全文获取类型
收费全文 | 5877篇 |
免费 | 408篇 |
国内免费 | 9篇 |
专业分类
6294篇 |
出版年
2023年 | 19篇 |
2022年 | 53篇 |
2021年 | 115篇 |
2020年 | 59篇 |
2019年 | 99篇 |
2018年 | 137篇 |
2017年 | 115篇 |
2016年 | 136篇 |
2015年 | 259篇 |
2014年 | 251篇 |
2013年 | 376篇 |
2012年 | 459篇 |
2011年 | 503篇 |
2010年 | 288篇 |
2009年 | 261篇 |
2008年 | 409篇 |
2007年 | 394篇 |
2006年 | 403篇 |
2005年 | 350篇 |
2004年 | 330篇 |
2003年 | 305篇 |
2002年 | 300篇 |
2001年 | 44篇 |
2000年 | 38篇 |
1999年 | 61篇 |
1998年 | 96篇 |
1997年 | 30篇 |
1996年 | 49篇 |
1995年 | 35篇 |
1994年 | 31篇 |
1993年 | 30篇 |
1992年 | 30篇 |
1991年 | 19篇 |
1990年 | 19篇 |
1989年 | 14篇 |
1988年 | 11篇 |
1987年 | 5篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 22篇 |
1981年 | 12篇 |
1980年 | 10篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 9篇 |
1976年 | 7篇 |
1975年 | 6篇 |
1974年 | 9篇 |
排序方式: 共有6294条查询结果,搜索用时 15 毫秒
991.
Frederick D Lewis Huihe Zhu Pierre Daublain Karsten Sigmund Torsten Fiebig Milen Raytchev Qiang Wang Vladimir Shafirovich 《Photochemical & photobiological sciences》2008,7(5):534-539
The mechanism and dynamics of charge separation and charge recombination in synthetic DNA hairpins possessing a stilbenedicarboxamide linker and a single guanine-cytosine base pair have been reinvestigated. The combination of femtosecond broad-band pump probe spectroscopy, nanosecond transient absorption experiments, and picosecond fluorescence decay measurements permits analysis of the formation and decay of the stilbene anion radical. Reversible hole injection resulting in the formation of the stilbene-adenine contact radical ion pair is found to occur on the picosecond time scale. The mechanism for charge separation across two or more base pairs is revised from single step superexchange to a multi-step process: hole injection followed by hole transport and hole trapping. The mechanism of charge recombination remains assigned to a superexchange process. 相似文献
992.
993.
Pravosudov VV 《Proceedings. Biological sciences / The Royal Society》2008,275(1630):55-61
Evolution of complex cognition in animals has been linked to complex social behaviour. One of the costs of sociality is increased competition for food which may be reduced by food caching, but cache theft may undermine the benefits of caching. In birds, sophisticated food-caching-related cognition has been demonstrated only for corvids and attributed to their highly social behaviour. Many non-corvid food-caching species exhibit similar complex social behaviour and here I provide experimental evidence that mountain chickadees (Poecile gambeli) adjust their caching strategies depending on social context. Chickadees were allowed to cache seeds in the presence of potential cache pilferer, either conspecific or heterospecific (red-breasted nuthatch, Sitta canadensis) and a non-pilferer (dark-eyed junco, Junco hyemalis) positioned at the opposite sides of the experimental arena. Available caching sites were either exposed to these observers or hidden from their view while the cacher could always see both observers. Chickadees chose caching sites that were hidden from direct view of the potential pilferers while caching in direct view of the non-pilferers. When no pilferers were present, chickadees made equal use of all available caching substrates and there were no differences in the amount of caching in the presence or absence of pilferers. These results suggest that (i) chickadees may be able to recognize potential cache thieves, both conspecific and heterospecific, and adjust their caching strategies to minimize potential cache pilferage and (ii) chickadees appear to discriminate between caching sites that can or cannot be seen by observers, which may allow them to control visual information available to potential pilferers. 相似文献
994.
Kvasnica M Budesinsky M Swaczynova J Pouzar V Kohout L 《Bioorganic & medicinal chemistry》2008,16(7):3704-3713
Twelve steroidal platinum(II) complexes were synthesized by reaction of potassium tetrachloroplatinate with steroidal esters of L-methionine and L-histidine. The steroidal esters coordinated as bidentate ligands via S and N donor atoms of L-methionine and via two N donor atoms of L-histidine. Cholesterol, cholestanol, diosgenine, pregnenolone, dehydroepiandrosterone, testosterone, estrone, and estradiol were used as the steroidal compounds. The esters and complexes prepared were characterized by infrared, mass, and (1)H NMR spectroscopy and elemental analysis. Platinum complexes were tested for in vitro cytotoxicity against several cancer cell lines: T-lymphoblastic leukemia CEM, breast carcinoma MCF-7, lung carcinoma A-549, multiple myeloma RPMI 8226, and one normal cell line human fibroblast BJ. 相似文献
995.
Erling S. Nordøy Lars P. Folkow Vladimir Potelov Vitaly Prischemikhin Arnoldus Schytte Blix 《Polar Biology》2008,31(9):1119-1135
Eight adult female harp seals (Pagophilus
groenlandicus) of the White Sea–Barents Sea stock were tagged with satellite-linked dive recorders during the nursing period and followed
from breeding in late February 1995 until moulting in late April 1995. Another ten adult harp seals of both sexes were tagged
and followed from moult in early May 1996 until breeding in late February the following year. Between breeding and moult the
seals were distributed along the coasts of Kola of Russia and eastern Finnmark of Norway, coinciding in time and space with
the spawning capelin (Mallotus villosus). Between moulting and breeding they encircled the entire Barents Sea, mostly in open water, using the water column from
20 to 300 m, and in so doing by and large reflecting the annual migrations of the capelin. Capelin is therefore assumed to
be the main source of prey for the White Sea–Barents Sea stock of harp seals, to be substituted, in part, by amphipods (e.g.
Themisto libellula) in mid-summer and polar cod (Boreogadus saida) and herring (Clupea pallasii) in late autumn and winter. These data provide a baseline for the evaluation of the effects of future climatic change in
the rich Barents Sea ecosystem. 相似文献
996.
Wesierska-Gadek J Wandl S Kramer MP Pickem C Krystof V Hajek SB 《Journal of cellular biochemistry》2008,105(5):1161-1171
Exposure of human HeLaS(3) cervix carcinoma cells to high doses of conventional cytostatic drugs, e.g. cisplatin (CP) strongly inhibits their proliferation. However, most cytostatic agents are genotoxic and may generate a secondary malignancy. Therefore, therapeutic strategy using alternative, not cytotoxic drugs would be beneficial. Inhibition of cyclin-dependent kinases (CDKs) by pharmacological inhibitors became recently a promising therapeutic option. Roscovitine (ROSC), a selective CDK inhibitor, efficiently targets human malignant cells. ROSC induces cell cycle arrest and apoptosis in human MCF-7 breast cancer cells. ROSC also activates p53 protein. Activation of p53 tumor suppressor protein is essential for induction of apoptosis in MCF-7 cells. Considering the fact that in HeLaS(3) cells wt p53 is inactivated by the action of HPV-encoded E6 oncoprotein, we addressed the question whether ROSC would be able to reactivate p53 protein in them. Their exposure to ROSC for 24 h induced cell cycle arrest at G(2)/M and reduced the number of viable cells. Unlike CP, ROSC in the used doses did not induce DNA damage and was not directly cytotoxic. Despite lack of detectable DNA lesions, ROSC activated wt p53 protein. The increase of p53 levels was attributable to the ROSC-mediated protein stabilization. Further analyses revealed that ROSC induced site-specific phosphorylation of p53 protein at Ser46. After longer exposure, ROSC induced apoptosis in HeLaS(3) cells. These results indicate that therapy of HeLaS(3) cells by ROSC could offer an advantage over that by CP due to its increased selectivity and markedly reduced risk of generation of a secondary cancer. 相似文献
997.
998.
999.
1000.
Francois Crampes Fabrice Marion‐Latard Alexia Zakaroff‐Girard Isabelle de Glisezinski Isabelle Harant Claire Thalamas Vladimir Stich Daniel Rivire Max Lafontan Michel Berlan 《Obesity (Silver Spring, Md.)》2003,11(2):247-256
Objective: The aim of this study was to determine how training modifies metabolic responses and lipid oxidation in overweight young male subjects. Research Methods and Procedures: Eleven overweight subjects were selected for a 4‐month endurance training program. Before and after the training period, they cycled for 60 minutes at 50% of their Vo 2max after an overnight fast or 3 hours after eating a standardized meal. Various metabolic and endocrine parameters, and respiratory exchange ratio values were evaluated. Results: Exercise‐induced plasma norepinephrine concentration increases were similar before and after training in fasted or fed conditions. After food intake, exercise promoted a decrease in plasma glucose and a higher increase in epinephrine than in fasting conditions. The increase in epinephrine after the meal was more marked after training (264 ± 32 vs. 195 ± 35 pg/mL). Training lowered the resting plasma nonesterified fatty acids. During exercise, changes in glycerol were similar to those found before training. Lipid oxidation during exercise was higher in fasting than in fed conditions (15.5 ± 1.4 vs. 22.3 ± 1.7 g/h). Training did not significantly increase fat oxidation when exercise was performed in fed conditions, but it did in fasting conditions (18.6 ± 1.4 vs. 27.2 ± 1.8 g/h). Discussion: Endurance training decreased plasma nonesterified fatty acids, cholesterol, and insulin concentrations. Training increased lipid oxidation during exercise, in fasting conditions, and not when exercise was performed after the meal. During exercise in overweight subjects, the fasting condition seems more suited to oxidizing fat and maintaining glucose homeostasis than a 3‐hour wait after a standard meal. 相似文献