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   4117篇
  免费   328篇
  2023年   25篇
  2022年   63篇
  2021年   112篇
  2020年   79篇
  2019年   96篇
  2018年   104篇
  2017年   102篇
  2016年   137篇
  2015年   216篇
  2014年   235篇
  2013年   311篇
  2012年   334篇
  2011年   316篇
  2010年   200篇
  2009年   147篇
  2008年   246篇
  2007年   226篇
  2006年   227篇
  2005年   198篇
  2004年   156篇
  2003年   143篇
  2002年   151篇
  2001年   24篇
  2000年   26篇
  1999年   38篇
  1998年   43篇
  1997年   36篇
  1996年   25篇
  1995年   19篇
  1994年   24篇
  1993年   32篇
  1992年   30篇
  1991年   21篇
  1990年   15篇
  1989年   25篇
  1988年   17篇
  1987年   19篇
  1986年   20篇
  1985年   20篇
  1984年   17篇
  1983年   17篇
  1982年   14篇
  1981年   18篇
  1980年   18篇
  1978年   9篇
  1977年   9篇
  1976年   14篇
  1975年   13篇
  1974年   12篇
  1973年   11篇
排序方式: 共有4445条查询结果,搜索用时 265 毫秒
41.
An experimental rodent model was used to demonstrate the viability of the coccoid form of Helicobacter pylori. Concentrated suspensions were prepared for the two different morphologies: at 2 days incubation for the bacillary forms and at 20 days incubation for the “dormant” forms. The strains used for incubation were two fresh isolates from humans with duodenal ulceration, and two collection strains. Five hundred microliters of culture (OD550 = 5 Mc Farland) of Helicobacter pylori with bacillary (2-5×109 CFU/ml) and coccoid (0 CFU/ml) morphology were inoculated intragastrically in BALB/c mice. The gastric mucosa of the mice was colonized by Helicobacter pylori with the administration of fresh bacillary and coccoid cultures and not with the established cultures. Helicobacter pylori was isolated at 1 week after inoculation with the administration of fresh bacillary cultures, while fresh coccoid Helicobacter pylori was recovered in mice stomachs after 2 weeks of inoculation. After colonization, histopathologic changes occurred after 1 month from inoculation; all colonized mice showed a systemic antibody response to Helicobacter pylori. These results support the thesis of the viability of coccoid Helicobacter pylori non-culturable in vitro and confirm that concentrated bacterial suspensions are able to colonize and to produce gastric alterations in this suitable animal model.  相似文献   
42.
Summary The nrtA gene, which has been proposed to be involved in nitrate transport of Synechococcus sp. PCC7942 (Anacystis nidulans R2), was mapped at 3.9 kb upstream of the nitrate reductase gene, narB. Three closely linked genes (designated nrtB, nrtC, and nrtD), which encode proteins of 279, 659, and 274 amino acids, respectively, were found between the nrtA and narB genes. NrtB is a hydrophobic protein having structural similarity to the integral membrane components of bacterial transport systems that are dependent on periplasmic substrate-binding proteins. The N-terminal portion of NrtC (amino acid residues 1–254) and NrtD are 58% identical to each other in their amino acid sequences, and resemble the ATP-binding components of binding protein-dependent transport systems. The C-terminal portion of NrtC is 30% identical to NrtA. Mutants constructed by interrupting each of nrtB and nrtC were unable to grow on nitrate, and the nrtD mutant required high concentration of nitrate for growth. The rate of nitrate-dependent O2 evolution (photosynthetic O2 evolution coupled to nitrate reduction) in wild-type cells measured in the presence of l-methionine d,l-sulfoximine and glycolaldehyde showed a dual-phase relationship with nitrate concentration. It followed saturation kinetics up to 10 mM nitrate (the concentration required for half-saturation = 1 M), and the reaction rate then increased above the saturation level of the first phase as the nitrate concentration increased. The high-affinity phase of nitrate-dependent O2 evolution was absent in the nrtD mutant. The results suggest that there are two independent mechanisms of nitrate uptake and that the nrtB-nrtC-nrtD cluster encodes a high-affinity nitrate transport system.  相似文献   
43.
44.
Acidiphilium aminolytica is proposed for a species of the genusAcidiphilium. Acidiphilium aminolytica can be phenotypically differentiated from all other species of the genusAcidiphilium. The seven strains of this species that have been studied are Gram-negative, aerobic, mesophilic, non-sporeforming, motile, and rod-shaped bacteria. They grow between pH 3.0 and 6.0, but not at pH 6.5. They yield positive results in tests for hippuric acid hydrolysis, catalase and urease production. Oxidase, esculin hydrolysis, and -galactosidase tests are negative. They can used-glucose,d-galactose, inositol, sorbitol,l-lysine,l-glutamate,l-arginine, -alanine,dl-4-aminobutyrate,dl-5-aminovalerate, sperimine, or diaminobutane as a sole carbon source, but cannot use elemental sulfur and ferrous iron as an energy source. The DNA base composition is 58.7–59.2 G+C mol%. The major isoprenoid quinone is ubiquinone with ten isoprene unit (Q-10). The major fatty acid is the C18:1 fatty acid. Two ornithine amide lipids, the C18:1 fatty acid esters of -N-3-hydroxystearylornithyltaurine and -N-3-hydroxystearylornithine, are detected as the polar aminolipid. DNA relatedness between this species and the other species ofAcidiphilium, the generaAcidomonas, andAcidobacterium was 29 to 2%. These results indicate, that this new species should be placed in the genusAcidiphilium. The type strain (strain 101) ofA. aminolytica is JCM 8796.  相似文献   
45.
Mechanism of antiviral activity of 1-β-d -arabinofuranosyl-E-5-(2-bromovinyl)uracil (BV-araU) against the YSR strain of varicella-zoster virus (VZV), which is a mutant derived from the wild YS strain and is completely deficient in viral thymidine kinase (TK), was searched in comparison with antiviral activity of other thymidine analogues, guanosine analogue and thymidylate synthase (TS) inhibitor in human embryo lung fibroblast cells. Thymidine analogues, such as BV-araU, 5-iododeoxyuridine (IUDR), 1-β-d -arabinofuranosylthymine (araT), and guanosine analogue, such as 9-(2-hydroxyethoxymethyl)guanine (ACV), showed higher antiviral activity to the YS strain than to the YSR strain. Though, BV-araU also had the antiviral activity of a microgram level against the YSR strain. In contrast to these results, TS inhibitor, 5-fluorodeoxyuridine (FUDR), had higher antiviral activity to the YSR strain than to the YS strain. Highly synergistic antiviral activities of FUDR to the YS strain and the YSR strain were observed in combination with IUDR, araT, or ACV. However, weakly synergistic or additive inhibition to the YSR strain was shown in combination of BV-araU and FUDR, in spite of highly synergistic effect of this combination to the YS strain. The viral and cellular TS activity was partially inhibited by BV-araU monophosphate, but not by BV-araU. These results indicate that BV-araU is converted into BV-araU monophosphate by cellular TK, and the inhibition of TS activity by BV-araU monophosphate in the YSR strain-infected cells results in the suppression of viral replication.  相似文献   
46.
Seeds of the water plant Trapa natans L. (water chestnut) can germinate in strict anoxia. The seedlings show seminal roots growing upwards while shoot buds remain quiescent until O2 becomes available. Trapa seedlings are highly tolerant to anoxia. The rate of ethanol fermentation was 21.2 μmol (g FW)−1 h−1, while production of lactate was negligible and lower than that of succinate. The seminal root of Trapa compares better to the rice coleoptile rather than to the rice root, both functionally and as to the metabolic response to anoxia. The anaerobic germination of Nuphar luteum L. and Scirpus mucronatus L. was also characterized by a limited developmental program.  相似文献   
47.
Capsiate is a nonpungent capsaicin analog, a recently identified principle of the nonpungent red pepper cultivar CH-19 Sweet. In the present study, we report that 2-wk treatment of capsiate increased metabolic rate and promoted fat oxidation at rest, suggesting that capsiate may prevent obesity. To explain these effects, at least in part, we examined uncoupling proteins (UCPs) and thyroid hormones. UCPs and thyroid hormones play important roles in energy expenditure, the maintenance of body weight, and thermoregulation. Two-week treatment of capsiate increased the levels of UCP1 protein and mRNA in brown adipose tissue and UCP2 mRNA in white adipose tissue. This dose of capsiate did not change serum triiodothyronine or thyroxine levels. A single dose of capsiate temporarily raised both UCP1 mRNA in brown adipose tissue and UCP3 mRNA in skeletal muscle. These results suggest that UCP1 and UCP2 may contribute to the promotion of energy metabolism by capsiate, but that thyroid hormones do not.  相似文献   
48.
The active nitrate transport system of the cyanobacterium Synechococcussp. PCC7942 is encoded by the four genes nrtA, nrtB, nrtC andnrtD. It is essential for the growth of the cyanobacterium atphysiological concentrations of nitrate and has been shown tobe involved in the active transport of nitrite as well. Thededuced amino acid sequences of the NrtB, NrtC and NrtD proteinsindicate that the transporter is a member of the ABC (ATP-bindingcassette) superfamily of active transporters. Among the prokaryoticABC transporters, the cyanobacterial nitrate/nitrite transporteris unique in having a membrane-bound protein NrtA and an NrtA-likeextra domain linked to one of the ATP-binding subunits (C-terminaldomain of NrtC). Molecular biological, biochemical and physiologicalstudies suggest that NrtA is the substrate-binding protein requiredfor the transport of nitrate/nitrite and that the C-terminaldomain of NrtC has a regulatory role. Comparison of the structuresof nitrate transporters from eukaryotic and prokaryotic, photosyntheticand non-photosynthetic organisms indicate that the nrt nitrate/nitritetransporter represents a prokaryotic nitrate transporter distinctfrom the nitrate transporters of eukaryotes. 1Recipient of the JSPP Young Investigator Award, 1994.  相似文献   
49.
De Castro LD 《Bioethics》1995,9(3-4):259-268
Relatively subtle forms of exploitation of human subjects may arise from the inefficiency or incompetence of a researcher, from the existence of a power imbalance between principal and subject, or from the uneven distribution of research risks among various segments of the population. A powerful and knowledgeable person (or institution) may perpetrate the exploitation of an unempowered and ignorant individual even without intending to. There is an ethical burden on the former to protect the interests of the vulnerable. Excessive or insufficient compensation may be exploitative. However, genuine economic imperatives motivating needy volunteers have to be considered. These forms of exploitation should be appreciated in the context of social and cultural factors suggesting that the relationship between researcher and subject cannot properly be appraised as a contractual undertaking. While compliance with pertinent codes and regulations minimises the exploitative potential, they cannot be enforced in a way that does not recognize a society's peculiar characteristics. The experience with some Filipino cultural traits illustrates this point.  相似文献   
50.
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