首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4862篇
  免费   181篇
  国内免费   1篇
  5044篇
  2023年   15篇
  2022年   14篇
  2021年   33篇
  2020年   17篇
  2019年   31篇
  2018年   83篇
  2017年   78篇
  2016年   98篇
  2015年   76篇
  2014年   110篇
  2013年   196篇
  2012年   420篇
  2011年   630篇
  2010年   276篇
  2009年   73篇
  2008年   420篇
  2007年   390篇
  2006年   334篇
  2005年   305篇
  2004年   233篇
  2003年   232篇
  2002年   171篇
  2001年   67篇
  2000年   146篇
  1999年   75篇
  1998年   47篇
  1997年   13篇
  1996年   12篇
  1995年   16篇
  1994年   9篇
  1993年   10篇
  1992年   20篇
  1991年   14篇
  1990年   11篇
  1989年   12篇
  1987年   17篇
  1985年   19篇
  1984年   22篇
  1982年   16篇
  1981年   9篇
  1980年   17篇
  1979年   14篇
  1978年   13篇
  1977年   9篇
  1976年   17篇
  1975年   12篇
  1974年   18篇
  1973年   9篇
  1971年   10篇
  1967年   8篇
排序方式: 共有5044条查询结果,搜索用时 15 毫秒
211.
This work examines the bioaccumulated organochlorine compounds in Muraena helena to draw attention to this species as a potential bioindicator organism. The morays were caught in the East Adriatic Sea, Elaphite Islands, near Dubrovnik, Croatia (lat: 42°45′38.8″; long: 17°45′53.6″) at the same location in summer (August, 2009) and winter (January, 2010), at depths from 5 to 10 m. Nine fish were caught by longline hooks per each field trip (total in summer and winter, N = 18). No single fish was free of organochlorines (OC), but not all pesticides were present in every fish. The concentration level was: Endrin > ppDDE > Heptachlor epoxide > Heptachlor > Aldrine > Lindane > βHCH > δHCH > Endrin aldehyde). Endrin was the only pesticide approaching the maximum concentrations proposed by the European Union and Croatia (summer max. concentration = 14.75 μg kg?1; winter max. concentration = 8.35 μg kg?1). All other pesticides ranged from 0.1 to 0.7 μg kg?1. Heptachlor was absent in winter and δ‐HCH and Endrin aldehyde in summer, while Lindane and pp‐DDE had higher (P ≤ 0.05) concentrations in summer. Other residues had similar seasonal concentration levels. The p‐DDT was similar as in other biota from the Adriatic or Mediterranean. HCH concentrations corresponded to those found in organisms from the Italian area of the Adriatic. The endocrine and reproductive disrupting potential of the detected OCs are bases for future studies. Results show that morays are a good bioindicator model, with slight interspecies and seasonal variability in OC bioaccumulation as a consequence of diverse food consumption.  相似文献   
212.
Synthesis and biological activities of a series of homo- or substituted piperidine unsymmetrical diethers are described. The novel compounds were evaluated for histamine H3 receptor binding affinities at recombinant human H3 receptor stably expressed in HEK-293 cells. All diethers showed in vitro affinities in nanomolar concentration range. The most potent compounds are 1-[3-(3-(4-chlorophenoxy)propoxy)propyl]-3-methylpiperidine 11 (Ki = 3.2 nM) and 1-[3-(3-(4-chlorophenoxy)propoxy)propyl]azepane 13 (Ki = 3.5 nM).  相似文献   
213.
In previous studies it has been shown that callus cell cultures of Arabidopsis thaliana respond to changes in gravitational field strengths by altered gene expression. In this study an investigation was carried out into how different g conditions affect the proteome of such cells. For this purpose, callus cells were exposed to 8 g (centrifugation) and simulated microgravity (2-D clinorotation: fast rotating clinostat, yielding 0.0016 g at maximum; and 3-D random positioning) for up to 16 h. Extracts containing total soluble protein were subjected to 2-D SDS-PAGE. Image analysis of Sypro Ruby-stained gels showed that approximately 28 spots reproducibly and significantly (P <0.05) changed in amount after 2 h of hypergravity (18 up- and 10 down-regulated). These spots were analysed by electrospray ionization tandem mass spectrometry (ESI-MS/MS). In the case of 2-D clinorotation, 19 proteins changed in a manner similar to hypergravity, while random positioning affected only eight spots. Identified proteins were mainly stress related, and are involved in detoxification of reactive oxygen species, signalling, and calcium binding. Surprisingly, centrifugation and clinorotation showed homologies which were not detected for random positioning. The data indicate that simulation of weightlessness is different between clinorotation and random positioning.  相似文献   
214.
A total of 45 racing pigeons were genotyped using PCR-RFLP method. PCR product of the LDH-A gene was amplified according to the Long-PCR procedure. The amplification products were digested with restriction enzymes. PCR-RFLPs for two restriction enzymes, HaeIII and NlaIV, were observed. Two pairs of alleles LDH-A(A) and LDH-A(B) for LDH-A-NlaIV polymorphism and LDH-A(C) and LDH-A(D) for LDH-A-HaeIII polymorphism were detected in the homozygous and heterozygous states. Frequencies of alleles were as follows: A - 0.622, B - 0.378 and C - 0.256, D - 0.744.  相似文献   
215.
216.
In vitro culture of Chenopodium murale L. (ecotype 197) green and herbicide SAN 9789 - treated "white" plants was established and the effects of benzylaminopurine (BAP), indole-3-acetic acid (IAA) and gibberellic acid (GA3) on growth and flowering were tested. Green plants did not flower on glucose free media, while 17 % of plants flowered on 5 % glucose-containing medium. SAN 9789 (10–5 M) inhibited growth and flowering. BAP and IAA (0.1 – 5 mg dm–3) also inhibited growth and flowering of green and "white" plants. GA3 (10 mg dm–3) stimulated leaf development in green plants, but had no significant effect on "white" plants, and stimulated flowering of green (41 %) and "white" (33 %) plants.  相似文献   
217.
Mutations in the DES gene coding for the intermediate filament protein desmin may cause skeletal and cardiac myopathies, which are frequently characterized by cytoplasmic aggregates of desmin and associated proteins at the cellular level. By atomic force microscopy, we demonstrated filament formation defects of desmin mutants, associated with arrhythmogenic right ventricular cardiomyopathy. To understand the pathogenesis of this disease, it is essential to analyze desmin filament structures under conditions in which both healthy and mutant desmin are expressed at equimolar levels mimicking an in vivo situation. Here, we applied dual color photoactivation localization microscopy using photoactivatable fluorescent proteins genetically fused to desmin and characterized the heterozygous status in living cells lacking endogenous desmin. In addition, we applied fluorescence resonance energy transfer to unravel short distance structural patterns of desmin mutants in filaments. For the first time, we present consistent high resolution data on the structural effects of five heterozygous desmin mutations on filament formation in vitro and in living cells. Our results may contribute to the molecular understanding of the pathological filament formation defects of heterozygous DES mutations in cardiomyopathies.  相似文献   
218.
The main objectives of this work were to investigate the evolution of some principal physico-chemical properties (temperature, carbon dioxide, oxygen, ammonia, pH, electrical conductivity, organic matter) and microbial population (mesophilic and thermophilic bacteria and fungi) during composting poultry manure with wheat straw in a reactor system, and to evaluate the optimum mixture ratio for organic substrate production. The experiments were carried out in four small laboratory reactors (1 l) and one large reactor (32 l) under adiabatic conditions over 14 days. During the process the highest temperature was 64.6°C, pH varied between 7.40 and 8.85, electrical conductivity varied between 3.50 and 4.31 dS m−1 and the highest value of organic matter (dry weight) degradation was 47.6%. Mesophilic bacteria and fungi predominated in the beginning, and started the degradation with generation of metabolic heat. By increasing the temperature in reactors, the number of thermophilic microorganisms also increased, which resulted in faster degradation of substrate. The application of a closed reactor showed a rapid degradation of manure/straw mixture as well as a good control of the emissions of air polluting gases into atmosphere. The results showed that the ratio of manure to straw 5.25:1 (dry weight) was better for composting process than the other mixture ratios.  相似文献   
219.
Starting from the D-homo lactones of androst-4-en-3-one 3 and 4, prepared from 1 and 2, the new 17a homolactones 5-12, 14 and 15, were synthesized. The 4-hydroxy compounds 9 and 10 were obtained through the reaction of 4alpha,5alpha- (5 and 7) and 4beta,5beta- (6 and 8) epoxides with formic acid. The epoxides 5 and 6 were prepared from compound 3, and epoxides 7 and 8 from compound 4 by oxidation with H(2)O(2) under basic conditions. Compound 1 served as a starting substance for obtaining lactones 11-13. Oxidation of compound 1 with m-chloroperbenzoic acid yielded 11 and 12, but compound 13 gave 14. Compound 15 was obtained from 13 by oxidation with H(2)O(2) under basic conditions. The structures of epoxides 6 and 14 were confirmed by X-ray structural analysis. Cytotoxic activity against three tumor cell lines (human breast adenocarcinoma ER+, MCF-7, human breast adenocarcinoma ER-, MDA-MB-231, and prostate cancer PC3) was evaluated. Compounds 6 and 14 showed strong activity against PC3, the IC(50) being 10.6 and 2.2 microM, respectively, whereas compounds 3 and 8 showed strong activity against MDA-MB-231 (IC(50) is 9.3 and 3.6 microM, respectively). Aromatase inhibition assay showed that the tested compounds 9, 10, and 14 possess lower activity compared to formestane.  相似文献   
220.
The averaging method was applied for the analysis of the effect of intermittent light as well as for the analysis of the spontaneous spike activities of burster neurons in Helix pomatia and Aplysia depilans ganglia. Owing to such analyses by means of which the unwanted effects of permanently present noise (i.e., irregular frequency modulation of neuron impulse activity) were diminished, the dynamics of the neurons reaction to light and the longer time effects of intermittent illumination on neurons spontancous activity could be observed. The functional characteristics of burster neurons were determined by averaging and the least squares and equalization methods. The results were presented in the table and in two average curves which are asymmetrical with a horizontal asymptote. From these data it was concluded that the dynamics of the spontaneous activity of the two burster neurons in both specia is similar. The possibilities and limitations of the method in physiological analyses are discussed.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号