全文获取类型
收费全文 | 45552篇 |
免费 | 4079篇 |
国内免费 | 32篇 |
出版年
2023年 | 182篇 |
2022年 | 340篇 |
2021年 | 696篇 |
2020年 | 438篇 |
2019年 | 554篇 |
2018年 | 705篇 |
2017年 | 609篇 |
2016年 | 1127篇 |
2015年 | 1897篇 |
2014年 | 2049篇 |
2013年 | 2649篇 |
2012年 | 3204篇 |
2011年 | 3268篇 |
2010年 | 2066篇 |
2009年 | 1785篇 |
2008年 | 2674篇 |
2007年 | 2654篇 |
2006年 | 2386篇 |
2005年 | 2445篇 |
2004年 | 2347篇 |
2003年 | 2250篇 |
2002年 | 2196篇 |
2001年 | 537篇 |
2000年 | 430篇 |
1999年 | 535篇 |
1998年 | 620篇 |
1997年 | 472篇 |
1996年 | 417篇 |
1995年 | 404篇 |
1994年 | 367篇 |
1993年 | 357篇 |
1992年 | 396篇 |
1991年 | 341篇 |
1990年 | 283篇 |
1989年 | 282篇 |
1988年 | 284篇 |
1987年 | 256篇 |
1986年 | 243篇 |
1985年 | 310篇 |
1984年 | 325篇 |
1983年 | 262篇 |
1982年 | 356篇 |
1981年 | 279篇 |
1980年 | 257篇 |
1979年 | 193篇 |
1978年 | 226篇 |
1977年 | 213篇 |
1976年 | 183篇 |
1974年 | 192篇 |
1973年 | 203篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Synopsis The most important factor affecting the potential range of 14 non-native fishes in Florida appears to be their lack of tolerance to low temperatures. In this study, temperatures associated with reduction in feeding, cessation of feeding, loss of equilibrium and death were identified by decreasing water temperature 1°C day–1. Fishes tested and their mean lower lethal temperatures were: Astronotus ocellatus (12.9°C), Belonesox belizanus (9.7°C), Cichlasoma bimaculatum (8.9°C), C. cyanoguttatum (5.0°C), C. meeki (10.3°C), C. octofasciatum (8.0°C), C. trimaculatum (10.9°C), Clarias batrachus (9.8°C), Hemichromis bimaculatus (9.5°C), Hypostomus sp. (11.2°C), Tilapia aurea (6.2°C), T. mariae (11.2°C), T. melanotheron (10.3°C) and T. mossambica (9.5°C). These data indicate that temperature is less limiting for these fishes in Florida than was previously recognized.Contribution Number 18, Non-Native Fish Research Laboratory, Florida Game and Fresh Water Fish Commmission, 801 N. W. 40th Street, Boca Raton, FL 33431, U.S.A. 相似文献
992.
A quantitative autoradiographic method for the measurement of local rates of brain protein synthesis
We have developed a new method for measuring local rates of brain protein synthesis in vivo. It combines the intraperitoneal injection of a large dose of low specific activity amino acid with quantitative autoradiography. This method has several advantages: 1) It is ideally suited for young or small animals or where immobilizing an animal is undesirable. 2) The amino acid injection floods amino acid pools so that errors in estimating precursor specific activity, which is especially important in pathological conditions, are minimized. 3) The method provides for the use of a radioautographic internal standard in which valine incorporation is measured directly. Internal standards from experimental animals correct for tissue protein content and self-absorption of radiation in tissue sections which could vary under experimental conditions.A preliminary report of this work was presented at the 11th annual meeting of the Society for Neuroscience, Los Angeles, California, October 18–23, 1981. 相似文献
993.
Leon Milewich Terry S. Hendricks Bettie Sue Masters Rene A. Frenkel Paul C. Mac Donald 《Archives of biochemistry and biophysics》1981,211(2):530-536
The estrogen synthetase present in human placental microsomes appears to be dependent on the cooperative interaction of the reduced cofactors NADPH and NADH for optimal activity. Using steady-state concentrations of either cofactor, it was found that while the estrogen synthetase activity followed hyperbolic saturation kinetics with NADPH (Kmapp = 14 μM), the enzyme followed sigmoidal saturation kinetics when the cofactor was NADH, with the half-maximum velocity attained at a cofactor concentration of 1.1 mm. The maximum velocity obtained with NADPH as the cofactor was greater than with corresponding concentrations of NADH. Estrogen synthetase activity in the presence of NADH was not due to NADPH contamination. NADH, in the presence of small concentrations of NADPH (0.5 to 5 μm), stimulated significantly the rate of estrogen formation from androstenedione by placental microsomes and, in addition, the enzyme saturation kinetics changed from sigmoidal to hyperbolic, thus mimicking the effect of NADPH. Estrogen synthetase activity, measured in the presence of 1 mm NADH, was stimulated in a dose-dependent manner by NADPH (Kmapp = 0.4 μM NADPH) and, when the enzyme was measured in the presence of 5 μm NADPH, the activity was stimulated in a dose-dependent manner by NADH (Kmapp = 45 μM NADH). Estrogen synthetase activity measured in the presence of NADH, without and with NADPH (1 μm) remained linear both with time of incubation for approximately 15 min and with microsomal protein concentration up to 3 mg/ml. The apparent Km of estrogen synthetase for androstenedione, when measured in the presence of NADH, was 1 μm. The synergistic interaction between NADH and NADPH in stimulating placental estrogen synthetase activity observed in vitro may, conceivably, take place in vivo in the intact placenta. 相似文献
994.
Glucosephosphate isomerase (EC 5.3.1.9) of Schistosoma mansoni is inhibited competitively by a number of tetrose, pentose, and hexose phosphates with inhibitor constant (Ki) values in the range of 0.5 to 400 μM. The most potent inhibitor is 5-phospho-d-arabinonate which resembles the cis-enediolate transition state intermediate of the reaction. These analogs were also found to be effective inhibitors of the production of lactate from glucose by suitably supplemented worm homogenates. The rank order of potency of inhibition of glycolysis was inversely related to the magnitudes of the Ki values for glucosephosphate isomerase. These Ki values were similar to those previously reported for mammalian glucosephosphate isomerase, suggesting similarities in the steric and electronic characteristics of the active sites of these isofunctional enzymes. This conclusion was further supported by the observed pH dependence of the inhibition by 5-phospho-d-arabinonate. Although glucosephosphate isomerase is not a rate-limiting enzyme of glycolysis, in the conventional sense, its selective inhibition could be of chemotherapeutic importance, in part because of the accumulation in glycolyzing systems of glucose 6-phosphate which is a potent feedback inhibitor of hexokinase. 相似文献
995.
996.
Morphology and lactose synthesis in tissue culture of mammary alveoli isolated from lactating mice 总被引:1,自引:0,他引:1
Polly R. Cline Paul O. Zamora Howard L. Hosick 《In vitro cellular & developmental biology. Plant》1982,18(8):694-702
Summary Mammary epithelial cells from lactating mice synthesize and secrete lactose in culture and retain many features of their in
vivo morphology if mammary glands are only partially dissociated to alveoli, rather than completely dissociated to single
cells. After 5 d in culture lactose synthesis by alveoli cultured on floating collagen gels is 10 to 20 times higher than
in cultures of single cells on floating collagen gels. Moreover, mammary alveoli in culture retain sensitivity to lactogenic
hormones; the synthesis of lactose by alveoli depends on the continued presence of insulin and either hydrocortisone or prolactin.
In addition, within alveoli the original juxtaposition of constituent epithelial cells is retained, and cells are cuboidal
and have many microvilli and fat droplets. In contrast, alveoli on attached gels flatten and lose their secretory morphology.
These results indicate that the shape of the cells, presence of lactogenic hormones, and maintenance of epithelial:epithelial
cell contacts are required for maintenance of mammary epithelial cell differentiation in culture.
This research was supported by Grants CA-16392 and AG-02909 from the National Institutes of Health and Institutional Grant
IN 119 from the American Cancer Society. 相似文献
997.
998.
999.
Paul A. Andrews Merrill J. Egorin Matthew E. May Nicholas R. Bachur 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1982,227(1):83-91
6-Thioguanine (6TG) and its metabolites were analyzed in human plasma with a reversed-phase high-performance liquid chromatographic method. 6TG and related compounds were extracted from plasma with an equal volume of 2 N perchloric acid at a 50–100% recovery efficiency. The neutralized extracts were chromatographed on a μBondapak C18 column by two separate isocratic conditions. 6TG, 6-thiouric acid, 6-thioxanthine, 6-thioguanosine, and 6-methylthiouric acid were analyzed with 0.01 M sodium acetate, pH 3.5–10% methanol as the mobile phase and 340 nm for detection. 6-Methylthioguanine and three unknown metabolites were separated with acetate—25% methanol and 310 nm detection. One of the unknowns was identified as 6-methylthioguanosine. External standard calibration was used for quantitation. The 6TG detection limit was 0.8 nmol/ml in plasma. 相似文献
1000.