全文获取类型
收费全文 | 3398篇 |
免费 | 237篇 |
国内免费 | 1篇 |
专业分类
3636篇 |
出版年
2012年 | 339篇 |
2011年 | 371篇 |
2010年 | 63篇 |
2009年 | 26篇 |
2008年 | 296篇 |
2007年 | 321篇 |
2006年 | 296篇 |
2005年 | 313篇 |
2004年 | 288篇 |
2003年 | 253篇 |
2002年 | 180篇 |
2001年 | 179篇 |
2000年 | 244篇 |
1999年 | 91篇 |
1998年 | 17篇 |
1997年 | 15篇 |
1996年 | 6篇 |
1995年 | 12篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 7篇 |
1990年 | 12篇 |
1989年 | 16篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1978年 | 7篇 |
1973年 | 4篇 |
1971年 | 6篇 |
1969年 | 4篇 |
1959年 | 17篇 |
1958年 | 25篇 |
1957年 | 17篇 |
1956年 | 15篇 |
1955年 | 15篇 |
1954年 | 10篇 |
1953年 | 16篇 |
1952年 | 8篇 |
1951年 | 11篇 |
1950年 | 5篇 |
1949年 | 6篇 |
1948年 | 5篇 |
1939年 | 7篇 |
1937年 | 4篇 |
1935年 | 4篇 |
1933年 | 6篇 |
排序方式: 共有3636条查询结果,搜索用时 0 毫秒
1.
2.
3.
Bark storage proteins (BSPs) accumulate in the inner bark parenchyma of many woody plants during autumn and winter. We investigated the effect of a short-day (SD) photoperiod on the accumulation of the 32-kilodalton bark storage protein of poplar (Populus deltoides Bart. ex Marsh.) under controlled environmental and natural growing conditions. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and protein gel blot analysis revealed that 10 days of SD exposure (8 hours of light) resulted in a 20% increase in the relative abundance of the 32-kilodalton bark storage protein of poplar. After 17 days of SD exposure, the 32-kilodalton bark storage protein accounted for nearly one-half of the soluble bark proteins. In natural field conditions, accumulation of the 32-kilodalton bark storage protein was observed to start by August 18 (daylength 14.1 hours). Immunoprecipitation of in vitro translation products with anti-BSP serum revealed that the SD protein accumulation was correlated with changes in the pool of translatable mRNA. A survey of poplar clones from different geographic origins revealed the presence of the 32-kilodalton BSP in the dormant bark of all the clones tested. These results demonstrate that a SD photoperiod induces, whether directly or indirectly, rapid changes in woody plant gene expression, leading to the accumulation of BSP. 相似文献
4.
5.
The mutual diffusional interference between adjacent stomata in laminar flow over a leaf is shown to play a decisive role in determining overall transpiration. The magnitude of this interference varies with the interaction of the vapor diffusional shells forming above each stoma and the air flow over the leaf. The interference decreases with increasing incident radiation and wind velocity. The effect of interference on the stomatal resistance to diffusion plays a major role in the overall variations in transpiration. 相似文献
6.
Carbon Oxysulfide Is an Inhibitor of Both CO(2) and HCO(3) Uptake in the Cyanobacterium Synechococcus PCC7942 总被引:2,自引:2,他引:0 下载免费PDF全文
Carbon oxysulfide (COS) was reinvestigated as an inhibitor of active inorganic carbon transport in cells of Synechococcus PCC7942 adapted to growth at low inorganic carbon. COS inhibited both CO2 and HCO3− transport processes in a reversible (in the short term) and mixed competitive manner. The inhibition of COS was established using both silicone oil centrifugation experiments and O2-evolution studies. The Ki for COS inhibition was 29 micromolar for CO2 transport and 110 micromolar for HCO3− transport. These results support a model of inorganic carbon transport with a central CO2 pump and an inducible HCO3− utilizing accessory protein which supplies CO2 to the primary pump. 相似文献
7.
Abstract Three-dimensional structures of cytochrome P450 2B1 were modeled based on the crystallographic structure of P450(cam). The effect of the alignment, loop choice, and minimization with or without water was assessed. Although final models were similar in overall structure, the identity of active site residues depended upon the alignment. An example is Phe-206, which may or may not form part of the active site. The choice of the loop conformation had a lesser effect, while including water in the final minimization step was essential for preserving the shape and size of the active site. The best model (model 2) was in good agreement with the data from site-directed mutagenesis studies, and correctly predicted the effect of substitutions at 9 out of 10 amino acid positions. Thus, residues important for P450 2B1 activity, such as Ile- 114, Phe-206, Ile-290, Thr-302, Val-363, and Gly-478, constitute part of the active site and are able to interact with the substrate androstenedione through hydrophobic interactions. On the other hand, Ser-303, Ser-360 and Lys-473 are far from the active site and/or cannot interact with the substrate, in agreement with experimental data. The model indicates other residues likely to be important for enzyme function, such as Tyr- 111, Leu-209, Ile-477, and Ile- 480, which can be tested experimentally. The substrate may assume numerous binding orientations consistent with observed patterns of hydroxylation at C(5) and C(6). The replacement in the model of certain amino acid residues to mimic residue substitutions from site-directed mutagenesis studies and docking of the substrate into the modified active site allowed a plausible explanation for alterations in regio- and stereospecificities of some mutants of P450 2B1, such as Gly-478 → Ala or Val-363 Ala. 相似文献
8.
Physiological and Environmental Requirements for Poplar (Populus deltoides) Bark Storage Protein Degradation 总被引:7,自引:3,他引:4 下载免费PDF全文
In poplar (Populus deltoides Bartr. ex Marsh), a 32-kD bark storage protein (BSP) accumulates in the bark during autumn and winter and declines during spring shoot growth. We investigated the physiological and environmental factors necessary for the degradation of poplar BSP. Poplar plants were exposed to short-day (SD) photoperiods for either 28 or 49 d. Plants exposed to short days for 28 d formed a terminal bud but were not dormant, whereas exposure to short days for 49 d induced bud dormancy. BSP accumulated in bark of plants exposed to both SD treatments. The level of BSP declined rapidly when nondormant plants were returned to long days. BSP levels did not decline in dormant plants that were exposed to long-day (LD) conditions. If dormant plants were first treated with either low temperatures (0[deg]C for 28 d) or with 0.5 M H2CN2 to overcome dormancy and then returned to long days, the level of BSP declined. Removal of buds from non-dormant or dormant plants in which dormancy had been overcome inhibited the degradation of BSP in LD conditions. BSP mRNA levels rapidly declined in plants exposed to long days, irrespective of the dormancy status of the plants or the presence or absence of buds. These results indicate that the buds of poplars are somehow able to communicate with bark storage sites and regulate poplar BSP degradation. These results further support an association of BSP mRNA levels with photoperiod because short days stimulate BSP mRNA accumulation, whereas long days result in a decline of BSP mRNA abundance. 相似文献
9.
This study has demonstrated that in the ameba, Pelomyxa carolinensis Wilson (Chaos chaos L.) the limiting membranes of mitochondria and postdivision nuclei are often continuous. The morphological relationship may be functional in that it permits an exchange of material resulting directly or indirectly in an increased enzyme content of the mitochondria. It is suggested that through a series of progressive foldings of its envelope, the nucleus may be a site of formation of mitochondria. 相似文献
10.