全文获取类型
收费全文 | 32972篇 |
免费 | 2767篇 |
国内免费 | 1919篇 |
出版年
2024年 | 43篇 |
2023年 | 323篇 |
2022年 | 813篇 |
2021年 | 1435篇 |
2020年 | 914篇 |
2019年 | 1143篇 |
2018年 | 1191篇 |
2017年 | 1066篇 |
2016年 | 1351篇 |
2015年 | 2002篇 |
2014年 | 2274篇 |
2013年 | 2596篇 |
2012年 | 3086篇 |
2011年 | 2835篇 |
2010年 | 1769篇 |
2009年 | 1624篇 |
2008年 | 1837篇 |
2007年 | 1599篇 |
2006年 | 1553篇 |
2005年 | 1275篇 |
2004年 | 1166篇 |
2003年 | 962篇 |
2002年 | 937篇 |
2001年 | 493篇 |
2000年 | 361篇 |
1999年 | 391篇 |
1998年 | 334篇 |
1997年 | 269篇 |
1996年 | 232篇 |
1995年 | 216篇 |
1994年 | 216篇 |
1993年 | 183篇 |
1992年 | 162篇 |
1991年 | 134篇 |
1990年 | 122篇 |
1989年 | 98篇 |
1988年 | 92篇 |
1987年 | 65篇 |
1986年 | 59篇 |
1985年 | 73篇 |
1984年 | 68篇 |
1983年 | 38篇 |
1982年 | 36篇 |
1981年 | 35篇 |
1980年 | 31篇 |
1979年 | 20篇 |
1978年 | 17篇 |
1977年 | 20篇 |
1976年 | 19篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
人肝癌细胞表皮生长因子受体以及佛波酯对它的调度 总被引:1,自引:0,他引:1
Using radioligand binding assay, the presence of epidermal growth factor (EGF) receptors in cells of two human liver cancer cell lines, BEL-7402 and SMMC-7721, was demonstrated. The ligand binding data were analyzed by a computer program. The dissociation constants (KD) of the ligand-receptor binding complex at equilibrium for 7402 and 7721 cells were 1.2 nM and 0.8 nM respectively, and their number of EGF receptors per cell were 6.2 x 10(4) and 2.5 x 10(4) respectively. After the treatment of cells with phorbol 12-myristate 13-acetate (PMA), no change either in the affinity or in the number of EGF receptors was found in 7721 cells. However, in the case of 7402 cells, while the number of receptors, like 7721 cells, remained unchanged, the affinity of EGF receptors displayed a time dependent modulation after PMA treatment. It dropped within the first hour to a KD value of 3.0 nM and then gradually returned to the normal control value at 48 hours or even slightly higher than normal (0.95 nM) at 96 hours of treatment. The modulation or down-regulation of EGF receptors by PMA in 7402 cells was paralleled by the simultaneous inhibition of DNA synthesis in these cells as evidenced from their reduction of 3H-TdR uptake. It is not clear what is the basis for the differences found between 7402 cells and 7721 cells in their number of EGF receptors per cell and their responsiveness to PMA treatment. It might be related to their difference in autocrine secretion of alpha-transforming growth factors.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
102.
103.
104.
Shi Ronglin 《古脊椎动物学报》1989,(2)
<正> This paper describes Mammalian fossils of 18 species discovered at Donghuangzhuang,about 15 km from NE of Qufu county,Shandong Province.The materials were collected bya farmer in 1984 and were sent to The Shandong Provincial Museum for studing.A short fieldinvestigation was made by Wang Jinwen,Sha Yesue(BGMRSD),Han Qingwen(SDM)andthe present author in same year.The age of the fauna is tentatively considered as the late Late Eocene,probably correlatedas the Heti Fauna(Yuanqu Basin).It represents the first occurrence of Late Eocene fossils inthis province. 相似文献
105.
The low ethylene yield in a cell-free ethylene-forming system from olive tree leaves ( Olea europaea L. cv. Picual) was investigated. During the incubation, 1-aminocyclopropane-1-carboxylic acid (ACC) was extensively transformed into 3-hydroxypropyl amide (HPA). Enzyme extract, Mn2+ and oxygen are responsible for this reaction. Horseradish peroxidase (EC 1.11.1.7) can substitute for the enzyme extract in this reaction. HPA formation could be one reason for the poor in vitro conversion efficiency of ACC to ethylene. 相似文献
106.
107.
108.
The specific binding of vasoactive intestinal peptide (VIP) to bovine thyroid plasma membranes is inhibited by guanine nucleotides. Guanosine 5-triphosphate (GTP) and the non-hydrolyzable GTP analogs guanosine 5-,-imidotriphosphate (Gpp(NH)p) and guanosine 5-O-(3-thiotriphosphate) (GTP--S) inhibited markedly the binding of VIP to its receptors. This inhibition was higher with GTP than with Gpp(NH)p and GTP--S and was due to an increase of the rate of dissociation of peptide bound to membranes. Other nucleotides did not show any effect. 相似文献
109.
Electron spin resonance (ESR) measurements provide evidence for the formation of Cr(V) intermediates in the enzymatic reduction of Cr(VI) by glutathione reductase (GSSG-R) in the presence of NADPH, indicating an initial single-electron transfer step in the reduction mechanism. Depending on the pH, at least two different Cr(V) species are generated which are relatively long-lived. In addition, we have detected the hydroxyl (.OH) radical formation during the GSSG-R catalyzed reduction of Cr(VI) by spin trapping, employing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) as spin traps. Superoxide dismutase (SOD) causes only a minor effect on the .OH radical and Cr(V) formation, indicating that the O2- is not significantly involved in the reaction mechanism. Catalase enhances the Cr(V) formation and substantially inhibits the .OH radical formation, indicating the involvement of hydrogen peroxide (H2O2) in the reaction mechanism. Addition of H2O2 suppresses Cr(V) and enhances the .OH radical formation. Measurements involving N-ethylmaleimide show that the Cr(V) species, produced enzymatically by the reduction of Cr(VI) by GSSG-R, react with H2O2 to generate .OH radicals, which might participate in the initiation of Cr(VI) carcinogenicity. 相似文献
110.
A method has been developed for the study of somatostatin (SS) binding to dissociated cells from rat cerebral cortex. Binding of [125I][Tyr11]SS to cells obtained by mechanical dissociation of rat cerebral cortex was dependent on time and temperature, saturable, reversible and highly specific. Under conditions of equilibrium, i.e., 60 min at 25°C, native SS inhibited tracer binding in a dose-dependent manner. The Scatchard analysis of binding data was linear and yielded a dissociation constant of 0.60±0.08 nM with a maximal binding capacity of 160±16 fmol/mg protein. The binding of [125I][Tyr11]SS was specific as shown in experiments on tracer displacement by the native peptide, SS analogues, and unrelated peptides. 相似文献