全文获取类型
收费全文 | 23347篇 |
免费 | 2427篇 |
国内免费 | 3538篇 |
专业分类
29312篇 |
出版年
2024年 | 89篇 |
2023年 | 385篇 |
2022年 | 936篇 |
2021年 | 1378篇 |
2020年 | 1073篇 |
2019年 | 1257篇 |
2018年 | 1164篇 |
2017年 | 871篇 |
2016年 | 1084篇 |
2015年 | 1561篇 |
2014年 | 1849篇 |
2013年 | 1814篇 |
2012年 | 2131篇 |
2011年 | 1992篇 |
2010年 | 1298篇 |
2009年 | 1121篇 |
2008年 | 1277篇 |
2007年 | 1116篇 |
2006年 | 997篇 |
2005年 | 831篇 |
2004年 | 838篇 |
2003年 | 780篇 |
2002年 | 725篇 |
2001年 | 484篇 |
2000年 | 381篇 |
1999年 | 348篇 |
1998年 | 244篇 |
1997年 | 175篇 |
1996年 | 160篇 |
1995年 | 152篇 |
1994年 | 116篇 |
1993年 | 108篇 |
1992年 | 97篇 |
1991年 | 72篇 |
1990年 | 72篇 |
1989年 | 71篇 |
1988年 | 46篇 |
1987年 | 39篇 |
1986年 | 34篇 |
1985年 | 20篇 |
1984年 | 23篇 |
1983年 | 20篇 |
1982年 | 22篇 |
1981年 | 16篇 |
1980年 | 8篇 |
1979年 | 7篇 |
1977年 | 5篇 |
1975年 | 4篇 |
1972年 | 3篇 |
1958年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 19 毫秒
141.
植物细胞程序性死亡(PCD)在植物生长发育和逆境适应中发挥重要作用。半胱氨酸蛋白酶(caspase)调控动物PcD的启动、执行及信号转导。通过人工合成底物、动物caspase抑制剂等方法已证实在植物中存在类caspase,可分为metacas.pases、VPEs(vacuolar processing enzymes)和saspases等。本文综述了植物类caspase的种类、结构、定位、功能及其调控PCD的研究进展,提出植物PCD中类caspase作用的调控途径,为深入研究植物PCD提供参考。 相似文献
142.
Carbon accumulation and distribution in Pinus massoniana and Cunninghamia lanceolata mixed forest ecosystem in Daqingshan, Guangxi, China 总被引:1,自引:0,他引:1 下载免费PDF全文
Carbon accumulation and distribution were studied at three sampling plots in a 13-year-old mixed planatation of Pinus massoniana and Cunninghamia lanceolata in Daqingshan, Guangxi, China. The results showed that carbon content varied with tissues and tree species, but the total carbon content of Pinus massoniana was higher than that of Cunninghamia lanceolata. The average tissue carbon contents of Pinus massoniana were: wood (58.6%) > root (56.3%) > branch (51.2%) > bark (49.8%) > leaf (46.8%), while those of Cunninghamia lanceolata were: bark (52.2%) > leaf (51.8%) > wood (50.2%) > root (47.5%) > branch (46.7%). The carbon contents of the soil (at a depth of 60cm) ranged from 1.45% to 1.84% with an average of 1.70%. Carbon contents were higher in the surface soil (0–20cm) than in the deep layer (below 20cm). The average carbon contents were the highest for trees (51.1%), followed by litter (48.3%), shrubs (44.1%), and herbs (33.0%). The biomass of the trees in the three plots ranged from 85.35 t hm-2 to 101.35 t hm-2 with an average of 93.83 t hm-2, in which 75.7%–82.6% was Pinus massoniana. The biomass of the understory was 2.10–3.95 t hm-2 with an average of 2.72 t hm-2, while the standing stock of ground litter was 5.49–7.91 t hm-2 with an average of 6.75 t hm-2. The carbon storage in the mixed plantation reached the maximum in the soil layer (69.02%), followed by vegetation (29.03%), and standing litter (1.82%). The carbon storage in the tree layer occupied 23.90% of the total ecosystem and 97.7% of the vegetation layer. Pinus massoniana accounted for 65.39% of the total carbon storage in the tree layer. Tissue carbon storage was directly related to the corresponding amount of biomass. Trunks had the highest carbon storage, accounting for 53.23% of the trees in Pinus massoniana and 55.57% in Cunninghamia lanceolata, respectively. Roots accounted for about 19.22% of the total tree carbon. The annual net productivity of the mixed plantation was 11.46 t hm-2a-1, and that of sequestered carbon was 5.96 t hm-2a-1, which was equivalent to fixing CO2 of 21.88 t hm-2a-1. The plantation was found to be an important sink of atmospheric CO2. 相似文献
143.
Wen X Wang T Wang Z Li L Zhao C 《International journal of biological macromolecules》2008,42(3):256-263
In this study, hydrogels for DNA-controlled release was prepared with konjac glucomannan (KGM), a water-soluble non-ionic polysaccharide, by means of deacetylated reaction and physically cross-linking method under mild conditions. The properties of the KGM hydrogels were analyzed by FTIR spectra and scanning electron microscopy (SEM). The integrality of the released DNA was investigated by circular dichroism (CD). The DNA release kinetics was performed using the DNA-loaded KGM gels in buffer solutions of pH 7.4 at 37+/-0.5 degrees C. Peppas model and Higuchi model were used to analysis the DNA release mechanism; the data indicated that the DNA release can be controlled by changing the preparation conditions and the structure parameters of the gels. This study suggested that the KGM hydrogels have a potential use for advanced controlled release. 相似文献
144.
145.
146.
147.
Tetramethylpyrazine scavenges superoxide anion and decreases nitric oxide production in human polymorphonuclear leukocytes 总被引:14,自引:0,他引:14
Tetramethylpyrazine is one of the active ingredients of the Chinese herb Ligusticum wallichii Franchat. By electron spin resonance spin trapping methods, effects of tetramethylpyrazine on superoxide anion and nitric oxide generated by human polymorphonuclear leukocytes were studied. During the respiratory burst of polymorphonuclear leukocytes induced by N-formylmethionyl-leucyl-phenylalanine, tetramethylpyrazine scavenges superoxide anion dose-dependently, and decreases the production of nitric oxide significantly, but shows no influence on oxygen consumption. These results suggest that the effective protection of tetramethylpyrazine against ischemic brain injury might be due to its scavenging of reactive oxygen species and regulation on nitric oxide production, and consequent prevention of peroxynitrite formation. 相似文献
148.
To interpret visual scenes, visual systems need to segment or integrate multiple moving features into distinct objects or surfaces. Previous studies have found that the perceived direction separation between two transparently moving random-dot stimuli is wider than the actual direction separation. This perceptual “direction repulsion” is useful for segmenting overlapping motion vectors. Here we investigate the effects of motion noise on the directional interaction between overlapping moving stimuli. Human subjects viewed two overlapping random-dot patches moving in different directions and judged the direction separation between the two motion vectors. We found that the perceived direction separation progressively changed from wide to narrow as the level of motion noise in the stimuli was increased, showing a switch from direction repulsion to attraction (i.e. smaller than the veridical direction separation). We also found that direction attraction occurred at a wider range of direction separations than direction repulsion. The normalized effects of both direction repulsion and attraction were the strongest near the direction separation of ∼25° and declined as the direction separation further increased. These results support the idea that motion noise prompts motion integration to overcome stimulus ambiguity. Our findings provide new constraints on neural models of motion transparency and segmentation. 相似文献
149.
目的:以PEI1.8kDa为基础连接戊二醛合成新型可降解PEI衍生物GPEI后,研究GPEI包裹质粒转染巨噬细胞后对巨噬细胞合成蛋白及表型的影响。方法:合成GPEI后,检测了GPEI质粒聚合物的粒径及电位,并通过透射电镜观察聚合物形态,通过免疫荧光、WB、RT-PCR、Elisa及Transwell实验检测巨噬细胞RNA、蛋白、终产物水平变化及表型改变。结果:GPEI能够包裹质粒,形成100 nm左右带有正电荷的聚合物,WB、RT-PCR结果表明GPEI转染较PEI25kDa有优势(P0.05),巨噬细胞转染后6天持续分泌PGI2(P0.05)。结论:以PEI1.8kDa为基础合成的GPEI能够包裹PTGIS质粒并将其转染进巨噬细胞后对巨噬细胞的表型有明显影响。 相似文献
150.
Xin Li Tian-Tian Li Xiao-Hui Zhang Li-Fei Hou Xiao-Qian Yang Feng-Hua Zhu Wei Tang Jian-Ping Zuo 《PloS one》2013,8(8)