全文获取类型
收费全文 | 8234篇 |
免费 | 603篇 |
国内免费 | 1篇 |
出版年
2021年 | 63篇 |
2020年 | 44篇 |
2019年 | 50篇 |
2018年 | 111篇 |
2017年 | 112篇 |
2016年 | 196篇 |
2015年 | 326篇 |
2014年 | 386篇 |
2013年 | 388篇 |
2012年 | 551篇 |
2011年 | 535篇 |
2010年 | 348篇 |
2009年 | 263篇 |
2008年 | 426篇 |
2007年 | 393篇 |
2006年 | 354篇 |
2005年 | 359篇 |
2004年 | 353篇 |
2003年 | 313篇 |
2002年 | 314篇 |
2001年 | 223篇 |
2000年 | 278篇 |
1999年 | 194篇 |
1998年 | 96篇 |
1997年 | 62篇 |
1996年 | 78篇 |
1995年 | 54篇 |
1994年 | 62篇 |
1993年 | 52篇 |
1992年 | 121篇 |
1991年 | 123篇 |
1990年 | 120篇 |
1989年 | 74篇 |
1988年 | 104篇 |
1987年 | 87篇 |
1986年 | 82篇 |
1985年 | 83篇 |
1984年 | 77篇 |
1983年 | 58篇 |
1982年 | 56篇 |
1981年 | 51篇 |
1980年 | 39篇 |
1979年 | 69篇 |
1978年 | 43篇 |
1977年 | 56篇 |
1975年 | 51篇 |
1974年 | 36篇 |
1973年 | 48篇 |
1972年 | 37篇 |
1971年 | 43篇 |
排序方式: 共有8838条查询结果,搜索用时 406 毫秒
21.
22.
Crystallization and preliminary x-ray diffraction studies of recombinant human interleukin-1 beta 总被引:3,自引:0,他引:3
Recombinant human interleukin-1 beta has been crystallized into a tetragonal cell. The unit cell constants are a = b = 54.9 A, c = 76.8 A, and alpha = beta = gamma = 90 degrees. The crystals diffract to better than 1.9 A and are suitable for high resolution data collection. The crystallization conditions and general crystal data are presented. 相似文献
23.
InCallithrix, Saguinus, Aotus, andCallicebus other group members than the mother participate in infant care. Differences among these species are obvious in respect to the time of being off any caregiver, and in respect to the carrying position (Aotus andCallicebus). Alloparental care, a basic behaviour pattern in these species, has evolved from the parking behaviour of the prosimians. The infants are «parked» at other group members. 相似文献
24.
Summary The morphology of the tibio-tarsal antenna cleaner (strigilis) of 30 species of Formicidae, 14 species of Mutillidae and 9 species of Tiphiidae was investigated and is described comparatively. In Formicidae, there is a common type of strigilis. The spur has a posterior-dorsal comb and its anterior side is covered with squamae, which usually form a brush. Posteriorly, the basitarsal notch bears a comb and anteriorly specialized paddle-shaped hairs. These characters may be apomorphic for Formicidae. In several Ponerinae and in Myrmecia there is also a velum on the spur. In two species of ants which are strongly parasitic (Teleutomyrmex schneideri and Anergates atratulus ) there are reduced antenna cleaners. Mutillidae (except Myrmosa) have another common type of strigilis: the spur bears a velum with a smooth rim and a clear apex with two rows of teeth. The notch in the basitarsus is usually deeper than that of ants; there is a comb, but no paddle-shaped hairs. The strigilis of Myrmosa females has no velum but there are two prominent rows of teeth on the spur. In the male, the velum is reduced to a slender strip. In Tiphiidae the antenna cleaners show considerable diversity. In Methocha the spur bears a comb but no velum; the spur of Tiphia has a velum with a serrated rim; the spur of Myzinum is equipped with a velum with a smooth rim; in Thynnus the surface of the velum is wrinkled or undulating. An apex (without teeth) is present in all investigated Tiphiidae. The notch of the basitarsus bears a comb except in female Myzinum, where the teeth seem to have fused, thus forming a rim. It is suggested that a velum with a serrated or toothed edge and an apex with two rows of teeth are plesiomorphic for aculeate Hymenoptera. The antenna cleaners in Mutillidae are remarkably similar to those in some bees. This fact is interpreted as partly due to convergence and as partly symplesiomorphic. In several species of ants, there is a specialized cuticular area (bande poreuse) anterior to the comb of the notch, which is characterized by fissures or holes. These are presumably openings of an excretory gland. 相似文献
25.
Summary An analysis was made of the effects of different sampling and extraction techniques on the amounts and pattern of monoterpenoids isolated from needles of Norway spruce. The following isolation and analysis procedure was finally adopted: liquid nitrogen-cooled needles were pulverized by a microdismembrator, extracted with pentane overnight at 2°–3°C and concentrated to a volume not less than 3 ml/g fresh weight on a Vigreux column. The crude extract was injected splitless (with solvent split) onto a cold programmed temperature vaporized (PTV) precolumn of a gas chromatograph and the vaporizable compounds heated to a capillary column. This method was tested for production of artefacts and quantitative extraction and applied to needles of eleven 80-year-old spruce trees. 相似文献
26.
27.
Exposure of mouse peritoneal macrophages to ethanol induces a rapid release of arachidonic acid to the extracellular medium. All major classes of phospholipids, phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol contribute to this release. Ethanol-induced mobilization of arachidonic acid occurs by deacylation, but it is not accompanied by eicosanoid synthesis. These data suggest that at least two signals are necessary for the release and metabolism of arachidonic acid. Ethanol also activates a phospholipase C which hydrolyzes only phosphatidylinositol, and not its phosphorylated derivatives. 相似文献
28.
In order to test the Methanochondrion concept, uptake of adenine nucleotides in various membrane preparations of Methanobacterium thermoautotrophicum was studied. The uptake showed properties which are in general interpreted as indicative of a transport mechanism: (i) kinetics in the time range of minutes, (ii) temperature dependence, (iii) substrate specificity and (iv) failure to remove the substrate by extensive washing.However, nucleotide transport as an interpretation of this uptake can definitely be excluded. Not only an exchange mechanism of the mitochondrial type, but also a general exchange or an uniport mechanism was ruled out. In contrast, the nucleotide uptake was shown to be actually a tight and specific binding of ADP and ATP to binding sites at the interior side of the cell membrane. This was conclusively demonstrated in protoplasts obtained from M. thermoautotrophicum cells. In these protoplasts which do not contain internal membranes also nucleotide binding was observed, but only after disruption of the plasma membrane by osmotic lysis, which leads to the exposure of binding sites. 相似文献
29.
Glucose and amino acid metabolism in 1- and 30-day-old chick telencephalon slices was studied in two incubation media in the presence or in the absence of a continuous oxygenation. Medium 1 has a composition and a tonicity similar to cerebrospinal fluid, medium 2 is hypertonic and does not contain any K+ ions. The incorporation of glucose carbon into amino acids and the distribution of radioactivity between the different amino acids are close to the ones observed in the chick brain in vivo only when the slices are incubated in medium 1, with oxygen at 30 days and without oxygen for the 1-day-old chick. It also appears that if oxygenation is necessary for incubation of mature brain tissue in vitro, the absence of the medium oxygenation is more suitable for the study of glucose metabolism in 1-day-old chick brain slices. 相似文献
30.