全文获取类型
收费全文 | 20774篇 |
免费 | 1022篇 |
国内免费 | 27篇 |
专业分类
21823篇 |
出版年
2024年 | 36篇 |
2023年 | 176篇 |
2022年 | 408篇 |
2021年 | 755篇 |
2020年 | 453篇 |
2019年 | 478篇 |
2018年 | 706篇 |
2017年 | 645篇 |
2016年 | 859篇 |
2015年 | 976篇 |
2014年 | 1219篇 |
2013年 | 1720篇 |
2012年 | 1830篇 |
2011年 | 1616篇 |
2010年 | 927篇 |
2009年 | 824篇 |
2008年 | 950篇 |
2007年 | 924篇 |
2006年 | 782篇 |
2005年 | 692篇 |
2004年 | 568篇 |
2003年 | 478篇 |
2002年 | 412篇 |
2001年 | 348篇 |
2000年 | 310篇 |
1999年 | 284篇 |
1998年 | 118篇 |
1997年 | 92篇 |
1996年 | 99篇 |
1995年 | 92篇 |
1994年 | 68篇 |
1993年 | 63篇 |
1992年 | 177篇 |
1991年 | 162篇 |
1990年 | 134篇 |
1989年 | 102篇 |
1988年 | 147篇 |
1987年 | 115篇 |
1986年 | 103篇 |
1985年 | 102篇 |
1984年 | 102篇 |
1983年 | 70篇 |
1982年 | 52篇 |
1981年 | 60篇 |
1980年 | 50篇 |
1979年 | 73篇 |
1978年 | 45篇 |
1977年 | 55篇 |
1974年 | 41篇 |
1972年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Anabaena sp. grew with mono- and di-ester phosphate compounds as sources of phosphate, indicating the presence of phosphomonoesterase (PMEase) and phosphodiesterase (PDEase) activities. Cell-bound PMEase and PDEase activities were detected during growth in 0.5 and 10 mg PO4l–1 only when the cellular phosphate concentration fell to 0.46% of cell protein and the activities increased as cellular phosphate content decreased. The Km values for these enzymes were 0.3mm forp-nitrophenyl phosphate and 0.2mm for bis-p-nitrophenyl phosphate, respectively. Only PMEase activity was found extracellularly. The pH optima for PMEase and PDEase were 10.2 and 10.4, respectively, and the temperature optima at pH 10.2 were 37°C and 40°C, respectively. Ca2+ increased the enzyme activities while Zn2+ caused marked inhibition. The inorganic phosphate repressed the cellular PMEase activity after a lag of 4 h. 相似文献
92.
93.
94.
Prem Kumar Dantu Sant S. Bhojwan 《Journal of plant biochemistry and biotechnology.》1992,1(2):115-118
Effect of some physical and chemical factors on in vitro shoot multiplication in Gladioius was investigated. A modified MS medium with NH4NO3 reduced to half strength, KH2PO4 replaced with 300 mg/l of NaH2PO4 2 H2O and Kl omitted completely, was found to be better than the original MS. BAP was better than either kinetin or 2iP. Of the various physical factors studied, the propagule size, light and container volume influenced the rate of shoot multiplication. On the modified medium containing 0.5 mg/l BAP in dark, the propagules bearing 20 buds showed 8-fold multiplication. 相似文献
95.
Summary Four strains of Fusarium oxysporum and a strain of Monilia brunnae were screened for their ability to convert cellulosic substrates into ethanol/acetic acid. These strains were found to utilize cellulose and produce extracellular cellulases. However, only F. oxysporum 841 was found to convert glucose, xylose, and cellulose into ethanol and acetic acid as major end-products under microaerobic conditions. Acetic acid at a level of 4.7 g/l resulted in a single-step process on potato pulp medium, indicating the potential of the strain for converting cellulosic substrates into acetic acid.
Offprint requests to: K. Schügerl 相似文献
96.
Mini-mu derivatives carrying plasmid replicons can be used to clone genesin vivo. This method was adopted to generate phasmid clones which were later screened for their ability of restore nitrofurantoin
sensitivity of a nitrofuran-resistant host by eliciting nitroreductase activity. One phasmid-derived clone (pAJ101) resulted
in considerable increase in nitroreductase activity when introduced into a nitrofurantoin-resistant mutant ofEscherichia coli with reduced nitroreductase activity. Subsequently, a 1.8 kb fragment obtained from pAJ101 by partial digestion with 5au3A,
was subcloned into pUC18 to yield pAJ102. The nitroreductase activity attributable to pAJ102 was capable of reducing both
nitrofurantoin and nitrofurazone. The polypeptides encoded by pAJ102 were identified by the minicell method. A large, well-defined
band corresponding to 37 kDa and a smaller, less-defined band corresponding to 35 kDa were detected. Tnl000 mutagenesis was
used to delineate the coding segment of the 1.8 kb insert of pAJ102. A 0.8 kb stretch of DNA was shown to be part of the nitroreductase
gene. The gene was mapped at 19 min on theEscherichia coli linkage map. 相似文献
97.
Regulation of phosphorylation of the c-erbB-2/HER2 gene product by a monoclonal antibody and serum growth factor(s) in human mammary carcinoma cells. 总被引:6,自引:0,他引:6 下载免费PDF全文
Monoclonal antibody (MAb) 4D5 was used to analyze the phosphorylation of p185HER2, the gene product of c-erbB-2/HER2, in SK-BR-3 cells. Culture in the continuous presence of 4D5 reduced the in vivo steady-state levels of p185HER2 phosphorylation by 80% in a dose-dependent manner, suggesting that MAb 4D5 may have interfered with the activation of phosphorylation of p185HER2. The observed MAb-mediated reduction of p185HER2 phosphorylation could not be completely accounted for by down-regulation. When cultures were grown under serum-free conditions, the steady-state levels of p185HER2 phosphorylation were reduced by 56%, and addition of 4D5 further inhibited phosphorylation to 20% of steady-state levels. With continuous exposure to increasing concentrations of newborn calf serum in these cultures, there was a linear increase in tyrosine-specific phosphorylation of p185HER2, reaching a 5.4-fold increase with 10% newborn calf serum. Phosphorylation of p185HER2 in the presence of newborn calf serum was not attributable to stimulation of the epidermal growth factor receptor by epidermal growth factor or by transforming growth factor-alpha. Extension of these observations to two other mammary carcinoma cell lines. MDA-MB-453 and BT-474, also demonstrated a significant capacity of serum to induce p185HER2 phosphorylation. The demonstration of antibody-mediated partial inhibition of phosphorylation under serum-free conditions suggests that mammary carcinoma cells may also produce and secrete a factor or factors which may activate p185HER2. Our observation that growth-inhibitory MAb 4D5 is able to reduce the phosphorylation of p185HER2 by newborn calf serum and by a cellular-derived factor(s) suggests the existence of a growth factor(s) which uses phosphorylation of p185HER2 as a signal transduction pathway to regulate cell proliferation. 相似文献
98.
Sujata Misra Kshudiram Naskar Dwijen Sarkar Dilip Kumar Ghosh 《Molecular and cellular biochemistry》1991,102(1):13-18
Summary
Leishmania donovani, the etiological agent for the disease visceral leishmaniasis, attach themselves to the macrophages for initiation of the disease. The attachment process has been found to be regulated by Ca2+ ions. Verapamil, a Ca2+-channel blocker inhibits Leishmania-macrophage attachment. The inhibitory effect is increased with time. Nifedipine, another Ca2+-channel blocker exhibits the same effect. The attachment process is stimulated by Ca2+-ionophore alone. The inhibitory effects of the calcium channel blockers are reversed by the ionophore. 相似文献
99.
Luminescence of ruthenium(II) polypyridyls: evidence for intercalative binding to Z-DNA. 总被引:3,自引:0,他引:3 下载免费PDF全文
Photophysical studies have been undertaken to characterize the binding interactions of enantiomers of Ru(phen)3(2+), Ru(DIP)3(2+), and racemic Ru(bpy)2dppz2+ (where phen = 1,10-phenanthroline, DIP = 4,7-diphenylphenanthroline, and dppz = dipyridophenazine) with Z-form poly d(GC). Parallel enhancements in steady state luminescent intensity and a lengthening of luminescent lifetimes are seen for ruthenium enantiomers with Z-DNA as for B-DNA but with enantioselectivities reversed. Greater enhancements are seen for delta-isomers with the right-handed helix but for lambda-isomers with the left-handed helix. Ru(bpy)2dppz2+, an avid intercalator in B-DNA, displays no luminescence free in aqueous solution, but luminesces brightly bound to either B- or Z-poly d(GC). Stern-Volmer quenching studies also support the enantioselective preference in binding to B-DNA by delta-isomers and a reversal with binding to Z-DNA preferentially by the lambda-isomers. Steady state polarization studies indicate a rigid association of the complexes with both B- and Z-DNA on the time-scale of their emission and again with symmetrical enantioselectivities for the left and right-handed helices. Given the well characterized intercalative association of the complexes with B-DNA, the parallel results seen here with Z-DNA point strongly to a comparable intercalative association with the Z-form helix. That molecules may interact with Z-DNA through intercalation has not been demonstrated previously and now requires consideration in describing the range of interactions of small molecules and proteins with Z-DNA. 相似文献
100.
The cytopathologic features of two cases of peritoneal endometriosis (secondary to ruptured ovarian endometrial cysts) are described. Both patients presented with abdominal distension and tenderness and were clinically thought to have an abdominal tumor. Preoperative cytologic examination of peritoneal fluids gave a diagnosis of endometriosis in both cases. The endometrial tissue was present in the smears as honeycombs, syncytia and tight clusters of both epithelial and stromal cells. Subsequent surgery confirmed the cytodiagnosis in both cases. These cases emphasize the need to include endometriosis in the differential diagnosis of peritoneal effusions, especially in women. 相似文献