全文获取类型
收费全文 | 11031篇 |
免费 | 1003篇 |
国内免费 | 5篇 |
出版年
2023年 | 60篇 |
2022年 | 161篇 |
2021年 | 324篇 |
2020年 | 156篇 |
2019年 | 221篇 |
2018年 | 258篇 |
2017年 | 221篇 |
2016年 | 326篇 |
2015年 | 621篇 |
2014年 | 613篇 |
2013年 | 692篇 |
2012年 | 934篇 |
2011年 | 919篇 |
2010年 | 552篇 |
2009年 | 511篇 |
2008年 | 670篇 |
2007年 | 634篇 |
2006年 | 534篇 |
2005年 | 502篇 |
2004年 | 498篇 |
2003年 | 512篇 |
2002年 | 448篇 |
2001年 | 84篇 |
2000年 | 52篇 |
1999年 | 87篇 |
1998年 | 114篇 |
1997年 | 59篇 |
1996年 | 65篇 |
1995年 | 66篇 |
1994年 | 58篇 |
1993年 | 55篇 |
1992年 | 54篇 |
1991年 | 60篇 |
1990年 | 43篇 |
1989年 | 42篇 |
1988年 | 36篇 |
1987年 | 31篇 |
1986年 | 38篇 |
1985年 | 65篇 |
1984年 | 48篇 |
1983年 | 53篇 |
1982年 | 45篇 |
1981年 | 42篇 |
1980年 | 37篇 |
1979年 | 35篇 |
1978年 | 36篇 |
1977年 | 35篇 |
1975年 | 29篇 |
1974年 | 36篇 |
1973年 | 29篇 |
排序方式: 共有10000条查询结果,搜索用时 546 毫秒
991.
The influence of fundamental design parameters on ciliates community structure in Irish activated sludge systems 总被引:1,自引:0,他引:1
The protozoan community in eleven activated sludge wastewater treatment plants (WWTPs) in the greater Dublin area has been investigated and correlated with key physio-chemical operational and effluent quality parameters. The plants represented various designs, including conventional and biological nutrient removal (BNR) systems. The aim of the study was to identify differences in ciliate community due to key design parameters including anoxic/anaerobic stages and to identify suitable bioindicator species for performance evaluation. BNR systems supported significantly different protozoan communities compared to conventional systems. Total protozoan abundance was reduced in plants with incorporated anoxic and anaerobic stages, whereas species diversity was either unaffected or increased. Plagiocampa rouxi and Holophrya discolor were tolerant to anoxic/anaerobic conditions and associated with high denitrification. Apart from process design, influent wastewater characteristics affect protozoan community structure. Aspidisca cicada was associated with low dissolved oxygen and low nitrate concentrations, while Trochilia minuta was indicative of good nitrifying conditions and good sludge settleability. Trithigmostoma cucullulus was sensitive to ammonia and phosphate and could be useful as an indicator of high effluent quality. The association rating assessment procedure of Curds and Cockburn failed to predict final effluent biological oxygen demand (BOD5) indicating the method might not be applicable to treatment systems of different designs. 相似文献
992.
993.
994.
The response to signals, including scent marks, from opposite-sex conspecifics can be affected by the nutritional state of
both the sender and receiver of these signals. Protein content of the diet affects how meadow voles (Microtus pennsylvanicus) respond to single scent marks, but it is unknown how it affects an individual’s response to the overlapping scent marks
of two donors (an over-mark). In experiment 1, we tested the hypothesis that protein content of the diet affects the amount
of time voles spend investigating the marks of the top- and bottom-scent donors of an over-mark. Males and females fed a 22%
protein diet spent more time investigating the scent mark of the top-scent donor than that of the bottom-scent donor; voles
fed 9% and 13% protein diets spent similar amounts of time investigating the top- and bottom-scent donors. In experiment 2,
we tested the hypothesis that protein content of the diet of the top- and bottom-scent donors affects the amount of time conspecifics
spend investigating their scent marks. Female voles spent more time investigating the mark of the top-scent male than that
of the bottom-scent male, independent of the differences in protein content of the diets of the top- and bottom-scent donors.
Male voles, however, spent more time investigating the top-scent female when she was fed a diet higher in protein content
than that of the bottom-scent female. Our results are discussed within the context of the natural history of voles. 相似文献
995.
Flynn JM Choi SW Day NU Gerencser AA Hubbard A Melov S 《Free radical biology & medicine》2011,50(7):866-873
Presynaptic nerve terminals require high levels of ATP for the maintenance of synaptic function. Failure of synaptic mitochondria to generate adequate ATP has been implicated as a causative event preceding the loss of synaptic networks in neurodegenerative disease. Endogenous oxidative stress has often been postulated as an etiological basis for this pathology, but has been difficult to test in vivo. Inactivation of the superoxide dismutase gene (Sod2) encoding the chief defense enzyme against mitochondrial superoxide radicals results in neonatal lethality. However, intervention with an SOD mimetic extends the life span of this model and uncovers a neurodegenerative phenotype providing a unique model for the examination of in vivo oxidative stress. We present here studies on synaptic termini isolated from the frontal cortex of Sod2 null mice demonstrating impaired bioenergetic function as a result of mitochondrial oxidative stress. Cortical synaptosomes from Sod2 null mice demonstrate a severe decline in mitochondrial spare respiratory capacity in response to physiological demand induced by mitochondrial respiratory chain uncoupling with FCCP or by plasma membrane depolarization induced by 4-aminopyridine treatment. However, Sod2 null animals compensate for impaired oxidative metabolism in part by the Pasteur effect allowing for normal neurotransmitter release at the synapse, setting up a potentially detrimental energetic paradigm. The results of this study demonstrate that high-throughput respirometry is a facile method for analyzing specific regions of the brain in transgenic models and can uncover bioenergetic deficits in subcellular regions due to endogenous oxidative stress. 相似文献
996.
Oxidative damage of the endothelium disrupts the integrity of the blood-brain barrier (BBB). We have shown before that alcohol exposure increases the levels of reactive oxygen species (ROS; superoxide and hydroxyl radical) and nitric oxide (NO) in brain endothelial cells by activating NADPH oxidase and inducible nitric oxide synthase. We hypothesize that impairment of antioxidant systems, such as a reduction in catalase and superoxide dismutase (SOD) activity, by ethanol exposure may elevate the levels of ROS/NO in endothelium, resulting in BBB damage. This study examines whether stabilization of antioxidant enzyme activity results in suppression of ROS levels by anti-inflammatory agents. To address this idea, we determined the effects of ethanol on the kinetic profile of SOD and catalase activity and ROS/NO generation in primary human brain endothelial cells (hBECs). We observed an enhanced production of ROS and NO levels due to the metabolism of ethanol in hBECs. Similar increases were found after exposure of hBECs to acetaldehyde, the major metabolite of ethanol. Ethanol simultaneously augmented ROS generation and the activity of antioxidative enzymes. SOD activity was increased for a much longer period of time than catalase activity. A decline in SOD activity and protein levels preceded elevation of oxidant levels. SOD stabilization by the antioxidant and mitochondria-protecting agent acetyl-L-carnitine (ALC) and the anti-inflammatory agent rosiglitazone suppressed ROS levels, with a marginal increase in NO levels. Mitochondrial membrane protein damage and decreased membrane potential after ethanol exposure indicated mitochondrial injury. These changes were prevented by ALC. Our findings suggest the counteracting mechanisms of oxidants and antioxidants during alcohol-induced oxidative stress at the BBB. The presence of enzymatic stabilizers favors the ROS-neutralizing antioxidant redox of the BBB, suggesting an underlying protective mechanism of NO for brain vascular tone and vasodilation. 相似文献
997.
Young DD Jockush S Turro NJ Schultz PG 《Bioorganic & medicinal chemistry letters》2011,21(24):7502-7504
The site-specific incorporation of unnatural amino acids (UAAs) into proteins in bacteria is made possible by the evolution of aminoacyl-tRNA synthetases that selectively recognize and aminoacylate the amino acid of interest. Recently we have discovered that some of the previously evolved aaRSs display a degree of polyspecificity and are capable of recognizing multiple UAAs. Herein we report the polyspecificity of an aaRS evolved to encode a comarin containing amino acid. This polyspecificity was then exploited to introduce several UAAs into the fluorophore of GFP, altering its photophysical properties. 相似文献
998.
Colchicine is a known tubulin binding agent enabling necrosis in tumors. A novel tubulin-directed DO3A-colchicine conjugate and its Gd(III) complex were prepared from N-deacetylcolchicine, coupling alkaloid and polyaza-alicyclic functions via a peptide coupling methodology. The longitudinal proton relaxivity of the Gd(III) complex in water at 4.7 T is 2.86 mM−1 s−1 and a similar efficacy as colchicine towards ovarian carcinoma cells in vitro. 相似文献
999.
Firooznia F Cheung AW Brinkman J Grimsby J Gubler ML Hamid R Marcopulos N Ramsey G Tan J Wen Y Sarabu R 《Bioorganic & medicinal chemistry letters》2011,21(7):1933-1936
The highly potent but modestly selective N-(2-amino-4-methoxy-benzothiazol-7-yl)-N-ethyl-acetamide derivative 2 was selected as the starting point for the design of novel selective A2B antagonists, due to its excellent potency, and good drug-like properties. A series of compounds containing nonaromatic amides or ureas of five- or six-membered rings, and also bearing an m-trifluoromethyl-phenyl group (shown to impart superior potency) was prepared and evaluated for their selectivity against the A2A and A1 receptors. This work resulted in the identification of compound 30, with excellent potency and high selectivity against both A2A and A1 receptors. 相似文献
1000.
Tran TD Pryde DC Jones P Adam FM Benson N Bish G Calo F Ciaramella G Dixon R Duckworth J Fox DN Hay DA Hitchin J Horscroft N Howard M Gardner I Jones HM Laxton C Parkinson T Parsons G Proctor K Smith MC Smith N Thomas A 《Bioorganic & medicinal chemistry letters》2011,21(8):2389-2393
The synthesis and structure-activity relationships of a series of novel interferon inducers are described. Pharmacokinetic studies and efficacy assessment of a series of 8-oxo-3-deazapurine analogues led to the identification of compound 33, a potent and selective agonist of the TLR7 receptor with an excellent in vivo efficacy profile in a mouse model. 相似文献