全文获取类型
收费全文 | 1306824篇 |
免费 | 115654篇 |
国内免费 | 1431篇 |
专业分类
1423909篇 |
出版年
2021年 | 17787篇 |
2020年 | 12594篇 |
2019年 | 16152篇 |
2018年 | 17473篇 |
2017年 | 16292篇 |
2016年 | 27934篇 |
2015年 | 42349篇 |
2014年 | 50333篇 |
2013年 | 76625篇 |
2012年 | 35188篇 |
2011年 | 23177篇 |
2010年 | 43089篇 |
2009年 | 44545篇 |
2008年 | 22239篇 |
2007年 | 20222篇 |
2006年 | 25743篇 |
2005年 | 26839篇 |
2004年 | 26270篇 |
2003年 | 23860篇 |
2002年 | 22152篇 |
2001年 | 31290篇 |
2000年 | 28505篇 |
1999年 | 29383篇 |
1998年 | 25169篇 |
1997年 | 24932篇 |
1996年 | 24585篇 |
1995年 | 22748篇 |
1994年 | 22581篇 |
1993年 | 21627篇 |
1992年 | 25947篇 |
1991年 | 24177篇 |
1990年 | 22846篇 |
1989年 | 23749篇 |
1988年 | 21770篇 |
1987年 | 20687篇 |
1986年 | 19583篇 |
1985年 | 21538篇 |
1984年 | 21734篇 |
1983年 | 19292篇 |
1982年 | 20182篇 |
1981年 | 19397篇 |
1980年 | 18149篇 |
1979年 | 18116篇 |
1978年 | 17055篇 |
1977年 | 16545篇 |
1976年 | 15668篇 |
1975年 | 15011篇 |
1974年 | 15420篇 |
1973年 | 15901篇 |
1972年 | 13477篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
971.
972.
973.
C Gerdemann C Eicken A Magrini H E Meyer A Rompel F Spener B Krebs 《Biochimica et biophysica acta》2001,1548(1):94-105
The amino acid sequences of two isozymes of catechol oxidase from sweet potatoes (Ipomoea batatas) were determined by Edman degradation of BrCN cleavage fragments of the native protein and by sequencing of amplified cDNA fragments. Sequence alignment and phylogenetic analysis of plant catechol oxidases revealed about 80% equidistance between the two I. batatas catechol oxidases and approximately 40--60% to catechol oxidases of other plants. When H(2)O(2) was applied as substrate the 39 kDa isozyme, but not the 40 kDa isozyme, showed catalase-like activity. The structure of the 40 kDa isozyme was modeled on the basis of the published crystal structure of the 39 kDa isozyme [T. Klabunde et al., Nat. Struct. Biol. 5 (1998) 1084]. The active site model closely resembled that of the 39 kDa isozyme determined by crystallography, except for a mutation of Thr243 (40 kDa isozyme) to Ile241 (39 kDa isozyme) close to the dimetal center. This residue difference affects the orientation of the Glu238/236 residue, which is thought to be responsible for the catalase-like activity of the 39 kDa isozyme for which a catalytic mechanism is proposed. 相似文献
974.
975.
Human spermatozoa with normal structure and with different axonemal deficiencies (absence of axoneme, of arms, or of central structures) were studied by electron microscopy, SDS-polyacrylamide gel electrophoresis, and ATPase activity measurements. Normal human sperm possess a complement of high molecular weight polypeptides with an electrophoretic migration similar to that of sea urchin and other mammalian sperm dyneins. Human high molecular weight bands are numbered one to four in order of increasing of electrophoretic mobility; all of them are absent in spermatozoa that lack axoneme. The absence of doublet arms, coincides with the absence of bands 2, 3, and 4; the absence of central structures coincides with a reduction in intensity of band 2. In the latter two abnormal conditions, band 1 has an increased intensity. The data are tentatively interpreted by attributing the polypeptides forming bands 3 and 4 to the arm structure, whereas band 2 is supposed to contain a mixture of polypeptides localized in the arms and in the central structures; these abnormal sperm contain modified polypeptides which gather in band 1. Histochemical ATPase stainings indicate that this enzyme is localized mainly in the doublet arms and, to a minor extent, in the central structures. 相似文献
976.
Tryptophan, tryptamine, or indolepyruvic acid were applied to 2 systems: a bacterial (pea stem sections containing the epiphytic bacteria) and a plant system (pea stem sections under sterile conditions). In the plant system, the production of indoleacetic acid and indoleethanol (tryptophol) from each applied indole derivative is clearly reduced by the aldehyde reagents bisulfite and dimedon, respectively. Indoleacetaldehyde is chromatographically detected after alkaline liberation from its bisulfite addition product. In the bacterial system, the production of indoleacetic acid and indoleethanol is likewise reduced by bisulfite and dimedon. However, after tryptophan or tryptamine application, we could not detect indoleacetaldehyde in the described way. In one case only, namely tryptamine application to the bacterial system, indoleethanol production (contrary to indoleacetic acid production) is scarcely reduced by the aldehyde reagents. This indicates a bacterial pathway tryptamine → indoleethanol which bypasses indoleacetaldehyde. 相似文献
977.
ARNOLD G. KLUGE 《Zoological Journal of the Linnean Society》1993,107(4):293-351
Two major subgroups of erycine snakes, designated Charina and Eryx , are delimited with a cladistic analysis of 75 morphological characters. The hypotheses of species relationships within the two clades are ( reinhardtii ( bottae, trivirgata )) and ( colubrinus, conicus, elegans, jayakari, muelleri, somalicus ( miliaris ( tataricus ( jaculus, johnii )))), respectively. This pattern of grouping obtains without assuming multistate character additivity. At least 16 synapomorphies indicate that reinhardtii is an erycine and that it is the sister lineage of the ( bottae, trivirgata ) clade. Calabaria and Lichanura are synonymized with Charina for reasons of taxonomic efficiency, and to emphasize the New-Old World geographic distribution of the three species in that assemblage. Further resolution of Eryx species relationships is required before Gongylophis (type species conicus ) can be recognized. 相似文献
978.
979.
980.