全文获取类型
收费全文 | 61357篇 |
免费 | 4720篇 |
国内免费 | 5748篇 |
专业分类
71825篇 |
出版年
2024年 | 481篇 |
2023年 | 1103篇 |
2022年 | 1396篇 |
2021年 | 1673篇 |
2020年 | 1619篇 |
2019年 | 2281篇 |
2018年 | 2047篇 |
2017年 | 1682篇 |
2016年 | 1672篇 |
2015年 | 1720篇 |
2014年 | 2940篇 |
2013年 | 3671篇 |
2012年 | 2246篇 |
2011年 | 2712篇 |
2010年 | 2084篇 |
2009年 | 2778篇 |
2008年 | 2868篇 |
2007年 | 3111篇 |
2006年 | 2847篇 |
2005年 | 2391篇 |
2004年 | 2068篇 |
2003年 | 1967篇 |
2002年 | 1825篇 |
2001年 | 1493篇 |
2000年 | 1286篇 |
1999年 | 1166篇 |
1998年 | 1120篇 |
1997年 | 1041篇 |
1996年 | 1007篇 |
1995年 | 1017篇 |
1994年 | 1005篇 |
1993年 | 879篇 |
1992年 | 867篇 |
1991年 | 838篇 |
1990年 | 651篇 |
1989年 | 649篇 |
1988年 | 590篇 |
1987年 | 577篇 |
1986年 | 527篇 |
1985年 | 800篇 |
1984年 | 1063篇 |
1983年 | 807篇 |
1982年 | 880篇 |
1981年 | 701篇 |
1980年 | 710篇 |
1979年 | 626篇 |
1978年 | 470篇 |
1977年 | 434篇 |
1976年 | 406篇 |
1973年 | 270篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
211.
Following is the first publication of a previously unknown translation by L.S. Vygotsky from Hebrew of a fictional short story by M.J. Berdyczewski, a Jewish writer popular at the beginning of the twentieth century. The commentary by the authors of this article reconstructs the context of Vygotsky’s life situation during the prerevolutionary period. Particular attention is focused on the questions of ethnic and cultural self-determination that were confronting him. The cultural contexts essential for an understanding and interpretation of the artistic and world-view features of the text of the short story are explored. A comparison is drawn to show the connection between the themes addressed in the story and the substantive problems that Vygotsky would subsequently deal with as a professional psychology: the artistic inner experience, moral choice, and cultural mediation. 相似文献
212.
T. Togashi Taizo Motomura Terunobu Ichimura Paul Alan Cox 《Sexual plant reproduction》1999,12(3):158-163
The role of phototactic behavior of gametes was tested experimentally in the slightly anisogamous marine green alga Monostroma angicava Kjellman, and the effect of phototaxis on mating efficiency was discovered. Both male and female gametes showed positive
phototaxis in response to a white light source. In contrast, they did not respond to a red light source. Their swimming velocity
did not differ between these two illuminating light sources. It was, therefore, suggested that the search ability of the gamete
itself might not vary between phototactic and non-phototactic conditions. The number of zygotes formed during the mating process
may be expressed as the product of the number of encounters between male and female gametes and the fraction of encounters
that result in sexual fusion. In this study, with high densities of male and female gametes mixed in test tubes, almost all
minor (fewer in number) gametes fused sexually within 10 min. After dilution of the gamete suspensions by half, mating efficiency
in test tubes illuminated by white light from above was higher than that in dark controls. This suggests that male and female
gametes gathered at the water surface through their positive phototaxis, thus increasing the rate of encounters. Mating efficiency
also decreased if the test tubes were illuminated from above by white light and also shaken. Since negative phototaxis is
clearly shown in planozygotes, we suggest that positive phototaxis of male and female gametes in M. angicava is an adaptive trait for increasing the rate of gametic encounters rather than for the dispersal of zygotes as previously
reported for zoospores of some marine algae.
Received: 12 February 1999 / Revision accepted: 24 May 1999 相似文献
213.
G-protein-coupled receptors (GPCRs) are the largest family of cell surface receptors that, via trimetric guanine nucleotide-binding proteins (G-proteins), initiate some signaling pathways in the eukaryotic cell. Many diseases involve malfunction of GPCRs making their role evident in drug discovery. Thus, the automatic prediction of GPCRs can be very helpful in the pharmaceutical industry. However, prediction of GPCRs, their families, and their subfamilies is a challenging task. In this article, GPCRs are classified into families, subfamilies, and sub-subfamilies using pseudo-amino-acid composition and multiscale energy representation of different physiochemical properties of amino acids. The aim of the current research is to assess different feature extraction strategies and to develop a hybrid feature extraction strategy that can exploit the discrimination capability in both the spatial and transform domains for GPCR classification. Support vector machine, nearest neighbor, and probabilistic neural network are used for classification purposes. The overall performance of each classifier is computed individually for each feature extraction strategy. It is observed that using the jackknife test the proposed GPCR–hybrid method provides the best results reported so far. The GPCR–hybrid web predictor to help researchers working on GPCRs in the field of biochemistry and bioinformatics is available at http://111.68.99.218/GPCR. 相似文献
214.
Muhammed Akif Akgz 《化学与生物多样性》2019,16(12)
Phytochemicals, which are commonly found at different levels in many medicinal plants, are natural strong antioxidants used in traditional medicine. In this research, determination of differences of phytochemical compositions and biological properties were aimed as periodically (pre‐, full and post flowering) and daily (6 am, 1 pm and 8 pm) in Achillea gypsicola Hub.‐Mor . The volatile oils belonging to A. gypsicola were obtained by hydrodistillation and analyzed by gas chromatography‐flame ionization detection (GC‐FID) and gas chromatography‐mass spectrometry (GC/MS). The antimicrobial activities of the volatile oils were determined with disc diffusion method. The microdilution method was used to determine minimum inhibitory concentration (MIC). Total phenolic and flavonoid contents were determined by spectrophotometric methods and antioxidant capacities were evaluated by 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH) free radical, reducing power (RP) and metal chelating activity (MCA) assay. In addition, the phenolic acid and flavonoid compositions were evaluated by reversed phase‐high‐performance liquid chromatography (RP‐HPLC). This study presented a comprehensive report for the first time on evaluation of the phytochemical composition and the biological properties of A. gypsicola at different phenological stages. Thirty‐two compounds, containing the major component as camphor, 1,8‐cineole and borneol, were detected. Designated harvest time for the highest yield of volatile oils was found to be at full flowering stage‐1 pm. It has been observed that the volatile oil composition changes periodically and even daily. Also, in this research, menthol and menthone were found as the composition of volatile oil in Achillea species for the first time. Full flowering stage was found as the richest period in terms of phenolic acid and flavonoid compositions of A. gypsicola for the first time. The species examined in this research showed a high antioxidant and antimicrobial activity in comparison to other studies with Achillea species. The volatile oils exhibited high performances with range of inhibition zones (8.3–42.3 mm) and minimum inhibitory concentration values (2.25—144 μg/ml). Besides, a high correlation between antioxidant activity and phenolic content of A. gypsicola was found. These results suggest that A. gypsicola can be used as a safe source in the cosmetic, food and pharmaceutical industries. 相似文献
215.
“雄茭”、灰茭形成规律的初步研究 总被引:4,自引:2,他引:4
茭白栽培过程中,常会出现“雄茭”和灰茭分蘖。根据对茭白的形态学观察,认识到:“雄茭”分蘖的产生是分蘖期母茎中茭白黑粉菌菌丝未能入侵新生分蘖腋芽而导致的结果;灰茭分蘖是因其菌丝的潜育期比正常茭短,故在苗端的膨大部分较早地产生了不同程度的黑粉孢子堆。 相似文献
216.
The Mi-1.2 resistance gene in tomato (Solanum lycopersicum) confers resistance against several species of root-knot nematodes (Meloidogyne spp.). This study examined the impact of M. javanica on the reproductive fitness of near-isogenic tomato cultivars with and without Mi-1.2 under field and greenhouse conditions. Surprisingly, neither nematode inoculation or host plant resistance impacted the yield of mature fruits in field microplots (inoculum=8,000 eggs/plant), or fruit or seed production in a follow-up greenhouse bioassay conducted with a higher inoculum level (20,000 eggs/plant). However, under heavy nematode pressure (200,000 eggs/plant), greenhouse-grown plants carrying Mi-1.2 had more than ten-fold greater fruit production than susceptible plants and nearly forty-fold greater estimated lifetime seed production, confirming prior reports of the benefits of Mi-1.2. In all cases Mi-mediated resistance significantly reduced nematode reproduction. These results indicated that tomato can utilize tolerance mechanisms to compensate for moderate levels of nematode infection, but that the Mi-1.2 resistance gene confers a dramatic fitness benefit under heavy nematode pressure. No significant cost of resistance was detected in the absence of nematode infection. 相似文献
217.
Synthesis of alpha and gamma interferons by a human cutaneous lymphoma with helper T-cell phenotype 总被引:3,自引:0,他引:3
A cloned human cutaneous lymphoma Hut102-B2 with helper T-cell phenotype (Leu1+, Leu2a?, Leu3a+) was found to produce substantial quantities of interferon (IFN) on induction with the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA). Whereas only trace amounts of IFN were secreted by Hut102-B2 cells spontaneously, up to 8000 laboratory units/ ml of IFN were synthesized under the optimal conditions of TPA induction. Characterization studies including neutralization by specific antisera to IFNs and determination of the activities in human and bovine cells disclosed that the IFN produced by Hut102-B2 cells exposed to TPA was a mixture of immune IFN (IFN-γ) and leukocyte IFN (IFN-α) made in approximately equal amounts in terms of antiviral activity. 相似文献
218.
Nancy L. Charó Natalia M. Galigniana Graciela Piwien-Pilipuk 《Biochimica et Biophysica Acta (BBA)/Molecular Cell Research》2018,1865(2):432-443
Confocal and electron microscopy images, and WB analysis of cellular fractions revealed that HP1γ is in the nucleus but also in the cytoplasm of C2C12 myoblasts, myotubes, skeletal and cardiac muscles, N2a, HeLa and HEK293T cells. Signal specificity was tested with different antibodies and by HP1γ knockdown. Leptomycin B treatment of myoblasts increased nuclear HP1γ, suggesting that its nuclear export is Crm-1-dependent. HP1γ exhibited a filamentous pattern of staining partially co-localizing with actin in the cytoplasm of myotubes and myofibrils. Immunoelectron microscopic analysis showed high-density immunogold particles that correspond to HP1γ localized to the Z-disk and A-band of the sarcomere of skeletal muscle. HP1γ partially co-localized with actin in C2C12 myotubes and murine myofibrils. Importantly, actin co-immunoprecipitated with HP1γ in the nuclear and cytosolic fractions of myoblasts. Actin co-immunoprecipitated with HP1γ in myoblasts incubated in the absence or presence of the actin depolymerizing agent cytochalasin D, suggesting that HP1γ may interact with G-and F-actin. In the cytoplasm, HP1γ was associated to the perinuclear actin cap that controls nuclear shape and position. In the nucleus, re-ChIP assays showed that HP1γ-actin associates to the promoter and transcribed regions of the house keeping gene GAPDH, suggesting that HP1γ may function as a scaffold protein for the recruitment of actin to control gene expression. When HP1γ was knocked-down, myoblasts were unable to differentiate or originated thin myotubes. In summary, HP1γ is present in the nucleus and the cytoplasm interacting with actin, a protein complex that may exert different functions depending on its subcellular localization. 相似文献
219.
Bowen L Aldridge BM Delong R Melin S Godinez C Zavala A Gulland F Lowenstine L Stott JL Johnson ML 《Molecular ecology》2006,15(2):529-533
Major histocompatibility complex (MHC) class II DRB genotypes were examined in two geographically isolated populations of California sea lions (Zalophus californianus) (Gulf of California and California coastal Pacific Ocean). Genomic DNA from 227 California sea lions was examined using eight sequence-specific primer (SSP) pairs flanking the putative peptide-binding site. A total of 40 different Zaca-DRB genotype configurations were identified among the 227 individuals. Using SSP-PCR, significant differences were found between coastal California and Gulf of California Zalophus populations in numbers of DRB sequences per individual and configuration of sequences within individuals. Additionally, unique local patterns of MHC diversity were identified among the Midriff Island animals. These population differences are consistent with either ecologically distinct patterns of selection pressures and/or geographical isolation. The consequences of these partitioned MHC configurations at the population level are as yet unknown; however, the worldwide increase in emerging marine diseases lends urgency to their examination. 相似文献
220.
Energy generation and consumption have always been an important component of social development. Interests in this field are beginning to shift to indoor photovoltaics (IPV) which can serve as power sources under low light conditions to meet the energy needs of rapidly growing fields, such as intelligence gathering and information processing which usually operate via the Internet‐of‐things (IoT). Since the power requirements for this purpose continue to decrease, IPV systems under low light may facilitate the realization of self‐powered high‐tech electronic devices connected through the IoT. This review discusses and compares the characteristics of different types of IPV devices such as those based on silicon, dye, III‐V semiconductors, organic compounds, and halide perovskites. Among them, specific attention is paid to perovskite photovoltaics which may potentially become a high performing IPV system due to the fascinating photophysics of the halide perovskite active layer. The limitations of such indoor application as they relate to the toxicity, stability, and electronic structure of halide perovskites are also discussed. Finally, strategies which could produce highly functional, nontoxic, and stable perovskite photovoltaics devices for indoor applications are proposed. 相似文献