全文获取类型
收费全文 | 613921篇 |
免费 | 67002篇 |
国内免费 | 979篇 |
出版年
2018年 | 15279篇 |
2017年 | 13703篇 |
2016年 | 12555篇 |
2015年 | 8464篇 |
2014年 | 9723篇 |
2013年 | 13621篇 |
2012年 | 18828篇 |
2011年 | 26989篇 |
2010年 | 21057篇 |
2009年 | 16989篇 |
2008年 | 22417篇 |
2007年 | 24527篇 |
2006年 | 13272篇 |
2005年 | 13152篇 |
2004年 | 13484篇 |
2003年 | 13110篇 |
2002年 | 12428篇 |
2001年 | 22911篇 |
2000年 | 23343篇 |
1999年 | 18611篇 |
1998年 | 6653篇 |
1997年 | 7067篇 |
1996年 | 6843篇 |
1995年 | 6574篇 |
1994年 | 6553篇 |
1993年 | 6498篇 |
1992年 | 16255篇 |
1991年 | 16154篇 |
1990年 | 15803篇 |
1989年 | 15372篇 |
1988年 | 14371篇 |
1987年 | 13814篇 |
1986年 | 12914篇 |
1985年 | 13058篇 |
1984年 | 10969篇 |
1983年 | 9707篇 |
1982年 | 7473篇 |
1981年 | 7007篇 |
1980年 | 6585篇 |
1979年 | 10960篇 |
1978年 | 8521篇 |
1977年 | 7847篇 |
1976年 | 7454篇 |
1975年 | 8264篇 |
1974年 | 8558篇 |
1973年 | 8399篇 |
1972年 | 8174篇 |
1971年 | 7173篇 |
1970年 | 6055篇 |
1969年 | 5713篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
Two color light scattering identifies physical differences between lymphocyte subpopulations 总被引:1,自引:0,他引:1
Forward angle light scattering of two different wavelengths by cells in a flow cytometer was used to investigate physical differences between lymphocytes of different lineage, functional subclass and developmental stage. Correlation of the ultraviolet (UV: 351 nm and 364 nm) and 488 nm light scattering signals produced by lymphoid cells demonstrated that the two signals were not equivalent and that they placed different emphasis on the physical parameters characterizing lymphocytes. Both small T and B lymphocytes from peripheral lymphoid tissues and mitogenically activated large T and B lymphocyte blasts were discriminated by both wavelengths. Differences between the Lyt-2 negative and Lyt-2 positive T lymphocyte subsets were also apparent. Two color light scattering could also discriminate between immature thymocytes and mature peripheral T cells and between small bone marrow cells and mature peripheral B cells. In bone marrow an increase in UV light scattering coincided with the appearance of cell surface immunoglobulin on small cells. These data establish that two color light scattering is a sensitive probe for distinguishing cells of apparently similar morphology and that it can be used to study the physical changes that occur during lymphoid cell differentiation. 相似文献
73.
Isolation of zymogen granules from rat pancreas and characterization of their membrane proteins 总被引:3,自引:0,他引:3
A zymogen granule fraction has been isolated from rat pancreas, and its purity has been assessed by biochemical and morphological criteria. Specific activities of two marker enzymes, amylase and chymotrypsin, are increased by 4.6 and 5.4-fold, respectively, as compared to the homogenate. The purified fraction is devoid of detectable RNA, DNA and 5'-nucleotidase, glucose-6-phosphatase, and cytochrome c oxidase activities. Electron micrographs confirm the absence of mitochondria, lysosomes, and rough endoplasmic reticulum fragments. Zymogen granule membranes were isolated from this fraction on a sucrose gradient following lysis in alkaline buffer. Secretory contaminants were efficiently removed from the membranes as indicated by experiments in which labeled secretory proteins were added during the isolation procedure and secondly by measuring residual levels of amylase and chymotrypsin. Three enzyme activities were found in the membranes: thiamine pyrophosphatase, ATP-diphosphohydrolase, and low levels of acid phosphatase. Membrane proteins were solubilized by urea-Triton X-100 and separated in double-dimension (isoelectric focusing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis). Isoelectric point and molecular weight of each protein band were determined. 相似文献
74.
The absence of centromeric bands in the karyotype of Felis catus is confirmed. It is also confirmed that no satellite band is visible in CsCl density gradients. However, a satellite is observed both by recentrifuging the fraction of the DNA that bands at high density in CsCl and by using netropsin to enhance the resolution of a CsCl gradient containing total F. catus DNA. The satellite, about 0.5% of total DNA, was isolated by repeated centrifugation in CsCl alone and in CsCl with netropsin. Netropsin was removed and a pure satellite DNA obtained. The reassociation kinetics (C0t1/2 less than 10(-3) M . s) show that the satellite is of the simple sequence type and hence a candidate for centromeric heterochromatin. Its cytological localisation awaits in situ hybridisation experiments. 相似文献
75.
Several synthetic 7-hydroxyflavylium salts related to apigeninidin, a natural 3-deoxyanthocyanidin, have been studied in the Ames mutagenicity test using strain TA1537 of Salmonella typhimurium. Under the neutral pH conditions of the test, these flavylium salts are deprotonated through ionization of the C7-OH (pKa′ = 4.2–4.4) to form quinone methides. Only the quinone methides of 4-methyl-7-hydroxyflavylium chloride and 4′-methoxy-4-methyl-7-hydroxy-flavylium chloride showed mutagenicity. Responses of 4–8 times the background were observed at the higher doses (1000 μg/plate), both with and without metabolic activation. It was concluded that the induction of frameshift mutagenicity by this group of compounds is caused by those quinone methides that have non-ionic, stable polycyclic structures at neutral pH. 相似文献
76.
77.
A. Knob C. R. F. Terrasan E. C. Carmona 《World journal of microbiology & biotechnology》2010,26(3):389-407
β-Xylosidases are hydrolytic enzymes which play an important role in xylan degradation, hydrolyzing xylobiose and xylooligosaccharides to xylose from the non-reducing end. Filamentous fungi are particularly interesting producers of this enzyme from an industrial point of view, due to the fact that they secrete β-xylosidases into the medium. Besides, fungal β-xylosidases are highly advantageous for their elevated activity levels and specificity. Interest in xylanolytic enzymes has been increasing, for their possible application in many biotechnological processes. This fact has driven the isolation, purification and characterization of several β-xylosidases. In this review, the mechanisms of action, substrate specificities, physicochemical characteristics, regulation at molecular level, molecular cloning and classification of filamentous fungal β-xylosidases are described. The potential industrial applications of fungal β-xylosidases will also be presented. 相似文献
78.
Rice fields are an alternative habitat for waterbirds and provide food and shelter for many avian species, but there is a lack of information about how the use of rice fields translates into population level effects. The aim of this study was to test the relationship between the use of rice fields by European waterbirds and trends in their populations. We tested this relationship during the autumn migration season and during the breeding season. Based on counts conducted over the last 23 years in natural marshes and areas of rice fields in Doñana (SW Spain), an index of rice field use was constructed for 76 bird species, which was then compared to these species’ European population trends obtained from the literature. A positive relationship was found between waterbird population trends and the use of rice fields during autumn migration season. Our study suggests that changes in the Common Agriculture Policy in Europe leading to reductions in areas of rice cultivation may have important effects on waterbirds. The restoration of former marsh areas and the maintenance of rice cultivation would seem to be more environmentally friendly approaches than the use of these areas to grow alternative crops or solar farms. 相似文献
79.
N. V. Ozolina E. V. Kolesnikova V. N. Nurminsky I. S. Nesterkina L. V. Dudareva L. I. Donskaya R. K. Salyaev 《Biochemistry (Moscow) Supplemental Series A: Membrane and Cell Biology》2010,4(3):297-301
The changes in transport activity of tonoplast proton pumps under the influence of exogenous NO donator and modulation of
Ca2+ concentration jointly and separately were investigated at different stages of ontogenesis and under hyperosmotic stress.
The results suggest that both exogenous NO donator and Ca2+ ions can influence the activity of transport processes related to tonoplast and this influence is especially evident in the
period of growth and accumulation of metabolites. Under hyperosmotic stress, H+-pyrophosphatase plays a more important role than H+-ATPase: the activity of the former increases 2.3-fold compared to the control osmotic conditions, whereas the activity of
H+-ATPase is practically unchanged. H+-pyrophosphatase was more responsive to the presence of exogenous NO donator and to variations in Ca2+ concentration. The effects of exogenous NO donator on tonoplast proton pumps depended on the concentration of Ca2+, which apparently can mediate NO action. 相似文献
80.
K. Ferrazzoli Devienne M.S. Gonalves Raddi R. Gomes Coelho W. Vilegas 《Phytomedicine》2005,12(5):378-381
Three naturally occurring isocoumarins (paepalantine, paepalantine 9-O-beta-D-glucopyranoside and paepalantine 9-O-beta-D-allopyranosyl(1 --> 6) glucopyranoside) and two semi-synthetic analogues, 9,10-acylated compound and 9-OH-10-methylated compound, structurally similar to paepalantine, were evaluated for antimicrobial activity using a spectrophotometric microdilution technique. The paepalantine was active against S. aureus, S. epidermidis, and E. faecalis while the other four compounds proved ineffective against all microorganisms tested at concentrations of 500 microg/ml. Variations in phenolic substitution at OH-9 and/or OH-10 in the paepalantine molecule resulted in compounds without antimicrobial activity. A combination of structural features, two phenolic groups as cathecolic system, forms an oxygenated system arrangement that may reflect the potentially antimicrobial properties of paepalantine. 相似文献