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   95552篇
  免费   435篇
  国内免费   895篇
  96882篇
  2022年   25篇
  2021年   20篇
  2020年   17篇
  2019年   19篇
  2018年   11845篇
  2017年   10689篇
  2016年   7481篇
  2015年   634篇
  2014年   340篇
  2013年   424篇
  2012年   4311篇
  2011年   12905篇
  2010年   12073篇
  2009年   8276篇
  2008年   9861篇
  2007年   11440篇
  2006年   361篇
  2005年   594篇
  2004年   1071篇
  2003年   1143篇
  2002年   903篇
  2001年   377篇
  2000年   268篇
  1999年   117篇
  1998年   51篇
  1997年   51篇
  1996年   33篇
  1995年   25篇
  1994年   36篇
  1993年   47篇
  1992年   78篇
  1991年   113篇
  1990年   72篇
  1989年   74篇
  1988年   53篇
  1987年   66篇
  1986年   37篇
  1985年   39篇
  1984年   33篇
  1983年   33篇
  1982年   25篇
  1981年   17篇
  1980年   20篇
  1979年   21篇
  1978年   17篇
  1976年   17篇
  1972年   257篇
  1971年   288篇
  1965年   16篇
  1962年   25篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
A wide number of pesticides, including highly persistent organochlorine compounds, such as lindane (γ-Hexachlorocyclohexane), have deteriorative effect on fauna and flora by inducing oxidative stress. Lindane induces cell damage by producing free radicals and reactive oxygen species. Quercetin, a dietary flavonoid, is ubiquitous in fruits and vegetables and plays an important role in human health by virtue of its antioxidant function. In this study the flavonoid quercetin was used to investigate its antioxidative effect against lindane induced oxidative stress in rats. The level of lipid peroxidation, reduced glutathione (GSH) were analysed in addition to the antioxidant enzymes such as catalase (CAT), glutathione peroxidase (GPx), superoxide dismutase (SOD) and glutathione-s-transferase (GST) activities in the liver and kidney tissue. Levels of hepatic marker enzymes in serum like Aspartate transaminase (AST), Alanine transaminase (ALT), Alkaline phosphatase (ALP) and Lactate dehydrogenase (LDH) and renal markers like serum creatinine and serum urea were estimated. Administration of Lindane induced histopathological alterations and increased levels of serum hepatic and renal markers and malondialdehyde (MDA) with a significant decrease in GSH content and CAT, SOD, GPx and GST activities. Cotreatment of quercetin along with lindane significantly decreased the lindane induced alteration in histology, serum hepatic and renal markers and MDA and also improved the cellular antioxidant status. The results show that Quercetin ameliorates Lindane induced oxidative stress in liver and kidney. The quercetin exhibited chemopreventive effect when administered along with lindane.  相似文献   
992.
Synthesis by Ribosomes of Viral Coat Protein containing Ester Linkages   总被引:4,自引:0,他引:4  
Peptidyl transferase, the ribosomal enzyme which synthesizes peptide bonds, can catalyse the formation of ester linkages between α-hydroxyacyl-tRNA and aminoacyl-tRNA when ribosomes carry out a specific translation of phage R17 RNA in vitro.  相似文献   
993.
A histological study was undertaken to clarify seasonal changes in the spermatogenic epithelium of Japanese macaques. Testicular tissue samples were excised by biopsies from five adult laboratory-maintained males in mating and non-mating seasons. The samples were fixed with Bouin's solution, embedded in paraffin, and stained with PAS and hematoxylin. Microscopic observations on cross-sections of seminiferous tubules revealed that the seminiferous epithelium in the mating season was thicker than in the non-mating season. PAS-stained granules were found in some of the dark A-type spermatogonia, which significantly increased in the non-mating season. Spermatids of the steps preceding the appearance of the acrosomic cap in stages I to III were observed significantly more often than those in the step coinciding with the formation of the acrosomic cap in stage IV. In stage I, the ratio of mature spermatids or spermatozoa to immature spermatids in the mating season was higher than that in the non-mating season. These findings suggest that spermiogenesis, as well as spermatocytogenesis, is inhibited in the non-mating season.  相似文献   
994.
The physical-chemical and carbohydrate binding specificity ofGriffonia simplicifolia I (GS I) isolectins, one of the 4 lectins isolated fromGriffonia simplicifolia seeds, are described.Association constants for the binding of methyl α- and β-D-galactopyranoside and methyl 2-acetamido-2-deoxy-α-D-galactopyranoside to the A4, A2 B2 and B4 isolectins are reported.Precipitation reactions of theGriffonia simplicifolia isolectins with guaran and type B blood group substance are described.The hypothesis that subunit B is a precursor of subunit A, a process involving proteolytic cleavage of the B subunit, was tested by conducting structural studies on the 2 subunits. The results indicated that the A and B subunits are probably products of 2 separate but closely related, possibly contiguous genes.  相似文献   
995.
The non-bilayer lipid monogalactosyldiacylglycerol (MGDG) is the most abundant type of lipid in the thylakoid membrane and plays an important role in regulating the structure and function of photosynthetic membrane proteins. In this study, we have reconstituted the isolated major light-harvesting complexes of photosystem II (PSII) (LHCIIb) and a preparation consisting of PSII core complexes and minor LHCII of PSII (PSIICC) into liposomes that consisted of digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG), with or without MGDG. Transmission electron microscopy and freeze-fracture studies showed unilamellar proteoliposomes, and demonstrated that most of the MGDG is incorporated into bilayer structures. The impact of MGDG on the functional interaction between LHCIIb and PSIICC was investigated by low temperature (77 K) fluorescence emission spectra and the photochemical activity of PSII. The additional incorporation of LHCIIb into liposomes containing PSIICC markedly increased oxygen evolution of PSIICC. Excitation at 480 nm of chlorophyll (Chl) b in LHCIIb stimulated a characteristic fluorescence emission of the Chl a in PSII (684.2 nm), rather than that of the Chl a in LHCIIb (680 nm) in the LHCIIb–PSIICC proteoliposomes, which indicated that the energy was transferred from LHCIIb to PSIICC in liposome membranes. Increasing the percentage of MGDG in the PSIICC–LHCIIb proteoliposomes enhanced the photochemical activity of PSII, due to a more efficient energy transfer from LHCIIb to PSIICC and, thus, an enlarged antenna cross section of PSII.  相似文献   
996.
An hydroponic culture was conducted to investigate the effect of saline stress on the essential oil and fatty acid composition of Tunisian coriander (Coriandrum sativum L.) roots. Ten days old coriander seedlings were treated during 3 weeks with different NaCl concentrations (0, 25, 50 and 75 mM). Roots volatile components and fatty acids were analyzed. The essential oil yield was 0.06% in the control, on the basis of dry matter weight, and did not changed at low concentration (25 mM), while it increased significantly with increasing NaCl concentrations to reach 0.12 and 0.21% at 50 and 75 mM NaCl, respectively. The major volatile component was (E)-2-dodecenal with 52% of total essential oil constituents, followed by decanal, dodecanal, (E)-2-tridecenal and (E)-2-dodecenal. Further, the amount of these compounds was affected differently by the NaCl level. Total fatty acid amount of coriander roots increased significantly only with 50 and 75 mM NaCl. Three major fatty acids: linoleic (43%), oleic (25.5%) and palmitic (21.6%) were identified. Linoleic acid amount remains unchanged at 25 mM, while it increased with raising NaCl concentrations. However, oleic acid amount decreased only at 25 mM and no effect was observed at 50 and 75 mM. Fatty acid percentages were differently affected by salt. The oleic/linoleic ratio was reduced with raising NaCl concentrations.  相似文献   
997.
The protective properties of a prenylated coumarin, umbelliprenin (UMB), on the human lymphocytes DNA lesions were tested. Lymphocytes were isolated from blood samples taken from healthy volunteers. DNA breaks and resistance to H2O2-induced damage were measured using a single-cell microgel electrophoresis technique under alkaline conditions (comet assay). Human lymphocytes were incubated in UMB (10, 25, 50, 100, 200, and 400 μM) alone or a combination of different concentrations of UMB (10, 25, 50, 100, 200, and 400 μM) and 25 μM H2O2. Untreated cells, ascorbic acid (AA; 25, 50, 100, 200, and 400 μM) and H2O2 (25 μM) were considered as negative control, positive control, and the standard antioxidant agent for our study, respectively. Single cells were analyzed with “TriTek Cometscore version 1.5” software. The DNA damage was expressed as percent tail DNA. UMB exhibited a concentration-dependent increase in protection activity against DNA damage induced by 25 μM H2O2 (from 67.28% to 39.17%). The antigenotoxic activity of AA, in the range 0–50 μM, was greater than that of UMB. However, no significant difference (p > 0.05) in the protective activity was found between UMB and AA at concentrations of approximately higher than 50 μM.  相似文献   
998.
Artificial cross between two genetically different populations of Japanese Misgurnus loach was made to examine the reproductive capacity of the artificial inter-populational hybrid females. Ploidy status and microsatellite genotypes of the eggs laid by these hybrids were inferred from those determined in progenies developed by normal fertilization with haploid loach sperm, induced gynogenesis with UV-irradiated goldfish sperm and/or hybridization with intact goldfish sperm. Some hybrid females laid unreduced diploid eggs genetically identical to the mother. However, these diploid eggs could not develop by spontaneous gynogenesis, but grow to triploid by incorporation of a sperm nucleus. Other hybrid females laid haploid eggs together with diploid eggs and/or various aneuploid and polyploid eggs. Thus, a disruption of normal meiosis occurred in inter-populational hybrid females. The results suggested that the two populations should be so distant as to give rise to atypical formation of unreduced and other unusual eggs in their hybrids.  相似文献   
999.
Previous sociobiological studies of tarsiers were invariably based on field observations. Sulawesi tarsiers are known for monogamous or facultative polygynous social mating systems, but, to date, no data exist to describe the genetic mating system. We here bring together behavioral studies and molecular tools for the first time to elucidate mating behavior and kinship within a tarsier population. We investigated the social system of the recently described Tarsius lariang, which researchers have never studied before. Between September and November 2005, we conducted field observations and sampling in central Sulawesi, Indonesia, where this species is endemic. Ten of 11 social groups of the focal population were composed of 1 adult male, 1 adult female, and putative offspring. To enlighten genetic relationships, we used 12 microsatellite loci and mitochondrial DNA sequences of 26 captured and sampled Lariang tarsiers for parentage and relatedness analyses. A significant number of young were the offspring of the sampled group adults, suggesting a predominantly monogamous social and genetic mating system. There is evidence for extrapair young in groups in which adult pairs exhibit close relationships, leading to the assumption that extrapair mating is solicited to avoid inbreeding. Ten of 11 social groups lived in monogamous social systems, indicating monogamy to be the rule in Tarsius lariang.  相似文献   
1000.
Salmonella typhimurium and Campylobacter jejuni pose significant risks to human health and poultry are a major vector for infection. Comparative in vivo infection models were performed to compare the avian host immune response to both bacterial species. Forty-five commercial broiler chickens were orally challenged with either C. jejuni or S. typhimurium whilst 60 similar control birds were mock challenged in parallel. Birds were sacrificed at 0, 6, 20 and 48 h post-infection and cloacal swabs, blood and tissue samples taken. Peripheral blood leukocytes were isolated for flow cytometric analyses and RNA was extracted for gene expression profiling. Colonisation patterns were markedly different between the two bacterial species, with systemic colonisation of Campylobacter outside the gastrointestinal tract. Salmonella infection induced significant changes in circulating heterophil and monocyte/macrophage populations, whilst Campylobacter infection had no effect on the heterophil numbers but caused a significant early increase in circulating monocytes/macrophages. Toll-like receptor 1 (TLR1) gene expression was decreased, and avian β-defensin (AvBD) gene expression (AvBD3, AvBD10 and AvBD12) was significantly increased in response to Salmonella infection (P < 0.05). In contrast, Campylobacter infection induced increased TLR21 gene expression but significantly reduced expression of seven antimicrobial peptide (AMP) genes (AvBD3, AvBD4, AvBD8, AvBD13, AvBD14, CTHL2 and CTHL3; P < 0.05). Considered together, microbiological, cellular and gene expression profiles indicate that the innate immune system responds differently to Salmonella and to Campylobacter infection. Furthermore, reduction in the expression of AMPs may play a role in the persistence of high level colonisation of the host by Campylobacter. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  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