全文获取类型
收费全文 | 10609篇 |
免费 | 833篇 |
国内免费 | 3篇 |
出版年
2024年 | 5篇 |
2023年 | 60篇 |
2022年 | 173篇 |
2021年 | 263篇 |
2020年 | 152篇 |
2019年 | 206篇 |
2018年 | 252篇 |
2017年 | 234篇 |
2016年 | 325篇 |
2015年 | 556篇 |
2014年 | 602篇 |
2013年 | 780篇 |
2012年 | 977篇 |
2011年 | 890篇 |
2010年 | 595篇 |
2009年 | 509篇 |
2008年 | 666篇 |
2007年 | 722篇 |
2006年 | 663篇 |
2005年 | 544篇 |
2004年 | 544篇 |
2003年 | 445篇 |
2002年 | 432篇 |
2001年 | 75篇 |
2000年 | 57篇 |
1999年 | 78篇 |
1998年 | 93篇 |
1997年 | 73篇 |
1996年 | 58篇 |
1995年 | 59篇 |
1994年 | 30篇 |
1993年 | 51篇 |
1992年 | 32篇 |
1991年 | 32篇 |
1990年 | 19篇 |
1989年 | 14篇 |
1988年 | 15篇 |
1987年 | 7篇 |
1986年 | 11篇 |
1985年 | 18篇 |
1984年 | 16篇 |
1983年 | 13篇 |
1982年 | 17篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 9篇 |
1977年 | 9篇 |
1976年 | 7篇 |
1975年 | 5篇 |
1974年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Elena P. Sawin Harold B. Dowse Melanie J. Hamblen-Coyle Jeffery C. Hall Marla B. Sokolowski 《Journal of Insect Behavior》1994,7(3):249-262
We examined the locomotor activity ofDrosophila melanogaster for the existence of circadian rhythms, using the wild type and two mutants of theperiod (per) gene,per
o andper
s. This was accomplished using a newly described apparatus for the recording and measurement of larval path lengths over a 96-h test period. None of the larvae examined exhibited appreciable diel rhythms under cycling conditions of light or temperature. Larvae were also not rhythmic under free-running conditions. Our results suggest that theper gene does not influence an observable locomotor behavioral phenotype in the larval stage of development. 相似文献
42.
Elena Driomina Igor Polnikov Victor Sharov Ofelia Azizova Yury Vladimirov 《Free radical research》1994,20(5):279-288
A chemiluminescence (CL) flash kinetics on the addition of Fe2+ ions into oxidized low density lipoprotein (LDL) suspension has been studied. LDL oxidation was carried out at 37°C without and in the presence of 5 or 50 μM of Cu.2+ It has been found that under certain experimental conditions (the addition of excess iron ions, more than 1 mM) the amplitude of CL flash depended almost linearly (1) on the concentration of oxidized LDL and (2) on the extent of LDL oxidation measured as diene conjugates (DC) and 2-thiobarbituric acid-reactive substance (TBARS) accumulation. The corresponding correlation coefficients were: for TBARS - 0.94 and for DC - 0.97, in the case of LDL autooxidation; 0.72 and 0.98, in the case of copper-induced LDL oxidation. A sensitivity of the CL method was shown to be significantly enhanced (by more than two orders) in the presence of CL sensitizer - 2, 3,5, 6-lH,4H-tetrahydro-9-(2' -benzoimidazolyl)-quinolizin-(9, 9a, 1 -gh)coumarin. 相似文献
43.
Manuela Aragno Elena Tamagno Giuseppe Poli Giuseppe Boccuzzi Enrico Brignardello Oliviero Danni 《Free radical research》1994,21(6):427-435
Dehydroepiandrosterone (DHEA), a lipid soluble steroid, administered to rats (100 mg/kg b.wt) by a single intraperitoneal injection, increases to twice its normal level in the liver microsomes. Microsomes so enriched become resistant to lipid peroxidation induced by incubation with carbon tetrachloride in the presence of a NADPH-regenerating system: also the lipid peroxidation-dependent inactivation of glucose-6-phosphatase and gamma-glutamyl transpetidase due to the haloalkane are prevented. Noteworthy, the liver microsomal drug-metabolizing enzymes and in particular the catalytic activity of cytochrome P450IIE1, responsible for the CCl4-activation, are not impaired by the supplementation with the steroid. Consistently, in DHEA-pretreated microsomes the protein covalent binding of the trichloromethyl radical (CCl3°), is similar to that of not supplemented microsomes treated with CCl4. It thus seems likely that DHEA protects liver microsomes from oxidative damage induced by carbon tetrachloride through its own antioxidant properties rather than inhibiting the metabolism of the toxin. 相似文献
44.
Elena B. Rodriguez de Turco Marie Therese Droy-Lefaix Nicolas G. Bazan 《Journal of neurochemistry》1993,61(4):1438-1444
Abstract: The effect of Ginkgo biloba extract (EGb 761) treatment (100 mg/kg/day, per os, for 14 days) on electroconvulsive shock (ECS)-induced accumulation of free fatty acids (FFA) and diacylglycerols (DAG) was analyzed in rat cerebral cortex and hippocampus. EGb 761 reduced the FFA pool size by 33% and increased the DAG pool by 36% in the hippocampus. These endogenous lipids were unaffected in cerebral cortex. During the tonic seizure (10 s after ECS) the fast accumulation of FFA, mainly 20:4, was similar in sham- and EGb 761 -treated rats, in both the cerebral cortex and hippocampus. However, further accumulation of free 18:0 and 20:4, observed in the hippocampus of sham-treated rats during clonic seizures (30 s to 2 min after ECS), did not occur in EGb 761-treated animals. The rise in DAG content triggered in the cortex and hippocampus by ECS was delayed by EGb 761 treatment from 10 s to 1 min, when values similar to those in sham animals were attained. Moreover, in the hippocampus the size of the total DAG pool was decreased by 19% during the tonic seizure. At later times, DAG content showed a faster decrease in EGb 761-treated rats. By 2 min levels of all DAG acyl groups decreased to values significantly lower than in sham animals in both cortex and hippocampus. This study shows that EGb 761 treatment affects, with high selectivity, lipid metabolism and lipid-derived second messenger release and removal in the hippocampus, while affecting to a lesser extent the cerebral cortex. 相似文献
45.
Coupling Among Energy Failure, Loss of Ion Homeostasis, and Phospholipase A2 and C Activation During Ischemia 总被引:1,自引:1,他引:0
Ken-ichiro Katsura‡ Elena B. Rodriguez de Turco† Jaroslava Folbergrov᧠Nicolas G. Bazan† Bo K. Siesjö 《Journal of neurochemistry》1993,61(5):1677-1684
Abstract— The objective of the present experiments was to correlate changes in cellular energy metabolism, dissipative ion fluxes, and lipolysis during the first 90 s of ischemia and, hence, to establish whether phospholipase A2 or phospholipase C is responsible for the early accumulation of phospholipid hydrolysis products. Ischemia was induced for 15–90 s in rats, extracellular K+ (K+ e ) was recorded, and neocortex was frozen in situ for measurements of labile tissue metabolites, free fatty acids, and diacylglycerides. Ischemia of 15-and 30-s duration gave rise to a decrease in phosphocreatine concentration and a decline in the ATP/free ADP ratio. Although these changes were accompanied by an activation of K+ conductances, there were no changes in free fatty acids until after 60s, when free arachidonic acid accumulated. An increase in other free fatty acids and in total diacylglyceride content did not occur until after anoxic depolarization. The results demonstrate that the early functional changes, such as activation of K+ conductances, are unrelated to changes in lipids or lipid mediators. They furthermore suggest that the initial lipolysis occurs via both phospholipase A2 and phospholipase C, which are activated when membrane depolarization leads to influx of calcium into cells. 相似文献
46.
Genetic bottlenecks and population passages cause profound fitness differences in RNA viruses. 总被引:15,自引:12,他引:3
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Repeated clone-to-clone (genetic bottleneck) passages of an RNA phage and vesicular stomatitis virus have been shown previously to result in loss of fitness due to Muller's ratchet. We now demonstrate that Muller's ratchet also operates when genetic bottleneck passages are carried out at 37 rather than 32 degrees C. Thus, these fitness losses do not depend on growth of temperature-sensitive (ts) mutants at lowered temperatures. We also demonstrate that during repeated genetic bottleneck passages, accumulation of deleterious mutations does occur in a stepwise (ratchet-like) manner as originally proposed by Muller. One selected clone which had undergone significant loss of fitness after only 20 genetic bottleneck passages was passaged again in clone-to-clone series. Additional large losses of fitness were observed in five of nine independent bottleneck series; the relative fitnesses of the other four series remained close to the starting fitness. In sharp contrast, when the same selected clone was transferred 20 more times as large populations (10(5) to 10(6) PFU transferred at each passage), significant increases in fitness were observed in all eight passage series. Finally, we selected several clones which had undergone extreme losses of fitness during 20 bottleneck passages. When these low-fitness clones were passaged many times as large virus populations, they always regained very high relative fitness. We conclude that transfer of large populations of RNA viruses regularly selects those genomes within the quasispecies population which have the highest relative fitness, whereas bottleneck transfers have a high probability of leading to loss of fitness by random isolation of genomes carrying debilitating mutations. Both phenomena arise from, and underscore, the extreme mutability and variability of RNA viruses. 相似文献
47.
Maria Grcia Bozzo Enric Ribes Elena Sagrista Montserrat Poquet Mercedes Durfort 《Molecular reproduction and development》1993,34(2):206-211
We describe sperm ultrastructure and acrosome differentiation during spermiogenesis in Crassostrea gigas (Mollusca Bivalvia). The sperm cell is a uniflagellated cell of the primitive type. The head region contains a rounded or conical nucleus surmounted by small acrosome. This organelle consists of a membrane-bound acrosomal granule, the contents of which have a homogeneous density, except in the anterior region, which is positive for PTA. The acrosome also surrounds the perforatorium, which includes oriented fibrillar elements: this is the axial body. The middle piece contains four mitochondria encircling two perpendicular centrioles. The distal centriole is provided with a system of mechanical fixation to the plasma membrane, consisting of nine fibers in radial arrangement. The tail flagellum, about 50 m?m long, contains the usual microtubular axoneme. © 1993 Wiley-Liss, Inc. 相似文献
48.
Marta Royo Vicente Chulvi Elena Mulet Laura Ruiz-Pastor 《Journal of Industrial Ecology》2023,27(2):562-586
One of the approaches followed by the circular economy (CE) to achieve sustainability through design is product life extension. Extending the life of products to make them useful for as long as possible is a means to reduce waste production and materials consumption, as well as the related impacts. For designers, conceptualizing products in a way that allows them to be used for longer is a challenge, and assessing how well they extend their lifespan can be helpful when it comes to choosing the best proposal. In this paper, 70 tools and methods related to eco-design and circular economy are studied to determine how many of them consider parameters related to life extension and which can be applied in the early stages of design. The results of the analysis show that most of the existing tools and methods are applicable to developed products, and only a few of them take into account parameters related to extending the useful life. Of the 70 tools and methods, only 14 include some parameter related to life extension and are applicable to concepts. CE toolkit, Eco-design PILOT, CE Designer, Circularity Assessment tool, Circularity Potential Indicator and Circular Design Tools take into consideration eight or more parameters to assess life extension in concepts. This will help designers select the most appropriate and will indicate the need for more complete tools to consider useful life extension in the early stages of design and thus enhance the selection of more sustainable products. 相似文献
49.
SruI restriction endonuclease from Selenomonas ruminantium 总被引:1,自引:0,他引:1
Ivan Vanat Peter Prista Elena Ryboovál rej Godány Peter Javorský 《FEMS microbiology letters》1993,113(2):129-132
Abstract Sru I, specific restriction endonuclease, has been characterized from Selenomonas ruminantium isolated from the rumen of fallow deer. Results from the study demonstrate that S. ruminantium 18D possesses a type II restriction endonuclease, which recognizes the sequence 5'-TTT↓AAA-3'. The recognition sequence of Sru I was identified using digestions on pBR322, pBR328, pUC18, M13mp18RF, pACYC184 and λDNA. The cleavage patterns obtained were compared with computer-derived data. Sru I recognises the palindromic hexanucleotide sequence and cleaves DNA after the third T in the sequence, producing blunt ends. The purification and characterization of restriction endonuclease Sru I presented here is the first described for Selenomonas ruminantium spp. and demonstrates that this microorganism pocesses a DNA-cleaving enzyme with the same specificity as Dra I or Aha III. 相似文献
50.
Enterocin 226NWC, a bacteriocin produced by Enterococcus faecalis 226, active against Listeria monocytogenes 总被引:2,自引:0,他引:2
F. Villani G. Salzano Elena Sorrentino Olimpia Pepe P. Marino S. Coppola 《Journal of applied microbiology》1993,74(4):380-387
F. VILLANI, G. SALZANO, E. SORRENTINO, O. PEPE, P. MARINO AND S. COPPOLA. 1993. Enterococcus faecalis 226, isolated from natural whey cultures utilized as starters in the manufacture of mozzarella cheese from water-buffalo milk, produces a bacteriocin designated enterocin 226NWC. The bacteriocin was isolated from culture supernatant fluids of the producer strain and was active against strains of the same species and Listeria monocytogenes, but not against useful lactic acid bacteria. Enterocin 226NWC is a protein with an apparent molecular weight of about 5800; it is relatively heat-stable and has a bactericidal mode of action. Listeria monocytogenes, growing in the presence of the enterocin 226NWC producer strain in broth and in reconstituted skim milk, was inhibited. 相似文献