全文获取类型
收费全文 | 34958篇 |
免费 | 2856篇 |
国内免费 | 1876篇 |
专业分类
39690篇 |
出版年
2024年 | 68篇 |
2023年 | 389篇 |
2022年 | 932篇 |
2021年 | 1633篇 |
2020年 | 1033篇 |
2019年 | 1249篇 |
2018年 | 1224篇 |
2017年 | 864篇 |
2016年 | 1324篇 |
2015年 | 2022篇 |
2014年 | 2363篇 |
2013年 | 2547篇 |
2012年 | 3082篇 |
2011年 | 2762篇 |
2010年 | 1718篇 |
2009年 | 1428篇 |
2008年 | 1661篇 |
2007年 | 1513篇 |
2006年 | 1350篇 |
2005年 | 1141篇 |
2004年 | 1020篇 |
2003年 | 864篇 |
2002年 | 743篇 |
2001年 | 677篇 |
2000年 | 681篇 |
1999年 | 655篇 |
1998年 | 386篇 |
1997年 | 335篇 |
1996年 | 359篇 |
1995年 | 333篇 |
1994年 | 325篇 |
1993年 | 216篇 |
1992年 | 351篇 |
1991年 | 270篇 |
1990年 | 306篇 |
1989年 | 257篇 |
1988年 | 189篇 |
1987年 | 171篇 |
1986年 | 152篇 |
1985年 | 134篇 |
1984年 | 121篇 |
1983年 | 95篇 |
1982年 | 77篇 |
1981年 | 61篇 |
1979年 | 75篇 |
1978年 | 55篇 |
1977年 | 53篇 |
1975年 | 59篇 |
1974年 | 47篇 |
1973年 | 50篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
Using cytochemical method,microspectrophotometry and image analysis,effects of va-soactive intestinal peptide(VIP)on activities of succinic dehydrogenase(SDH)and alkalinephosphatase(ALP)in rat hepatoma cells were studied in vitro.The results showed that thehepatoma cell expressed potent positive reactions of SDH and ALP,the positive positionswere located at the cell membranes and/or cytoplasm.Having been treated with VIP,ALPdecreased obviously in activity(P<0. 01,compared with hepatoma cells untreated by VIP).The sites of ALP activty were chiefly located at the cell membranes,particularly at the cell-cell contacts.Cultured rat hepatoma cells had intensive SDH activity in their cytoplasm.Compared with untreated eclls,there was no marked difference in the intensity of SDH activ-ity in VIP-treated hepatoma cells(P>0.05). 相似文献
42.
H. T. Huang 《Applied microbiology》1961,9(5):419-424
Two auxotrophic mutants of Escherichia coli have been shown to accumulate significant amounts of l-threonine in the culture medium. One mutant, 13071, is deficient in α,ε-diaminopimelic acid (DAP), and the other, 13070, is deficient in both DAP and methionine. 相似文献
43.
44.
45.
本文利用小鼠大脑机械损伤模型及体外培养的胶质细胞,采用同位素渗入法观察了细胞介素及其抗体对胶质细胞增生的影响。结果表明:体外培养时TNF-α在浓度为10~200u/ml培养液时均能促进胶质细胞增生(P<0.05),IL1-β在浓度为5~200u/ml培养液时能促进胶质细胞增生,TNF-α+IL1-β其促进胶质细胞增生作用更强烈,TNF-α抗体能完全阻断TNF-α的促增生作用,部分阻断TNF-α+IL1-β的促增生作用。在体实验时,IL1-β及TNF-α的作用与离体时相似,IL1-β及TNF-α亦能促进胶质细胞增生,二者共同作用时促细胞增生作用更强。以上结果提示,外源性TNF-α及IL1-β能促进中枢神经损伤后胶质细胞增生且具有协同作用。 相似文献
46.
Blackfoot disease is a peripheral vascular disease resulting in gangrene of the lower extremities. Although extensive epidemiological
study has implicated high arsenic content in artesian well water of the endemic area bears some important connection with
the disease, the etiology of the disease is still not clarified. In this study, attention is paid to chemical speciation of
arsenic in order to find out whether the concentrations of arsenic species in urine of Blackfoot disease patients are different
from those of controls. Experimental results indicate that the total arsenic, inorganic arsenic, monomethylarsonic acid, and
other forms of arsenic in the urine of patients are significantly higher than those of the contols. The possible connection
of those arsenic species with the etiology of the disease is discussed. 相似文献
47.
Jin-Cherng Lien Li-Jiau Huang Jih-Pyang Wang Che-Ming Teng Kuo-Hsiung Lee Sheng-Chu Kuo 《Bioorganic & medicinal chemistry》1997,5(12):2111-2120
A series of 2-substituted 3-chloro-1,4-naphthoquinones was synthesized, and the antiplatelet, antiinflammatory, and antiallergic activities of these compounds were evaluated. The structure-activity relationships in this series were also examined. Most of the 2-alkyl/arylcarboxamido derivatives of 3-chloro-1,4-naphthoquinone showed potent activities with similar trends in each of the activities evaluated. 相似文献
48.
The mechanism of enhancement of Ca2+ uptake by the T cell mitogen concanavalin A (Con A) was studied in murine thymocytes. Native Con A enhanced the rate of Ca2+ uptake as much as 9-fold, an increase being observed within five minutes after Con A addition. The effect of Con A was reversed completely by alpha-methyl mannopyranoside (alpha-MM). Increased Ca2+ uptake was observed with increasing concentrations of Con A, between 2 and 400 microgram/ml, indicating that the stimulation of Ca2+ uptake is not restricted to mitogenic lectin concentrations (0.5-2 microgram/ml). Succinyl Con A exhibits only a slight effect in the same concentration ranges as native Con A. Ca2+ uptake, both in the absence and presence of Con A, is strongly dependent on energy metabolism and is carrier mediated. The augmentation of Ca2+ uptake by native Con A is due to an enhanced Vmax. Uptake of the anion, CrO42-, by thymocytes, found to be a non-saturable process, was also enhanced by Con A. The effect of Con A on CrO42- permeability appears to be independent of its effect on Ca2+ uptake. 相似文献
49.
50.
The DNA of the nonoccluded baculovirus (Hz-1V) obtained from the IMC-Hz-1 cell line was characterized by physicochemical and restriction endonuclease techniques. Hz-1V DNA isolated from purified virus had buoyant densities of 1.58 and 1.54 g/ml in CsCl-ethidium bromide density gradients, which corresponded to supercoiled and to relaxed circular and linear DNA, respectively. Neutral CsCl equilibrium centrifugation indicated that the Hz-1V DNA had a buoyant density of 1.7024 g/ml, which corresponded to a guanine-plus-cytosine (G+C) content of 43%. Thermal denaturation indicated a high G+C domain(s) in the Hz-1V genomic DNA. The domain(s), which included about 11% of the total genomic DNA, exhibited a T(m) of 97 degrees C. The remaining portion (89%) of the DNA had a T(m) of 86.5 degrees C. The T(m)s corresponded to G+C contents of 42 and 67%, respectively. The mean genetic complexity of Hz-1V DNA determined by DNA reassociation kinetic analysis was found to be 152 x 10(6). A possible rapidly reassociating component comprising approximately 13% of the genome was observed. The mean molecular weights from restriction endonuclease digests were 159 x 10(6) for both HindIII and EcoRI. Genomic heterogeneity was found in both the wild-type Hz-1V stock and in two plaque isolates. Of 12 single-plaque isolates, 3 basic restriction endonuclease DNA fragment patterns were observed. The molecular size estimates from electron microscopic contour lengths of uncloned viral DNA ranged from 70 to 158 megadaltons, and the mode was the 130- to 140-megadalton class. 相似文献