全文获取类型
收费全文 | 98069篇 |
免费 | 6723篇 |
国内免费 | 24篇 |
专业分类
104816篇 |
出版年
2023年 | 600篇 |
2022年 | 805篇 |
2021年 | 1554篇 |
2020年 | 1261篇 |
2019年 | 1438篇 |
2018年 | 2596篇 |
2017年 | 2372篇 |
2016年 | 3170篇 |
2015年 | 4286篇 |
2014年 | 4507篇 |
2013年 | 6002篇 |
2012年 | 7089篇 |
2011年 | 6549篇 |
2010年 | 4304篇 |
2009年 | 3607篇 |
2008年 | 5291篇 |
2007年 | 5083篇 |
2006年 | 4713篇 |
2005年 | 4122篇 |
2004年 | 4108篇 |
2003年 | 3711篇 |
2002年 | 3397篇 |
2001年 | 2216篇 |
2000年 | 2150篇 |
1999年 | 1765篇 |
1998年 | 997篇 |
1997年 | 718篇 |
1996年 | 734篇 |
1995年 | 685篇 |
1994年 | 577篇 |
1993年 | 560篇 |
1992年 | 988篇 |
1991年 | 880篇 |
1990年 | 804篇 |
1989年 | 775篇 |
1988年 | 747篇 |
1987年 | 697篇 |
1986年 | 616篇 |
1985年 | 634篇 |
1984年 | 621篇 |
1983年 | 500篇 |
1982年 | 457篇 |
1981年 | 365篇 |
1980年 | 366篇 |
1979年 | 431篇 |
1978年 | 403篇 |
1975年 | 378篇 |
1974年 | 423篇 |
1973年 | 417篇 |
1972年 | 338篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
From the roots of Ferula loscosii the coumarins umbelliprenin, coladin and coladonin and the new natural isovaleryl derivative of the latter have been isolated. A study which confirms the structure of coladonin and its difference from farnesiferol A is also given. 相似文献
82.
Fabián Michelangeli Dalia M. Sulcas Marie-Christine Ruiz 《Cell and tissue research》1987,250(2):413-419
Summary This study is concerned with electron-microscopic observations on endocrine or paracrine cells in the fundic gastric mucosa of the bullfrog. Also, an attempt was made to identify the histamine-releasing cells involved in the secretagogue response. At least three distinct endocrine-like cell types were found. The classification is based on the appearance of secretory granules and other organelles, and the relationship of endocrine-like cells with other cells in the tissue. The amphibian endocrine-like cells resemble the ECL, D and EC cells of mammals. Type-I (ECL) cells showed degranulation after repeated stimulation with tetragastrin (TG), acetylcholine (ACh) and K+ depolarizing solution, all of which release histamine. 相似文献
83.
84.
Production of vanillic acid from vanillin by resting cells of Serratia marcescens. 总被引:2,自引:2,他引:0 下载免费PDF全文
Resting-cell suspensions of Serratia marcescens were able to convert, quantitatively, 0.3% vanillin to vanillic acid. The vanillic acid-producing activity reached a maximum after 28 h of incubation with 0.01% vanillin as an inducer. 相似文献
85.
Using gas chromatography, the authors studied the development of the proportion of 25 saturated and non-saturated fatty acids in the plasma of Wistar rats of their own breed aged 5, 10, 14 and 25 days and adult (both sexes), and in three age groups (5 and 14 day and adult) they determined the effect of acute altitude hypoxia. Fatty acids were determined as methylesters (FAME) (Base 1978), using the method already described by Smídová et al. (1984a), in mixed blood plasma obtained from decapitated control and experimental animals. Altitude hypoxia, which corresponded to 9 000 m, lasted 20 min. The temperature was kept stable at 25 degrees C. Marked changes occurred during ontogenesis in the proportion of total fatty acids (TFA) in the plasma; the proportion of sigma non-saturated FA fell significantly during maturation, chiefly in favour of monoene FA, with a resultant significant drop in their reciprocal index (sigma saturated/sigma monoene). The proportion of all saturated FA amounted in 5-day-old rats to 54% of the total quantity, but in adult rats to only 40%. Acute hypoxia had a demonstrable effect in all three age categories. In 5-day-old rats it led to significant changes in 7 FA and in adult rats in 10. In both 5-day-old and adult animals it produced an increase in the arachidonic acid level. The results are discussed in relation to previous findings on the effect of the same hypoxia on FA proportion in various parts of the CNS of rats of different ages. 相似文献
86.
87.
R. Brändén T. Nilsson S. Styring 《Biochemical and biophysical research communications》1980,92(4):1297-1305
A sensitive and reliable method to determine the stereochemical composition of 3-phosphoglyceric acid is presented. Results obtained with this method show that 3-phosphoglyceric acid formed in the ribulose-1,5-bisphosphate carboxylase reaction is a mixture of 10% L-3-PGA and 90% D-3-PGA. 相似文献
88.
89.
Genetic analysis of a proposed cis-acting temporal locus ( Adh-3t ), which regulates alcohol dehydrogenase C2 (ADH-C2 ) acitivity in mouse epididymis extracts, among F1 (ddN × BALB/c) × ddN male backcross progeny provided evidence for genetic distinctness between the structural ( Adh-3 ) and temporal ( Adh-3t ) loci on chromosome 3. Genetic analysis also confirmed the close, linkage of Adh-1 (encoding liver and kidney ADH-A2 ) and Adh-3 (encoding stomach ADH-C2 ) to within 0.3 centimorgans on the mouse genome. Evidence is presented for a proposed closely linked cis-acting temporal locus (designated Adh-1t ) for the A2 isozyme (encoded by Adh-1 ) controlling the activity of this enzyme in mouse kidney extracts, but having no apparent affect on liver and intestine ADH-A2 activities. An extensive survey of the distribution of Adh-1, Adh-3 and Adh-3t alleles among 65 strains of mice is reported — with the exception of two Japanese strains (ddN and KF), linkage disequilibrium between Adh-3 and Adh-3t was observed. Sex differences in mouse liver and kidney ADH-A2 activities were observed, with male/female ratios of approximately 0.6 and 3 respectively for these tissue extracts. 相似文献
90.
Summary A quantitative histochemical assay for NADPH-ferrihemoprotein (P450) reductase had been developed. For optimal activity, it is necessary to use a relatively electropositive tetrazolium salt such as neotetrazolium chloride as the final acceptor. The apparentK
m of the reaction is 0.83 mM. Its specificity has been proven in two ways: (i) activity is increased selectively in the pericentral zone of liver from rats treated with phenobarbitone, an inducer of the reductase, though not in liver of rats injected with 3-methylcholanthrene, which induces NAD(P)H dehydrogenase; (ii) it is competitively inhibited by NADP+ (K
i=1.50mm) though unaffected by dicumarol, an inhibitor of NAD(P)H dehydrogenase activity. An NADP+ concentration ten times greater than the substrate concentration inhibits the histochemical reaction and the reaction in a microsomal fraction assayed biochemically to the same degree (70% inhibition). The amount of inhibition is independent of temperature, of the zone of the acinus and of the treatment of the animal.Continuous microdensitometric monitoring of the reaction product as it is formed has shown that the specific reaction is linear with incubation up to 10 min, thus allowing end-point measurements to be used for cytophotometric analysis. 相似文献