全文获取类型
收费全文 | 6654篇 |
免费 | 513篇 |
专业分类
7167篇 |
出版年
2024年 | 8篇 |
2023年 | 56篇 |
2022年 | 104篇 |
2021年 | 204篇 |
2020年 | 107篇 |
2019年 | 137篇 |
2018年 | 185篇 |
2017年 | 187篇 |
2016年 | 226篇 |
2015年 | 394篇 |
2014年 | 392篇 |
2013年 | 526篇 |
2012年 | 644篇 |
2011年 | 556篇 |
2010年 | 346篇 |
2009年 | 296篇 |
2008年 | 379篇 |
2007年 | 377篇 |
2006年 | 312篇 |
2005年 | 293篇 |
2004年 | 268篇 |
2003年 | 247篇 |
2002年 | 235篇 |
2001年 | 51篇 |
2000年 | 32篇 |
1999年 | 47篇 |
1998年 | 67篇 |
1997年 | 40篇 |
1996年 | 44篇 |
1995年 | 35篇 |
1994年 | 23篇 |
1993年 | 19篇 |
1992年 | 17篇 |
1991年 | 16篇 |
1990年 | 47篇 |
1989年 | 9篇 |
1988年 | 12篇 |
1987年 | 15篇 |
1986年 | 13篇 |
1985年 | 13篇 |
1984年 | 23篇 |
1983年 | 20篇 |
1982年 | 14篇 |
1981年 | 21篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1975年 | 7篇 |
1974年 | 7篇 |
1973年 | 6篇 |
排序方式: 共有7167条查询结果,搜索用时 0 毫秒
41.
42.
Paula Tatiana Lopes Seixas Antonio Jacinto Demuner Luiz Claudio Almeida Barbosa Cristiane Isaac Cerceau Daiane Einhardt Blank Marcelo Henrique Dos Santos Elizeu de Sá Farias Marcelo Coutinho Picanço 《Journal of Applied Entomology》2023,147(2):116-125
Aedes aegypti L. (Diptera: Culicidae) is a vector for serious diseases in tropical regions. This pest is mainly controlled by commercial larvicides but the application of such products has led to environmental problems. Essential oils (EO) have been consistently reported as molecules with insecticidal activity and can be used to produce more environmentally friendly larvicides in the control of A. aegypti. In this study, the larvicidal effect of essential oils (EO) from the leaves of three Artemisia species was evaluated against A. aegypti. The oils were obtained from steam distillation and their chemical composition was determined by gas chromatography–mass spectrometry. The EO of Artemisia camphorata was the most active in the screening bioassay and presented LC50 and LC95 of 64.95 and 74.18 μg ml−1, respectively. In addition, we found that germacrene D-4-ol was the constituent responsible for the toxicity of this EO. Artemisia camphorata EO and its major constituent, germacrene D-4-ol, are promising for the development of natural larvicides against A. aegypti. 相似文献
43.
44.
Antigenic variation of pilin regulates adhesion of Neisseria meningitidis to human epithelial cells 总被引:20,自引:9,他引:11
Xavier Nassif Jonathan Lowy Paula Stenberg Peadar O'Gaora Amir Ganji Magdalene So 《Molecular microbiology》1993,8(4):719-725
Pili have been shown to play an essential role in the adhesion of Neisseria meningitidis to epithelial cells. However, among piliated strains, both inter- and intrastrain variability exist with respect to their degree of adhesion to epithelial cells in vitro (Virji et al., 1992). This suggests that factors other than the presence of pili per se are involved in this process. The N. meningitidis pilin subunit undergoes extensive antigenic variation. Piliated low- and high-adhesive derivatives of the same N. meningitidis strain were selected and the nucleotide sequence of the pilin gene expressed in each was determined. The highly adhesive derivatives had the same pilin sequence. The alleles encoding the pilin subunit of the low-adhesive derivatives were completely different from the one found in the high-adhesive isolates. Using polyclonal antibodies raised against one hyperadhesive variant, it was confirmed that the low-adhesive piliated derivatives expressed pilin variants antigenically different from the highly adhesive strains. The role of antigenic variation in the adhesive process of N. meningitidis was confirmed by performing allelic exchanges of the pilE locus between low-and high-adhesive isolates. Antigenic variation has been considered a means by which virulent bacteria evade the host immune system. This work provides genetic proof that a bacterial pathogen, N. meningitidis, can use antigenic variation to modulate their degree of virulence. 相似文献
45.
Janaine Almeida Neto Daniel Amando Nery Katia Simoni Bezerra Lima Maria Eduarda Gomes da Cruz Silva Tarcísio Cícero de Lima Araújo Nathália Andrezza Carvalho de Souza Rodolfo Hideki Vicente Nishimura Camila de Souza Araújo Ana Paula de Oliveira Jackson Roberto Guedes da Silva Almeida Larissa Araújo Rolim 《化学与生物多样性》2023,20(3):e202201039
This article describes the phytochemical study of Cannabis sativa roots from northeastern Brazil. The dried plant material was pulverized and subjected to exhaustive maceration with ethanol at room temperature, obtaining the crude ethanolic extract (Cs-EEBR). The volatile compounds were analyzed by gas chromatography coupled with mass spectrometry (GC/MS), which allowed to identify 22 compounds by comparing the linear retention index (LRI), the similarity index (SI) and the fragmentation pattern of the constituents with the literature. By this technique the major compounds identified were: friedelan-3-one and β-sitosterol. In addition, two fractions were obtained from Cs-EEBR by classical column chromatography and preparative thin layer chromatography. These fractions were analyzed by NMR and IR and together with the mass spectrometry data allowed to identify the compounds: epifriedelanol, friedelan-3-one, β-sitosterol and stigmasterol. The study contributed to the phytochemical knowledge of Cannabis sativa, specifically the roots, as there are few reports on the chemical constituents of this part of the plant. 相似文献
46.
João Ricardo Sato Claudinei Eduardo Biazoli Ana Paula Arantes Bueno Arthur Caye Pedro Mario Pan Marcos Santoro Jessica Honorato-Mauer Giovanni Abrahão Salum Marcelo Queiroz Hoexter Rodrigo Affonseca Bressan Andrea Parolin Jackowski Euripedes Constantino Miguel Sintia Belangero Luis Augusto Rohde 《Genes, Brain & Behavior》2023,22(2):e12838
47.
Huaibi Zhang Kathryn Joan Horgan Paul Hugh Stewart Reynolds Paula Elizabeth Jameson 《Physiologia plantarum》2003,117(2):264-269
Cytokinins (CKs) play essential roles in the regulation of plant growth and development. In the previous paper (Zhang et al. 2001), we reported the detection and identification of a wide spectrum of CKs, including several novel forms, in the buds of Pinus radiata D. Don. In this paper we examine the relationship between the CKs and buds from juvenile and adult trees of P. radiata. During development the morphology of buds alters significantly, from buds bearing primary needles during their juvenile phase to buds sealed in scales at the adult phase. The morphology of adult buds is a very stable character, as fascicle meristems released from apical dominance, or cultured in vitro, produced only secondary needles. However, exogenous CK causes the adult buds to revert to juvenile bud development in vitro . Analyses of the endogenous CKs revealed that juvenile buds had a relatively higher level of isopentenyladenine and isopentenyladenosine, extremely low levels of phosphorylated CKs and a relatively low level of novel CK glycosides. The adult buds contained lower levels of free base and riboside CKs but very high levels of phosphorylated CKs and novel CK glycosides. Possible roles for CKs in the regulation of bud development are discussed. 相似文献
48.
Paula S. Campos M. Salomé S. Pais 《In vitro cellular & developmental biology. Plant》1996,32(3):184-189
Summary Shoot propagation ofPersea indica (L.) K. Spreng was achieved using seedling axillary buds cultured on MS (Murashige and Skoog, 1962) medium with 1 mg/l (2.8
μM) N6-benzyladenine (BA). Forty percent of the obtained shoots did not elongate, but showed bud proliferation, which was maximal
(three axillary buds per shoot) at the end of the seventh subculture. Sixty percent of the shoots elongated, did not show
bud proliferation, and formed calluses at their base. Successful rooting (84.6%) was achieved dipping the base of each elongated
shoot in 3 g/l (16.11 mM) indolebutyric acid (IBA) for 1–2 s, and transferring to half strength MS medium without growth regulators. These shoots
presented an acclimatization success of 100%. Results suggest that micropropagated elongated shoots ofP. indica can be adequately used in reforestation programs. 相似文献
49.
Paula Veríssimo Cristina Esteves Carlos Faro Euclides Pires 《Biotechnology letters》1995,17(6):621-626
The flowers of cardoon (genus Cynara) are traditionally used in Portugal for cheese making. In this work the vegetable rennet of the species Cynara cardunculus L. was characterized in terms of enzymic composition and proteolytic specificity of its proteinases (cardosin A and cardosin B). Cardosin A was found to cleave insulin B chain at the bonds Leu15-Tyr16, Leu17-Val18 and Phe25-Tyr26. In addition to the bonds mentioned cardosin B cleaves also Glu13-Ala14, Ala14-Leu15 and Phe24-Phe25 indicating that it has a broader specificity. The kinetic parameters for the hydrolysis of the synthetic peptide Leu-Ser-Phe(NO2)-Nle-Ala-Leu-oMe were also determined and compared to those of chymosin and pepsin. The results obtained indicate that in terms of specificity and kinetic parameters cardosin A is similar to chymosin whereas cardosin B is similar to pepsin. It appears therefore that the enzyme composition of cardoon rennet closely resembles that of calf rennet. 相似文献
50.
Ethanol Promotes Apoptosis in Cerebellar Granule Cells by Inhibiting the Trophic Effect of NMDA 总被引:8,自引:3,他引:5
Abstract: When primary cultures of cerebellar granule neurons are grown in a physiological concentration of KCl (5 m M ) they undergo apoptosis, which can be prevented by growing the cells in the presence of N -methyl- d -aspartate (NMDA). We now show that ethanol inhibits this trophic effect of NMDA, i.e., promotes apoptosis, and also inhibits the NMDA-induced increase in intracellular Ca2+ concentration in cells grown in 5 m M KCl. Both effects of ethanol show a similar concentration dependence and are reversed by a high concentration of glycine, the co-agonist at the NMDA receptor. The data suggest that the effect of ethanol on apoptosis is mediated, at least in part, by inhibition of NMDA receptor function. This effect of ethanol to increase apoptosis could contribute to the previously described in vivo sensitivity of the developing cerebellum to ethanol-induced damage. 相似文献