全文获取类型
收费全文 | 68321篇 |
免费 | 4559篇 |
国内免费 | 8081篇 |
专业分类
80961篇 |
出版年
2023年 | 1148篇 |
2022年 | 1505篇 |
2021年 | 1721篇 |
2020年 | 1799篇 |
2019年 | 2587篇 |
2018年 | 2263篇 |
2017年 | 1940篇 |
2016年 | 1849篇 |
2015年 | 1765篇 |
2014年 | 3213篇 |
2013年 | 4217篇 |
2012年 | 2644篇 |
2011年 | 3042篇 |
2010年 | 2427篇 |
2009年 | 3081篇 |
2008年 | 3275篇 |
2007年 | 3614篇 |
2006年 | 3281篇 |
2005年 | 2758篇 |
2004年 | 2417篇 |
2003年 | 2347篇 |
2002年 | 2151篇 |
2001年 | 1835篇 |
2000年 | 1560篇 |
1999年 | 1378篇 |
1998年 | 1328篇 |
1997年 | 1225篇 |
1996年 | 1265篇 |
1995年 | 1116篇 |
1994年 | 1115篇 |
1993年 | 973篇 |
1992年 | 1005篇 |
1991年 | 986篇 |
1990年 | 779篇 |
1989年 | 739篇 |
1988年 | 674篇 |
1987年 | 607篇 |
1986年 | 616篇 |
1985年 | 861篇 |
1984年 | 1064篇 |
1983年 | 829篇 |
1982年 | 930篇 |
1981年 | 739篇 |
1980年 | 743篇 |
1979年 | 669篇 |
1978年 | 551篇 |
1977年 | 491篇 |
1976年 | 438篇 |
1974年 | 287篇 |
1973年 | 295篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Abstract: The enzymatic hydrolysis of UDP-galactose in rat and calf brain was studied. The hydrolysis occurs in two steps: The first is the conversion of UDP-galactose to galactose-1-phosphate catalyzed by nucleotide pyrophosphatase (EC 3.6.1.9), and the second is the conversion of the latter to free galactose by alkaline phosphatase (EC 3.1.3.1). The overall conversion has a pH optimum of 9.0, but there is considerable activity at pH 7.4, which is the optimum for UDP-galactose:ceramide galactosyltransferase in the synthesis of cerebrosides. Preparations from cytosol from calf brain cerebellum or stem that were enriched in UDP-galactose hydrolytic activity inhibit cerebroside synthesis under conditions optimal for the synthesis. Microsome-rich and nuclear debris fractions contain the highest apparent specific activity among the subcellular fractions studied. Hydrolysis of UDP-galactose occurs in all areas of brain, brainstem having the highest activity. The apparent specific activity in jimpy mouse brain homogenate is nearly twice as high as in the control brain homogenate. 相似文献
52.
In searching for differentially expressed genes in human uterine leiomyomas (ULs), suppression sub-tractive hybridization was used to construct an UL up-regulated library, which turned out to represent 88genes. After two rounds of screening by reverse Northern analysis, twenty genes were proved to be up-regulated, including seventeen known genes and three genes with unknown function. All these genes werefirstly associated with UL. Three genes with notable difference were selected for Northern confirmationOur results proved the authenticity of the twenty genes. One gene named Phospholipase A2 (PLA2) showedup-regulation in 4/6 of the patients and investigation of tissue distribution indicated that it had obviousexpression in prostate, testis, liver, heart and skeletal muscle. 相似文献
53.
G. Karrer 《Plant Ecology》1985,59(1-3):199-209
Five plant communities contrasting in successional status and human impact from the southern part of the ‘Wienerwald’ (Austria) are analyzed using vegetation relevés, spectra of area types and a newly proposed disjunction quotient. A climax community (Asperulo-Fagetum), a subclimax community (Querco-Carpinetum s.l.), an anthropogenous substitute community (Mesobromion) and two natural, non-climax permanent communities (Euphorbio saxatilis-Pinetum nigrae and Fumano-Stipetum eriocaulis) are recognized. The disjunction quotient is defined as the number of partial (discontinous) areas divided by the size of the total area of distribution of a species. In particular, the average disjunction quotients of the species in the first two communities reflect relatively table environments only slightly influenced by man, with many ancient, stable taxa. These communities are characterized by species with well-delimited, stable distribution areas. The species in the Mesobromion community have very low average disjunction quotients as its component species are widely and continuously distributed and are often promoted by man. In contrast to these communities, the species linked to the natural permanent, non-climax communities of extreme habitats, have high distribution quotients i.e. small, disconinuous areas; this illustrates the relic character of these plant communities and of the eastern edge of the Alps is a whole. Using the highly variable disjunction quotient of all species and communities examined, the concepts of climax and permanent communities (of different origin) are discussed with regard to European conditions. 相似文献
54.
55.
The influence of contact guidance on the orientation of colonies of subcultured vascular smooth muscle cells 总被引:1,自引:0,他引:1
Robert C. Buck 《In vitro cellular & developmental biology. Plant》1982,18(9):783-788
Summary Subcultures of smooth muscle cells derived from rat thoracic aorta were grown on plane plastic substrata and on plastic substrata
having ridges molded in them by a heated, ruled template. The cells were found to have a very high degree of contact guidance
when distributed sparsely on the ridged substrata. When the cell density increased multilayered, elongated colonies formed.
On plane substrata these were irregular, curved, and disposed in all directions. On the ridged substrata, however, the colonies
were straight, evenly spaced, and positioned at right angles to the ridges.
Supported by Grant MT1011 from the Medical Research Council of Canada. 相似文献
56.
57.
DefattedMadhuca butyraceae seeds contain 24% of crude protein and 10.4% of saponins. The solubility ofMadhuca seed proteins was determined in water and NaCl as a function of pH and minimum solubility occurred at pH 4.0. The proteins
consist of three components with S20,w values of 2.2, 9.8 and 15.4. On gel filtration the proteins gave three peaks and on diethylaminoethyl cellulose chromatography
they resolved into two components. Thein vitro digestibility ofMadhuca seed protein was found to be 69% when assayed with a pepsin-pancreatin system. 相似文献
58.
M. Tobler† 《Journal of fish biology》2005,66(3):877-881
Feigning death, a hunting strategy in which a healthy individual acts as if it was dead to trick prey into its reach, is reported for a population of the Central American cichlid Parachromis friedrichsthalii . Possible mechanisms leading to the evolution of such a behaviour are discussed. 相似文献
59.
Abstract: Slices cut from five frozen human brains were dissected into 2-mm cubes and assayed for choline acetyltransferase (ChAT) activity and protein content. A pattern of enrichment of ChAT activity was found ventral to the anterior commissure; this finding is consistent with the location of the enzyme in the cells of the nucleus basalis of Meynert. The region beneath the anterior commissure was the only place a discrete enrichment of activity could be found, and the precise topography of the enrichment was somewhat variable from brain to brain. The results are discussed in the light of recent knowledge concerning the source of the cortical cholinergic innervation. 相似文献
60.
The uptake of two different preparations of99mTechnetium-methylene diphosphonate in fetal rat calvaria is compared. The localization of99mTc after administration of99mTc(Sn)-MDP and99mTc-MDP showed equal distribution in autoradiography. 相似文献