全文获取类型
收费全文 | 538011篇 |
免费 | 55636篇 |
国内免费 | 436篇 |
专业分类
594083篇 |
出版年
2018年 | 5054篇 |
2017年 | 4876篇 |
2016年 | 6767篇 |
2015年 | 8423篇 |
2014年 | 10076篇 |
2013年 | 14364篇 |
2012年 | 16055篇 |
2011年 | 16707篇 |
2010年 | 11385篇 |
2009年 | 10234篇 |
2008年 | 14730篇 |
2007年 | 15309篇 |
2006年 | 14324篇 |
2005年 | 13774篇 |
2004年 | 13407篇 |
2003年 | 13055篇 |
2002年 | 12911篇 |
2001年 | 27150篇 |
2000年 | 27427篇 |
1999年 | 21139篇 |
1998年 | 6439篇 |
1997年 | 6739篇 |
1996年 | 6516篇 |
1995年 | 6025篇 |
1994年 | 5887篇 |
1993年 | 5671篇 |
1992年 | 17094篇 |
1991年 | 16700篇 |
1990年 | 16334篇 |
1989年 | 15798篇 |
1988年 | 14640篇 |
1987年 | 13698篇 |
1986年 | 12504篇 |
1985年 | 12695篇 |
1984年 | 10122篇 |
1983年 | 8801篇 |
1982年 | 6363篇 |
1981年 | 5732篇 |
1980年 | 5345篇 |
1979年 | 9561篇 |
1978年 | 7275篇 |
1977年 | 6595篇 |
1976年 | 6201篇 |
1975年 | 7161篇 |
1974年 | 7560篇 |
1973年 | 7468篇 |
1972年 | 6751篇 |
1971年 | 6022篇 |
1970年 | 5363篇 |
1969年 | 5019篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
212.
M Feneant-Thibault N Moatti J Maccario A Corriat A Lemonnier 《Biochemical and biophysical research communications》1987,142(1):221-227
Proline uptake was studied in human skin fibroblasts by simultaneous running of kinetic and inhibition experiments on the same cell lines. Two systems for proline uptake were shown: a high-affinity system not inhibited by alpha-(methylamino)isobutyric acid and a low affinity system inhibited by this amino acid (i.e. system A). These results appear to be of interest, firstly because up till now, system A was considered preferable for proline uptake in human fibroblasts, and secondly because they illustrate the need for combined inhibition and kinetic studies of amino acid uptake, especially when the substrate concentration range used and the respective Km of the systems do not allow their detection by kinetic analysis alone. Furthermore, this high-affinity system may have major physiological implications. 相似文献
213.
N D Goldberg T F Walseth J H Stephenson T P Krick G Graff 《The Journal of biological chemistry》1980,255(21):10344-10347
The hydrolysis of cGMP by phosphodiesterase was conducted in [18O]water to determine the site of bond cleavage and the stoichiometry of 18O incorporation into 5'-GMP. Three different forms of phosphodiesterase including a calmodulin-calcium-dependent enzyme in its basal and activated states were examined. The hydrolysis of cGMP catalyzed by each of the forms of phosphodiesterase proceeded with incorporation of 1 18O atom recoverable in the phosphate moiety of each molecule of 5'-GMP generated. No molecular species of phosphate deriving from the 5'-GMP generated containing two or three 18O were detectable. These results indicate that the phosphodiesterase-catalyzed hydrolysis of cGMP proceeds by nucleophilic substitution at phosphorus resulting in P-O bond cleavage. The stoichiometry of 18O incorporation indicates that the reaction proceeds without phosphate-water oxygen exchange when the hydrolytic reaction is catalyzed by diverse forms of phosphodiesterase in the basal or activated state. These considerations of the phosphodiesterase reaction help to establish the validity of monitoring the rate of enzyme-catalyzed hydrolysis of cGMP as a function of the rate of 18O-labeling of the phosphate of 5'-GMP when the reaction proceeds in a medium of predetermined 18O enrichment. 相似文献
214.
C. N. Page 《Plant Systematics and Evolution》1980,135(1-2):71-79
This study surveys the micromorphological surface structure of the leaves of the conifer genusAgathis (Araucariaceae) from throughout the range of the genus (Malaysia to New Zealand and Fiji) as seen with the scanning electron microscope. These data confirm that the vegetative parts ofAgathis are taxonomically highly conservative, but suggest the Melanesian and New Zealand elements to be distinctive from those of the rest of the genus, and from one another. Conclusions are compared with those that have been derived from studies based on other characters. 相似文献
215.
The action of trypsin at 55 degree C and pH 8.3 on pig IgM anti-Salmonella has been compared with the action of pepsin at 37 degree C and pH 4.6. Both processes cause the gradual removal of Fab arms and Cmu2 domains to produce eventually an (Fc)5 fragment. However, during tryptic digestion Fab arms are preferentially removed from the same subunit, whereas peptic digestion causes random removal from any subunit. At intermediate stages of digestion both processes produce partially fragmented molecules which consist of an (Fc)5 portion still attached to limited numbers of Fab arms. Both processes cause a gradual decrease in the ability of molecules to agglutinate Salmonella, but complement fixation by the complexes declines much more rapidly. A stage is reached where molecules having four Fab arms can still agglutinate but there is no complement fixation. However, the remaining arms on the tryptic molecules are distributed in pairs on the same subunit, whereas those on the peptic molecules are distributed randomly. Hence the number of remaining Fab arms, rather than their distribution, appears to be the critical factor which influences biological activity. A possible explanation for this is discussed. 相似文献
216.
Radioimmunoassay has been used to measure levels of wheat-germ agglutinin and barley-germ agglutinin during embryogenesis and germination. The two lectins exhibited similar patterns of accumulation during grain maturation in vivo and both decreased to low levels after imbibition of harvest-ripe grains for 3 d. Precocious germination of immature wheat and barley embryos excised and cultured in vitro could be prevented either by inclusion of abscisic acid or mannitol in the culture medium. Changes in the level of wheat-germ agglutinin induced by in vitro culture depended on the maturation stage of the embryo. No direct correlation was found between application of exogenous abscisic acid and accumulation of the lectin. 相似文献
217.
218.
Ten healthy middle-aged women volunteered for a study to test the effect of lactulose--a synthetic, non-absorbable disaccharide--on the colonic metabolism of bile acids and on bile lipid composition. Lactulose (60 g daily in eight cases, 39 g daily in two) was taken as a proprietary syrup for six weeks, and bile was collected by duodenal intubation before and immediately after six weeks. All subjects showed a fall in the percentage of the 7-alpha-dehydroxylated bile acid deoxycholic acid (mean 28.4 +/- SEM 3.7 to 15.6 +/- 2.4; p less than 0.002) and a rise in the percentage of the primary bile acid chenodeoxycholic acid (mean 33.2 +/- 42.9 +/- 2.9; p less than 0.001). The percentage of cholic acid rose in eight subjects but mean values did not differ significantly. Bile was initially super-saturated with cholesterol in most subjects and became less saturated with cholesterol in all but one (mean saturation index 1.40 +/- 0.11 to 1.19 +/- 0.07; p less these 0.005). These data support the theory colonic bacteria contribute to cholesterol gall-stone formation. 相似文献
219.
Cyclic nucleotide phosphodiesterase from wheat sprouts was isolated and partially purified. The molecular weight of the enzyme is about 83 000. The enzyme activity sharply rises as the inhibiting factors present in the homogenate are separated. The pH optimum of the enzymatic reaction is 4,8. Divalent cations (Mg2+, Mn2+, Cu2+) within the concentration range of 1--5 mM and complexons (EDTA, EGTA) at the concentration of 1 mM do not affect the PDE activity. The temperature optimum for the reaction is 60 degrees. The enzyme hydrolyzes 3' : 5'-AMP, 3' : 5'-GMP and 2':3'-AMP. The Km value for cAMP is 4 . 10(-3) M. The enzyme activity is inhibited by chemical agents possessing the fungicide activity, the strongest effect being exerted by anylate. 相似文献
220.