全文获取类型
收费全文 | 9785篇 |
免费 | 246篇 |
国内免费 | 299篇 |
专业分类
10330篇 |
出版年
2023年 | 139篇 |
2022年 | 149篇 |
2021年 | 232篇 |
2020年 | 274篇 |
2019年 | 433篇 |
2018年 | 349篇 |
2017年 | 225篇 |
2016年 | 182篇 |
2015年 | 152篇 |
2014年 | 614篇 |
2013年 | 1075篇 |
2012年 | 378篇 |
2011年 | 589篇 |
2010年 | 387篇 |
2009年 | 443篇 |
2008年 | 388篇 |
2007年 | 432篇 |
2006年 | 465篇 |
2005年 | 403篇 |
2004年 | 327篇 |
2003年 | 270篇 |
2002年 | 244篇 |
2001年 | 83篇 |
2000年 | 80篇 |
1999年 | 81篇 |
1998年 | 82篇 |
1997年 | 68篇 |
1996年 | 79篇 |
1995年 | 62篇 |
1994年 | 65篇 |
1993年 | 85篇 |
1992年 | 52篇 |
1991年 | 50篇 |
1990年 | 37篇 |
1989年 | 38篇 |
1988年 | 34篇 |
1986年 | 31篇 |
1985年 | 101篇 |
1984年 | 182篇 |
1983年 | 105篇 |
1982年 | 119篇 |
1981年 | 121篇 |
1980年 | 107篇 |
1979年 | 78篇 |
1978年 | 67篇 |
1977年 | 75篇 |
1976年 | 67篇 |
1975年 | 63篇 |
1974年 | 55篇 |
1973年 | 55篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Fruits of tomato incorporated [2-14C]mevalonic acid lactone into the steroidal glycoalkaloid α-tomatine. Young fruits showed the greatest alkaloid-synthesizing ability but this decreased as the fruits developed. Analysis of sap exuded from fruit stalks and also application of[4-14C]cholesterol to leaves confirmed that tomatine is not transported into fruits from vegetative organs. Accumulation of this alkaloid in fruits thus appears entirely due to synthesis. Excised fruits of all developmental stages degraded injected [14C]tomatine and rates were directly related to fruit age. The pattern of accumulation/decline in fruit tomatine may be explicable on the basis of changing capacity for synthesis/degradation during development. Label from injected [14C]tomatine was present mainly in chlorophylls and carotenoids where it increased with time as that in tomatine decreased. The significance of the relationship between tomatine disappearance and carotenoid development is briefly discussed. The aglycone tomatidine was not detected in green fruits but a Δ16-5α-pregnenolone-like compound was. 相似文献
32.
A new method which utilizes Procion Red MX 2B amylopectin for the detection of α-amylase in crude plant extracts is described. The substrate is specific only against α-amylase hydrolysis and β-amylase does not attack it. Paper containing Procion Red MX 2B amylopectin applied to gels after isoelectric focusing reveals α-amylase isoenzymes as white bands. When this technique is used, heat-inactivation of β-amylase is not required. 相似文献
33.
34.
In vitro induction and expression of interleukin 2 receptor in a clonal T helper cell differentiation model 总被引:1,自引:0,他引:1
Induction and expression of interleukin 2 (IL 2) receptor have been studied using a poly( Glu60 Ala30 Tyr10 ) (GAT)-specific T cell clone of mouse origin. This clone (52-3) has been characterized and it exhibits functional properties of T helper (TH) cells: it leads to a specific anti-DNP response in the presence of DNP-GAT and DNP-primed B cells and it secretes biological activities which can induce polyclonal B cell proliferation and IgM secretion. In vitro this clone mimics the activation stages of normal T lymphocytes and can be obtained under two states of differentiation. depending on the antigen-specific signal provided by antigen-presenting cells (APC). The expression of IL 2 receptor by this clone has been studied by (i) its response to IL 2, (ii) its capacity to absorb IL 2 bioactivity, and (iii) its reactivity with monoclonal antibody 7D4 specific for mouse IL 2 receptor. All the results indicate that the unstimulated state does not express the IL 2 receptor while the activated state does. Clone 52-3 has been compared with clone 14-1.6 that derives from a TH cell line and expresses the IL 2 receptor constitutively. 52-3 offers a good experimental model for studying in vitro, in a clonal TH cell population, the detailed mechanism of IL 2 receptor induction. 相似文献
35.
Charles N. Serhan Mats Hamberg Bengt Samuelsson 《Biochemical and biophysical research communications》1984,118(3):943-949
Addition of 15L-hydroperoxy-5,8,11,13-eicosatetraenoic acid (15-HPETE) to human leukocytes led to the formation of a novel series of compounds containing four conjugated double bonds. The yield of tetraenes was increased approx. 100-fold when ionophore A23187 (5 μM) was added simultaneously with 15-HPETE. The structure of the major tetraene was established by physical methods as well as by chemical degradation and found to be 5,6,15L-trihydroxy-7,9,11,13-eicosatetraenoic acid. 相似文献
36.
P Midoux J P Grivet F Delmotte M Monsigny 《Biochemical and biophysical research communications》1984,119(2):603-611
The synthesis of N-acetyl- and N-trifluoroacetyl-glucosaminides was reported. The interaction of these compounds with wheat germ agglutinin, a plant lectin specific for N-acetyl-glucosamine and sialic acid, was investigated by two complementary approaches: 1H and 19F NMR, and fluorescence spectroscopy. This last technique relies on the existence of a competitive equilibrium involving the protein, the ligand and O-(methylumbelliferyl)-N-acetyl-glucosaminide, a fluorescent saccharide. The binding constants and the chemical shifts in the complex were determined and were related to the protein structure. 相似文献
37.
Phytochrome (120 kdalton or 60 kdalton) was isolated from etiolated seedlings of Avena sativa L. cv. Pirol (Baywa München). Irradiation with red light of the Pr form at −23°C in aqueous medium or at −40°C in 66% glycerol leads to the intermediate meta-Rb. Acidification of the glycerol solution at −40°C leads to the absorption of the 15(E) phytochrome chromophore (= Pfr chromophore). Subsequent irradiation transforms this into the 15(Z) chromophore (= Pr chromophore). The presence of the 15(E) chromophore was demonstrated by the same methods also in phytochrome bleached either as Pfr in the dark by 4 M urea, methanol, acetone, ethylene glycol, 8-anilinonaphthalene-1-sulfonate, or as Pr by irradiation with red light in the presence of the same agents. Phytochrome bleached by sodium dodecylsulfate or by dehydration was also investigated. It was concluded that bleached phytochrome contains the Pfr chromophore without specific interaction with the protein. 相似文献
38.
Inhibition of ornithine decarboxylase induces embryonal carcinoma cell differentiation 总被引:2,自引:0,他引:2
J Schindler M Kelly P P McCann 《Biochemical and biophysical research communications》1983,114(1):410-417
Murine embryonal carcinoma cells can be induced to differentiate in vitro by various physical and chemical means. We report here that inhibition of ornithine decarboxylase activity with a specific enzyme-activated inhibitor, alpha-difluoromethylornithine, can induce differentiation in embryonal carcinoma cells. The differentiated phenotype can be distinguished from undifferentiated embryonal carcinoma cells by altered cellular morphology, biochemical and cell surface antigenic properties. These results suggest that alterations in the levels of cellular polyamines may play a role in embryonal carcinoma cell differentiation. 相似文献
39.
Isolation and characterization of a human interleukin 2 gene 总被引:1,自引:0,他引:1
S Mita S Maeda K Obaru N Nishino K Shimada T Hirano K Onoue T Ogawa H Ogawa 《Biochemical and biophysical research communications》1983,117(1):114-121
An interleukin 2 (IL-2) gene was isolated from a Charon 4A human gene library. Electron microscopic examination of 15 heteroduplexes formed between the genomic DNAs and the IL-2 cDNAs demonstrated that the size of the IL-2 gene is about 5.1 +/- 0.5 kb and that there are at least two introns in this gene. Nucleotide sequence of the 5' flanking region of the IL-2 gene showed a homology with that of the corresponding region of the human immune interferon gene. 相似文献
40.
Gideon Bach Elizabeth F. Neufeld 《Biochemical and biophysical research communications》1983,112(1):198-205
The biosynthesis of arylsulfatase A was studied in cultured fibroblasts by pulse-chase labeling with [2-3H]mannose; the enzyme was isolated by immunoprecipitation and denaturing polyacrylamide gel electrophoresis. In normal fibroblasts, and in fibroblasts from a patient with multiple sulfatase deficiency, the enzyme was synthesized as a glycoprotein of apparent molecular weight of 59,000; half of it was processed over a period of 4 days to Mr= 57,000. The precursor chain of Mr= 59,000 was secreted in the presence of 10 mM NH4Cl. An immunoprecipitable glycoprotein of normal size was synthesized by fibroblasts from two unrelated patients with metachromatic leukodystrophy, but this material disappeared within twenty hours. In fibroblasts from an individual with pseudodeficiency of arylsulfatase A, the immunoprecipitable precursor glycoprotein was smaller (Mr= 56,000). The synthesis of cross-reactive proteins with altered properties supports the concept of allelic mutations as the genetic basis of metachromatic leukodystrophy and of arylsulfatase A pseudodeficiency. 相似文献