 3599篇 |
2009年 | 2855篇 |
2008年 | 4504篇 |
2007年 | 4260篇 |
2006年 | 3966篇 |
2005年 | 3338篇 |
2004年 | 3285篇 |
2003年 | 2961篇 |
2002年 | 2703篇 |
2001年 | 1905篇 |
2000年 | 1935篇 |
1999年 | 1422篇 |
1998年 | 645篇 |
1997年 | 487篇 |
1996年 | 474篇 |
1995年 | 441篇 |
1994年 | 351篇 |
1993年 | 339篇 |
1992年 | 691篇 |
1991年 | 602篇 |
1990年 | 522篇 |
1989年 | 496篇 |
1988年 | 497篇 |
1987年 | 436篇 |
1986年 | 423篇 |
1985年 | 408篇 |
1984年 | 399篇 |
1983年 | 280篇 |
1982年 | 270篇 |
1981年 | 213篇 |
1980年 | 207篇 |
1979年 | 256篇 |
1978年 | 243篇 |
1975年 | 226篇 |
1974年 | 243篇 |
1973年 | 244篇 |
1972年 | 201篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
María-Carmen Rodríguez-Yoldi José-Emilio Mesonero Maríaa-Jesús Rodríguez-Yoldi 《Biological trace element research》1995,50(1):1-11
Zinc is an essential trace element necessary to life. This metal may exert some of its physiological effects by acting directly
on cellular membranes, either by altering permeability or by modulating the activity of membrane-bound enzymes. On the other
hand, calcium is an essential element in a wide variety of cellular activities. The aim of the present work was to study a
possible interaction between zinc and calcium on intestinal transport ofd-galactose in jejunum of rabbit in vitro. In media with Ca2+, when ZnCl2 was present at 0.5 or 1 mM, zinc was found to reduce thed-galactose absorption significantly. In Ca2+-free media, where CaCl2 was omitted and replaced isotonically with choline chloride, the sugar transport was not modified by zinc. Verapamil at 10−6
M (blocking mainly Ca2+ transport) did not modify the inhibitory effect of zinc ond-galactose transport. When 10−6
M of A 23187 (Ca2+-specific ionophore) was added with/without Ca2+ to the media, ZnCl2 produced no change in sugar transport. These results could suggest a possible interaction of calcium and zinc for the same
chemical groups of membrane, which could affect the intestinal absorption of sugars. 相似文献
992.
Jesús Zurdo Miguel A. Centeno José A. Odriozola Concepción Fernández-Cabrera Juan M. Ramírez 《Photosynthesis research》1995,46(1-2):363-369
In previous work (Zurdo J, Fernández-Cabrera C and Ramírez JM (1993) Biochem J 290: 531–537), it had been shown that selective extraction of the carotenoid from the light-harvesting protein 2 (LH2) of Rhodobacter capsulatus induced the dissociation of 800-nm absorbing bacteriochlorophyll (Bchl), a 10-nm red shift of 854-nm Bchl, and a decrease of the stability of the protein in detergent solution. In the present study, the Fourier transform Raman and near-infrared circular dichroism spectra of native and carotenoid-depleted LH2 membrane preparations were compared. It was found that while the coupled carbonyls of 854-nm Bchl remained specifically H-bonded to the peptides after carotenoid extraction, the optical activity of the near-infrared electronic transition was significantly altered. Given the excitonic origin of such optical activity, our data suggest that carotenoid extraction elicits a rearrengement of the chromophore cluster and of the associated polypeptide subunits. This implies a significant role of the carotenoid in maintaining the native quaternary structure of the protein, which would be consistent with the observed dissociation of 800-nm Bchl and the loss of solubilized LH2 stability that result from carotenoid removal. There is no evidence for a similar role of the carotenoid in the LH1 protein.Abbreviations Bchl
bacteriochlorophyll
- FT
Fourier transform
- CD
circular dichroism
- LH1 and LH2
the bacterial light-harvesting proteins 1 and 2
In memoriam of Daniel I. Arnon. 相似文献
993.
Angel García-Gutiérrez Francisco R. Cantón Fernando Gallardo Francisca Sánchez-Jiménez Francisco M. Cánovas 《Plant molecular biology》1995,27(1):115-128
Pine seedlings are able to accumulate chlorophylls and develop green plastids in a light-independent manner. In this work, we have characterized ferredoxin-dependent glutamate synthase (EC 1.4.7.1; Fd-GOGAT), a key enzyme in nitrogen interconversion during this process. Fd-GOGAT has been purified about 170-fold from cotyledons of maritime pine (Pinus pinaster). As occurs in angiosperms, the native enzyme is a single polypeptide with an apparent molecular mass of 163–168 kDa that is confined to the chloroplast stroma. Polyclonal antibodies generated against the purified enzyme were used to immunoscreen a gt11 expression library from Scots pine (Pinus sylvestris) seedlings and partial cDNA clones were isolated and characterized. The clone with the longest cDNA insert (pGOP44) contained the codification for the C-terminal (550 amino acids) of the pine Fd-GOGAT polypeptide. Immunological cross-reactivity and comparative amino sequence analysis revealed that Fd-GOGAT is a well conserved protein in higher plants. Western blot analyses showed that protein was expressed in chloroplast-containing pine tissues and this expression pattern was not affected by exogenously supplied nitrogen. Fd-GOGAT mRNA, polypeptide and enzyme activity accumulated in substantial amounts in dark-grown pine seedlings. The presence of a functional Fd-GOGAT may be important to provide the required glutamate for the biosynthesis of nitrogen compounds during chloroplast biogenesis in the dark. 相似文献
994.
M. E. Acuña-Argüelles M. Gutiérrez-Rojas G. Viniegra-González E. Favela-Torres 《Applied microbiology and biotechnology》1995,43(5):808-814
Three extracellular pectinases were produced byAspergillus niger CH4 by submerged and solid-state fermentation, and their physicochemical and kinetic properties were studied. The highest
productivities of endo- and exo-pectinase and pectin lyase were obtained with solid-state fermentation. The kinetic and physicochemical
properties of these enzymes were influenced by the type of culture method used. All activities were very different in terms
of pH and temperature optima, stability at different pH and temperature values and affinity for the substrate (K
m values). In solid-state fermentation, all pectinase activities were more stable at extreme pH and temperature values but
theK
m values of endo-pectinase and pectin lyase were higher with respect to those activities obtained by the submerged-culture
technique. The pectin lyase activity obtained by the submerged-culture technique showed substrate inhibition but the enzyme
obtained by solid-state fermentation did not. Electrophoresis, using sodium dodecyl sulphate/polyacrylamide gel with enzymatic
extracts obtained for both culture methods, showed the same number on protein bands but some differences were found in their
electrophoretic position. The results obtained in this work suggest that the culture method (submerged or solid-state) may
be responsible for inducing changes in some of the pectinolytic enzymes produced byA. niger. 相似文献
995.
J. M. Barrasa S. Camarero A. T. Martínez K. Ruel 《Applied microbiology and biotechnology》1995,43(4):766-770
The ultrastructural patterns characterizing wheat straw degradation by the ligninolytic fungi Phanerochaete chrysosporium and Trametes versicolor were studied. During fungal attack, the less lignified tissues were degraded first, whereas the xylematic and sclerenchymatic fibers underwent a delayed attack. In straw samples degraded by T. versicolor, partial delignification, defibrillation and swelling of cell walls, often causing separation between primary and secondary walls, were observed. By contrast, the formation of erosions and fissures, with minor lignin removal, characterized the attack to the cell wall by P. chrysosporium. At an advanced stage of decay, KMnO4 staining demonstrated abundant electron-dense material around hyphae and in the proximity of the cell-wall surface. In the case of P. chrysosporium, spherical black bodies were found in the erosions and fissures produced during fungal attack. 相似文献
996.
P J Hurlin C Quéva P J Koskinen E Steingrímsson D E Ayer N G Copeland N A Jenkins R N Eisenman 《The EMBO journal》1995,14(22):5646-5659
997.
998.
Yeast aminopeptidase I is post-translationally sorted from the cytosol to the vacuole by a mechanism mediated by its bipartite N-terminal extension. 总被引:2,自引:0,他引:2 下载免费PDF全文
Transport of aminopeptidase I (API) to the vacuole appears to be insensitive to blockage of the secretory pathway. Here we show that the N-terminal extension of the 61 kDa precursor of API (pAPI) is proteolytically processed in two sequential steps. The first step involves proteinase A (PrA) and produces a 55 kDa unstable intermediate (iAPI). The second step involves proteinase B (PrB) and converts iAPI into the 50 kDa stable, mature enzyme (mAPI). Reversion of the cup1 growth phenotype by a pAPI-CUP1 chimera indicates that pAPI is transported to the vacuole by a post-translational mechanism. Deletion of the first 16 amino acids results in accumulation of the truncated protein in the cytosol, indicating that pAPI is actively transported to the vacuole. The chimera pAPI-myc, constructed by fusing a myc tag to the C-terminus of pAPI, was exploited to dissect the mechanism of pAPI transport. Cell fractionation studies show the presence of iAPI-myc and mAPI in a fraction of vacuoles purified by density centrifugation. This and the sequential conversion of pAPI-myc into iAPI-myc and mAPI lacking the myc tag is consistent with insertion of pAPI into the vacuolar membrane through its N-terminal extension. The specific mechanism of API sorting demonstrates a new pathway of protein transport in vacuolar biogenesis. 相似文献
999.
1000.
Summary We tested the adaptive significance of flowering synchrony by means of a quantitative analysis of selection and by flowering induction experiments with the deciduous shrubErythroxylum havanense. Temporal schedules of flower and fruit production were determined for a local population (in three sites) in a Mexican seasonal forest for 2 years (1987–1988). The consequences of natural variation in flowering time (flowering initiation day) on maternal reproductive success (fecundity) were evaluated. We observed high levels of inter- and intraindividual flowering synchrony in 1987, but not in 1988 and this contrast was related to differences in rainfall patterns between the two years. A significant proportion (15.4%) of the phenotypic variation in flowering initiation day was accounted for by environmental variance. The expression of phenotypic variance of flowering time and, consequently, the opportunity for selection to act, are controlled by annual variation in rainfall. Despite the between-year difference in flowering synchrony, we detected a relatively intense directional selection on flowering initiation day in both years, but selection coefficients were of opposite sign (standardized directional gradients were –0.326 and 0.333 for 1987 and 1988, respectively). For both years there was a significant relationship between individual relative fitness and the number of neighbouring flowering plants in a given day, suggesting positive frequency-dependent selection. 相似文献