全文获取类型
收费全文 | 10809篇 |
免费 | 996篇 |
国内免费 | 8篇 |
出版年
2021年 | 90篇 |
2018年 | 113篇 |
2017年 | 125篇 |
2016年 | 178篇 |
2015年 | 333篇 |
2014年 | 310篇 |
2013年 | 428篇 |
2012年 | 568篇 |
2011年 | 517篇 |
2010年 | 380篇 |
2009年 | 303篇 |
2008年 | 487篇 |
2007年 | 508篇 |
2006年 | 453篇 |
2005年 | 539篇 |
2004年 | 466篇 |
2003年 | 449篇 |
2002年 | 435篇 |
2001年 | 169篇 |
2000年 | 149篇 |
1999年 | 158篇 |
1998年 | 150篇 |
1997年 | 114篇 |
1996年 | 117篇 |
1995年 | 112篇 |
1994年 | 93篇 |
1993年 | 129篇 |
1992年 | 126篇 |
1991年 | 136篇 |
1990年 | 131篇 |
1989年 | 139篇 |
1988年 | 138篇 |
1987年 | 110篇 |
1986年 | 117篇 |
1985年 | 90篇 |
1984年 | 96篇 |
1983年 | 88篇 |
1982年 | 90篇 |
1981年 | 123篇 |
1980年 | 95篇 |
1979年 | 117篇 |
1978年 | 106篇 |
1977年 | 99篇 |
1976年 | 80篇 |
1975年 | 83篇 |
1974年 | 107篇 |
1973年 | 112篇 |
1972年 | 86篇 |
1970年 | 96篇 |
1969年 | 86篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Walter Fahn Eva Laußermair Brigitte Deus-Neumann Joachim Stöckigt 《Plant cell reports》1985,4(6):337-340
From differentiated plants of Catharanthus roseus (L.) G. Don we have isolated a specific enzyme of the vindoline biosynthetic pathway catalysing the S-adenosylmethionine-dependent methylation of 11-O-demethyl-17-O-deacetyl-vindoline. The enzyme we named S-adenosyl-L-methionine : 11-O-demethyl-17-O-deacetylvindoline 11-O-methyltransferase. This transferase exhibits a high substrate specificity. Obviously the O-methylation at C-11 precedes the O-acetylation at the C-17 position during the biosynthesis of vindoline.A second enzyme was detected which hydrolyses the acetyl function of vindoline. The distribution of this acetylesterase in C. roseus plants demonstrates that the enzyme is not specifically associated with the vindoline distribution in the plant material. Most probably this enzyme plays no essential role in the biosynthesis of vindoline. 相似文献
122.
Walter Fahn Heidrun Gundlach Brigitte Deus-Neumann Joachim Stöckigt 《Plant cell reports》1985,4(6):333-336
From differentiated plants of Catharanthus roseus (L.) G. Don, a specific enzyme was isolated and named acetyl-CoA : 17-O-deacetylvindoline 17-O-acetyltransferase, acting on the biosynthetic formation of the Aspidosperma type alkaloid vindoline.The enzyme shows a high selectivity towards different substrates. The acetyl-CoA-dependent transferase also catalyses the reverse reaction by hydrolysis of the 17-O-acetyl group of vindoline in the presence of free CoA. This enzyme is localized only in vindoline-containing plant parts, but was so far not detectable in cell suspension cultures of C. roseus. The enzyme allows the synthesis of labelled vindoline with high specific activity, applicable for instance as tracer for radioimmunoassays of vindoline. 相似文献
123.
124.
125.
126.
Walter Pflumm 《Journal of Ornithology》1984,125(4):481-482
Summary By recording the rate of glancing (i. e. looking up), the number of seeds taken by a crossbill per unit time from a larch cone (Larix X eurolepis) was evaluated. Female crossbills took more seeds per unit time from the cones than males do. Because of similar bill (and body) sizes the sexual difference could be explained in the following ways: a) during the breeding season the females need more energy for egg production than males do for spermatogenesis; b) females are more vigilant than males. 相似文献
127.
Two species of Anabas Cuvier, 1816 (Osteichthyes: Anabantidae): A. testudineus (Bloch, 1795) and A. oligolepis Bleeker, 1855 are widely distributed in India. Since both species are represented in one habitat — Lake Kolleru, and are very closely related, they were compared using starch gel electrophoresis. Electrophoretic separation of proteins of eye lens, skeletal muscle and heart muscle revealed differences such as i) absence of one protein fraction in one of them, ii) mobility and iii) staining intensity of some of the bands. The esterase (non-specific) patterns of serum, liver, kidney, skeletal muscle, heart muscle and eggs also showed differences. The LDH fraction of eye lens showed a difference in mobility. 相似文献
128.
Karla J. Matteson Bon-chu Chung Walter L. Miller 《Biochemical and biophysical research communications》1984,120(1):264-270
P450scc is the rate-limiting hormonally regulated enzyme that cleaves the cholesterol side chain. Translation of bovine adrenocortical mRNA and immunoprecipitation with rabbit anti-bovine P450scc indicates P450scc mRNA represents 1% of the total. DNA complementary to bovine adrenocortical mRNA was cloned in the site of pBR322 by dC·dG tailing and high-efficiency transformation. A clone containing sequences complementary to P450scc mRNA was identified by hybrid-selected translation only when plasmid DNA was first purified by CsCl gradient centrifugation. As is often the case with hybrid-selected translation, the clone identified contains a small insert. 相似文献
129.
Filter mesh size and food particle uptake by Daphnia 总被引:7,自引:0,他引:7
Food size selection of four Daphnia, species (D. magna, D. hyalina, D. galeata, D. pulicaria) was investigated using spherical plastic beads as artificial food and with small bacteria. The size of the particles ranged from 0.1 to 35 m with special emphasis to the particle diameters between 0.1 and 1 m. In one set of experiments a mixture of differently sized particles was offered as food suspension and the selectivity of filtering was determined by comparing the size spectrum of the particles found in the gut contents with the spectrum in the food suspension. In a second series of experiments suspensions of uniformly sized particles were offered to single animals and their feeding activity was observed directly. In both types of experiments the mesh sizes of the filtering apparatus of the respective animals studied were measured after the experiments by, scanning electron microscopy. The mean sizes of the filter meshes were about 0.4–0.7 m. In all experiments the size of the particles found in the gut or those which caused high feeding activities were larger than the smallest mesh sizes of the filters. As a consequence simple mechanical sieving provides a sufficient explanation for the mechanism of particle retention of the filtering process in Daphnia. D. magna was found to feed with high efficiency on suspended freshwater bacteria, the residual species investigated showed low filtering efficiencies when bacteria were offered as food.The present study was supported by Deutscher Akademischer Austauschdients 相似文献
130.
J M Krueger M L Karnovsky S A Martin J R Pappenheimer J Walter K Biemann 《The Journal of biological chemistry》1984,259(20):12659-12662
The structures of components of the sleep-promoting material purified from human urine were established by fast atom bombardment-mass spectrometry, as reported in the accompanying paper (Martin, S. A., Karnovsky, M. L., Krueger, J. M., Pappenheimer, J. R., and Biemann, K. (1984) J. Biol. Chem. 259, 12652-12658). We report here that two substances isolated from that preparation, viz. N-acetylglucosaminyl-1,6 -anhydro-N-acetylmuramyl-Ala-gamma-Glu-diaminopimelyl-Ala) and that compound lacking the terminal alanine, are active as somnogens. Cerebro-intraventricular administration of 1 pmol of the glycotetrapeptide was sufficient to induce prolonged excess sleep in rabbits. A similar substance obtained from Brevibacterium divaricatum in which the free carboxyls of the glutamic and diaminopimelic moieties, indicated above, were amidated (N-acetylglucosaminyl-1,6-anhydro-N-anhydromura-myl-Ala-iso- Gln- epsilon-amido-diaminopimelyl -Ala-Ala) was not active as a promoter of slow-wave sleep. Deamidation of this peptide to a mixture of the free dicarboxylic forms produced a somnogenic substance. Our findings show that in addition to the muramyl form of peptidoglycan monomers, the anhydro muramyl form, with no reducing end, is compatible with somnogenic activity. Furthermore, the data obtained with a natural product amplify our earlier observations with smaller synthetic molecules of the importance of amidation/deamidation in the structure-activity relationships of muramyl peptides. 相似文献