全文获取类型
收费全文 | 19784篇 |
免费 | 1333篇 |
国内免费 | 5篇 |
出版年
2023年 | 151篇 |
2022年 | 263篇 |
2021年 | 525篇 |
2020年 | 389篇 |
2019年 | 429篇 |
2018年 | 638篇 |
2017年 | 557篇 |
2016年 | 770篇 |
2015年 | 1050篇 |
2014年 | 1158篇 |
2013年 | 1388篇 |
2012年 | 1536篇 |
2011年 | 1530篇 |
2010年 | 941篇 |
2009年 | 822篇 |
2008年 | 1112篇 |
2007年 | 1058篇 |
2006年 | 1011篇 |
2005年 | 849篇 |
2004年 | 804篇 |
2003年 | 670篇 |
2002年 | 611篇 |
2001年 | 335篇 |
2000年 | 314篇 |
1999年 | 240篇 |
1998年 | 146篇 |
1997年 | 104篇 |
1996年 | 100篇 |
1995年 | 102篇 |
1994年 | 86篇 |
1993年 | 88篇 |
1992年 | 136篇 |
1991年 | 119篇 |
1990年 | 115篇 |
1989年 | 80篇 |
1988年 | 76篇 |
1987年 | 68篇 |
1986年 | 65篇 |
1985年 | 59篇 |
1984年 | 57篇 |
1983年 | 52篇 |
1982年 | 46篇 |
1981年 | 43篇 |
1980年 | 35篇 |
1979年 | 48篇 |
1978年 | 47篇 |
1976年 | 26篇 |
1975年 | 47篇 |
1974年 | 27篇 |
1973年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
131.
Marcelo V. Sousa Michael Richardson Wagner Fontes Lauro Morhy 《Journal of Protein Chemistry》1994,13(8):659-667
Enterolobin, a 55-kDa cytolytic, inflammatory, and insecticidal protein isolated from seeds of the Brazilian treeEnterolobium contortisiliquum (Leguminosae-Mimosoideae) has been further purified and partially sequenced by using both manual and automated methods. A computational search of enterolobin partial amino acid sequence against the PIR database revealed possible sequence similarities with aerolysins, cytolytic proteins fromAeromonas species. An alignment of enterolobin partial sequence to the amino acid sequences ofA. hydrophila andA. sobria aerolysins showed several similar regions with many residue identites. The seed protein enterolobin and the bacterial aerolysins may be homologous proteins despite the distant phylogenetic relationship. 相似文献
132.
Lidia Jabłońska Michał Walski Urszula Rafałowska 《Cellular and molecular neurobiology》1994,14(6):701-709
Summary 1. The effect of lead (in vivo) on the uptake of GABA, dopamine, and histidine as a precursor of histamine in synaptosomes obtained from chronically lead-treated rats was studied.2. Lead decreased the uptake of GABA, increased the uptake of dopamine, and did not change the uptake of histidine. These effects were independent of calcium concentration.3. Lead administration to the rat changed the morphology of the synaptosomes, as manifested in the decreased number of synaptic vesicles and disturbed mitochondrial structure.4. The results suggest the existence of several mechanisms of lead toxicity on uptake, related to individual neurotransmitters, which are not necessarily connected with a Pb2+/Ca2+ interaction. 相似文献
133.
Collagen IV and laminin are important constituents of the basement membrane (BM). By use of immunocytochemistry we examined the occurrence and distribution of these two components in the BM beneath normal, mucoid and metaplastic epithelium of large bronchi in 22 adults suffering from chronic nonspecific lung diseases. Both collagen IV and laminin were expressed as a thin and continuous layer beneath the epithelium in most tissue specimens with normal epithelium. In a few specimens the layer showed interruptions with a patchy distribution of the immunoreactivity. Three patterns of distribution of BM components were found under the metaplastic epithelium. Total absence of immunoreactive collagen IV and laminin was the most common variant. Weak and scarce staining for both proteins in the BM characterized the second pattern. The third variant showed strong collagen IV immunoreactivity but lack of laminin. The BM beneath the mucoid epithelium was characterized by irregular distribution of collagen IV and laminin. We suggest that the occurrence and distributional pattern of the BM components are related to the type of overlying epithelium and connected with an altered synthesis of these components. 相似文献
134.
In this paper we use a dynamical systems approach to prove the existence of a unique critical value c
* of the speed c for which the degenerate density-dependent diffusion equation u
ct
= [D(u)u
x
]
x
+ g(u) has: 1. no travelling wave solutions for 0 < c < c
*, 2. a travelling wave solution u(x, t) = (x - c
*
t) of sharp type satisfying (– ) = 1, () = 0 *; '(*–) = – c
*/D'(0), '(*+) = 0 and 3. a continuum of travelling wave solutions of monotone decreasing front type for each c > c
*. These fronts satisfy the boundary conditions (– ) = 1, '(– ) = (+ ) = '(+ ) = 0. We illustrate our analytical results with some numerical solutions. 相似文献
135.
Research Notes on Avian Biology 1994: Selected Contributions from the 21st International Ornithological CongressGeneral Biology: Migration
Subject: Migration 相似文献136.
Rodrigo A. Cunha E. Milusheva †E. S. Vizi J. A. Ribeiro A. M. Sebastião 《Journal of neurochemistry》1994,63(1):207-214
Abstract: The modulation by adenosine analogues and endogenous adenosine of the electrically evoked release of [3H]acetylcholine ([3H]ACh) was compared in subslices of the three areas of the rat hippocampus (CA1, CA3, and dentate gyrus). The mixed A1/A2 agonist 2-chloroadenosine (CADO; 2–10 µM) inhibited, in a concentration-dependent manner, the release of [3H]ACh from the three hippocampal areas, being more potent in the CA1 and CA3 areas than in the dentate gyrus. The inhibitory effect of CADO (5 µM) on [3H]ACh release was prevented by the A1 antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 50 nM) in the three hippocampal areas and was converted in an excitatory effect in the CA3 and dentate gyrus areas. The A2A agonist CGS-21680 (30 nM) produced a greater increase of the evoked release of [3H]ACh in the CA3 than in the dentate gyrus areas, whereas no consistent effect was found in the CA1 area or in the whole hippocampal slice. The excitatory effect of CGS-21680 (30 nM) in the CA3 area was prevented by the adenosine receptor antagonist 3,7-dimethyl-1-propargylxanthine (10 µM). Both adenosine deaminase (2 U/ml) and DPCPX (250 nM) increased the evoked release of [3H]ACh in the CA1 and CA3 areas but not in the dentate gyrus. The amplitude of the effect of DPCPX and adenosine deaminase was similar in the CA1 area, but in the CA3 area DPCPX produced a greater effect than adenosine deaminase. It is concluded that the electrically evoked release of [3H]ACh in the three areas of the rat hippocampus can be differentially modulated by adenosine. In the CA1 area, only A1 inhibitory receptors modulate ACh release, whereas in the CA3 area, both A2A excitatory and A1 inhibitory adenosine receptors modulate ACh release. In the dentate gyrus, both A1 inhibitory and A2A excitatory adenosine receptors are present, but endogenous adenosine does not activate them. 相似文献
137.
Charles Coudray Sylvie Pucheu François Boucher Josiane Arnaud Joël de Leiris Alain Favier 《Biological trace element research》1994,41(1-2):69-75
The hypothesis that oxygen-derived free radicals play an important role in myocardial ischemic and reperfusion injury has received a lot of support. In the presence of catalytic amounts of transition metals such as iron, superoxide anions, and hydrogen peroxide can be transformed into a highly reactive hydroxyl radical °OH (Haber-Weiss reaction). In view of this, we have undertaken this study to investigate whether iron is involved in the reperfusion syndrome and therefore could aggravate free radicals injury. Coronary effluent iron concentrations and cardiac cytosolic iron levels were evaluated in rat hearts subjected to an ischemia/reperfusion sequences. In the case of total ischemia, iron concentration in coronary effluents peaked immediately in the first sample collected upon reperfusion. However, in the case of partial ischemia, iron concentration in coronary effluents peaked rather exclusively during ischemia period. Cardiac cytosolic iron level augmented significantly after 30 min of total ischemia and non significantly in the other ischemia protocols compared to perfused control hearts. It also appears that the iron released is not protein-bound, and could therefore have a marked catalytic activity. The results of the present study suggest that in the oxygen paradox, iron plays an important role in inducing alterations during reoxygenation. 相似文献
138.
Molecular characterization of a fourth isoform of the catalytic subunit of protein phosphatase 2A from Arabidopsis thaliana 总被引:1,自引:0,他引:1
Antonio Casamayor Encarna Pérez-Callejón Gemma Pujol Joaquín Ariño Albert Ferrer 《Plant molecular biology》1994,26(1):523-528
We have recently reported the existence of multiple isoforms of the catalytic subunit of protein phosphatase 2A (PP2A) in Arabidopsis thaliana and the molecular cloning of cDNAs encoding three of these proteins (PP2A-1, PP2A-2, PP2A-3). The reported cDNA encoding PP2A-3 was truncated at the 5 terminus, lacking a short fragment of the N-terminal coding sequence. We have now isolated a near full-length cDNA encoding the entire PP2A-3 protein (313 residues). The clone includes 188 nucleotides of 5-untranslated region, where a 44 bp long poly(GA) track is found. We also describe the cloning of a cDNA encoding a fourth isoform of PP2A (PP2A-4). The polypeptide contains 313 residues being 98% identical to PP2A-3 and only 80% identical to both PP2A-1 and PP2A-2. The mRNA for PP2A-4 is 1.4 kb in length and, although predominantly expressed in roots, it is also found in other organs. It is concluded that in A. thaliana the isoforms of PP2A can be grouped in two extremely conserved subfamilies. 相似文献
139.
Luís C. Duarte Alexandra P. Nobre Francisco M. Gírio M. T. Amaral-Collaço 《Biotechnology Techniques》1994,8(12):859-864
Summary The kinetic parameters of the yeastDebaryomyces hansenii grown in continous cultivation on D-xylose were determined by different methods. While the values obtained for μm by the steady state and the washout methods only gave a 3% difference, the determined Ks values by the steady state and the maximal biomass output methods led a to a 305% difference. The latter method was suggested
to overestimate the Ks value. 相似文献
140.
A polymorphic alpha satellite sequence specific for human chromosome 13 detected by oligonucleotide primed in situ labelling (PRINS) 总被引:8,自引:0,他引:8
Franck Pellestor Anne Girardet Brigitte Andréo Jean-Paul Charlieu 《Human genetics》1994,94(4):346-348
The centromeric alpha satellite DNA subfamilies from chromosomes 13 and 21 are almost identical in sequence and cannot be easily distinguished by mean of probes for Southern blot or in situ hybridisation. We have used the oligonucleotide-primed in situ (PRINS) labelling technique with primers defined from the alpha satellite sequence of chromosome 13. One primer was found to label specifically the centromeric region of chromosomes 13 and allowed the detection of a polymorphism between two chromosome 13 homologues in one individual. 相似文献