全文获取类型
收费全文 | 66213篇 |
免费 | 6104篇 |
国内免费 | 5361篇 |
专业分类
77678篇 |
出版年
2024年 | 140篇 |
2023年 | 643篇 |
2022年 | 1529篇 |
2021年 | 2525篇 |
2020年 | 1685篇 |
2019年 | 2192篇 |
2018年 | 2091篇 |
2017年 | 1695篇 |
2016年 | 2508篇 |
2015年 | 3894篇 |
2014年 | 4472篇 |
2013年 | 4903篇 |
2012年 | 6120篇 |
2011年 | 5668篇 |
2010年 | 3634篇 |
2009年 | 3152篇 |
2008年 | 4003篇 |
2007年 | 3704篇 |
2006年 | 3347篇 |
2005年 | 2962篇 |
2004年 | 2647篇 |
2003年 | 2468篇 |
2002年 | 2188篇 |
2001年 | 1005篇 |
2000年 | 847篇 |
1999年 | 850篇 |
1998年 | 635篇 |
1997年 | 493篇 |
1996年 | 457篇 |
1995年 | 374篇 |
1994年 | 359篇 |
1993年 | 304篇 |
1992年 | 372篇 |
1991年 | 316篇 |
1990年 | 274篇 |
1989年 | 261篇 |
1988年 | 209篇 |
1987年 | 198篇 |
1986年 | 203篇 |
1985年 | 227篇 |
1984年 | 158篇 |
1983年 | 176篇 |
1982年 | 169篇 |
1981年 | 155篇 |
1980年 | 140篇 |
1979年 | 152篇 |
1978年 | 145篇 |
1977年 | 99篇 |
1976年 | 110篇 |
1975年 | 90篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
111.
Glial fibrillary acidic (GFA) protein has been synthesized in an RNA-dependent cell-free system derived from rabbit reticulocytes. The cell-free synthesized product appears to have the same size as GFA protein isolated from bovine spinal cord, thus showing that GFA protein does not undergo detectable proteolytic processing. 相似文献
112.
113.
114.
115.
Citrate-tris(hydroxymethyl)aminomethane-mediated release of outer membrane sections from the cell envelope of a deep-rough (heptose-deficient lipopolysaccharide) strain of Escherichia coli O8. 总被引:11,自引:9,他引:2 下载免费PDF全文
A heptose-deficient lipopolysaccharide strain of Escherichia coli O8, strain F515, was found to release portions of its outer membrane when cells were exposed to 10 mM citrate buffer (pH 2.75) for 30 min and subsequently exposed to 100 mM tris(hydroxymethyl)aminomethane buffer (pH 8.00). The outer membrane component release was found to be composed of protein, lipopolysaccharide, phospholipid (cardiolipin, phosphatidylethanolamine, and phosphatidylglycerol), and alkaline phosphatase. The outer membrane component was released from the cell envelope in the absence of cell lysis, as no glucose-6-phosphate dehydrogenase activity or succinic dehydrogenase activity was detected. Morphologically, the outer membrane component appeared to consist of laminar fragments and vesicles which had an associated alkaline phosphatase activity. 相似文献
116.
117.
E S Kang R E Gates T M Chiang A H Kang 《Biochemical and biophysical research communications》1979,86(3):769-778
Intact adipocytes exhibit ectoprotein kinase activity as reflected by their ability to catalyze the transfer of the terminal phosphate of (γ-32P) ATP to histone added to a cell suspension. This activity is substrate, time and cell number dependent. Lineweaver-Burk plots gave Km and Vmax values for ATP of 5 × 10?5 M and 7.14 pmoles/min/1.5 × 105 cells. Cyclic AMP but not cyclic GMP in μM concentrations stimulates ectoprotein kinase activity. The controlled tryptic digestion of intact cells results in reduction of ectoprotein kinase activity. This activity is not due to leakage of intracellular protein kinases during the preparative procedure nor to penetration of histone into the cells. Additional phosphoproteins not accessible to endogenous protein kinase activity are also localized on the external surface of the intact fat cell. 相似文献
118.
A Genetically Distinct Form of Cyclic Amp Phosphodiesterase Associated with Chromomere 3d4 in DROSOPHILA MELANOGASTER 下载免费PDF全文
Two cyclic AMP phosphodiesterase enzymes (E.C.3.1.4.17) are present in homogenates of adult Drosophila melanogaster. The two enzymes differ from one another in heat stability, affinity for Mg++, Ca++ activation and molecular weight. They do not differ markedly in their affinities for cyclic AMP, and both exhibit anomalous Michaelis-Menten kinetics. The more heat-labile enzyme is controlled in a dosage-dependent manner by chromomere 3D4 of the X chromosome and is absent in flies that are deficient for chromomere 3D4. Chromomere 3D4 is also necessary for the maintenance of normal cAMP levels, for male fertility, and for normal female fertility and oogenesis. The structural gene(s) for the more heat-stable enzyme is located outside of chromomeres 3C12-3D4. Whether 3D4 contains a structural gene, or a regulatory gene necessary for the presence of the labile enzyme, remains to be determined. 相似文献
119.
The 14 and 18 S forms of acetylcholinesterase from the electric organ of Electrophorus electricus were purified by chromatography on an N-methyl-3-aminopyridinium derivative of Affi-Gel 202. a further increase in purity was seen when these forms were separated by density gradient sedimentation subsequent to the affinity step. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate demonstrated that the 14 and 18 S forms were highly purified following these procedures. Using [3H]diisopropyl fluorophosphate labeling and separation of labeled enzyme from unreacted [3H]diisopropyl fluorophosphate by gel filtration, active site numbers of 8.3 and 11.4 were determined for the 14 and 18 S forms, respectively. These numbers compare to 4.2 active sites determined for the 11.8 S globular form of acetylcholinesterase. These results are in accord with a proposed model of two and three tetrameric structures comprising the head groups of the 14 and 18 S forms of electric tissue acetylcholinesterase. 相似文献
120.
Aspartate transcarbamoylase from Escherichia coli is composed of six catalytic (c) and six regulatory (r) polypeptides. We have studied the structure and function of this enzyme using chymotrypsin as a probe. The protease inactivates the isolated catalytic subunit (c3) but has not effects on the native enzyme (c6r6). Under identical conditions, the c3r6 complex is inactivated at a much slower rate than c3. The presence of the substrate analogue succinate together with carbamoyl phosphate reduces substantially the rate of inactivation. Extended exposure to chymotrypsin converts the catalytic subunit into a partially active derivative with a fourfold higher Michaelis constant. This derivative is indistinguishable from the unmodified catalytic subnit in gell electrophoresis under nondenaturing conditions. However, in the presence of sodium dodecyl sulfate, the major fragment in the electropherogram is smaller than that of the intact catalytic polypeptide. The results could be explained by postulating the presence of a chymotrypsin-sensitive peptide bond at or near the active site. Since X-ray crystallographic studies have indicated that the active sites are located in a central cavity, the resistance of the native enzyme towards inactivation may be due to the inability of chymotrypsin to enter this cavity. 相似文献