全文获取类型
收费全文 | 5082篇 |
免费 | 352篇 |
国内免费 | 1篇 |
出版年
2024年 | 6篇 |
2023年 | 34篇 |
2022年 | 117篇 |
2021年 | 164篇 |
2020年 | 126篇 |
2019年 | 154篇 |
2018年 | 201篇 |
2017年 | 154篇 |
2016年 | 227篇 |
2015年 | 304篇 |
2014年 | 307篇 |
2013年 | 396篇 |
2012年 | 451篇 |
2011年 | 427篇 |
2010年 | 259篇 |
2009年 | 213篇 |
2008年 | 280篇 |
2007年 | 227篇 |
2006年 | 236篇 |
2005年 | 202篇 |
2004年 | 178篇 |
2003年 | 141篇 |
2002年 | 127篇 |
2001年 | 37篇 |
2000年 | 36篇 |
1999年 | 31篇 |
1998年 | 45篇 |
1997年 | 23篇 |
1996年 | 28篇 |
1995年 | 25篇 |
1994年 | 23篇 |
1993年 | 23篇 |
1992年 | 20篇 |
1991年 | 23篇 |
1990年 | 18篇 |
1989年 | 20篇 |
1988年 | 12篇 |
1987年 | 18篇 |
1986年 | 16篇 |
1985年 | 15篇 |
1984年 | 12篇 |
1983年 | 12篇 |
1982年 | 10篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1973年 | 5篇 |
1969年 | 4篇 |
1966年 | 3篇 |
排序方式: 共有5435条查询结果,搜索用时 15 毫秒
31.
The Lotka-Volterra competition model was used to represent the interaction between Laurencia obtusa and Hypnea spinella. A new model that considers effects of competition on algal carrying capacity is suggested. To test the models, data from field experiments conducted in an intertidal region at Cabo Frio Island, Rio de Janeiro, Brazil, were used. Both models showed that Hypnea was a stronger competitor than Laurencia. The model of interaction through the carrying capacity showed a stable coexistence between the algal populations and better represented the experimental data. 相似文献
32.
Carlos Hermenegildo Goizane Marcaida Carmina Montoliu Santiago Grisolía María-Dolores Miñana Vicente Felipo 《Neurochemical research》1996,21(10):1237-1244
We proposed that acute ammonia toxicity is mediated by activation of NMDA receptors. To confirm this hypothesis we have tested
whether different NMDA receptor antagonists, acting on different sites of NMDA receptors, prevent death of mice induced by
injection of 14 mmol/Kg of ammonium acetate, a dose that induces death of 95% of mice. MK-801, phencyclidine and ketamine,
which block the ion channel of NMDA receptors, prevent death of at least 75% of mice. CPP, AP-5, CGS 19755, and CGP 40116,
competitive antagonists acting on the binding site for NMDA, also prevent death of at least 75% of mice. Butanol, ethanol
and methanol which block NMDA receptors, also prevent death of mice. There is an excellent correlation between the EC50 for preventing ammonia-induced death and the IC50 for inhibiting NMDA-induced currents. Acute ammonia toxicity is not prevented by antagonists of kainate/AMPA receptors, of
muscarinic or nicotinic acetylcholine receptors or of GABA receptors. Inhibitors of nitric oxide synthase afford partial protection
against ammonia toxicity while inhibitors of calcineurin, of glutamine synthetase or antioxidants did not prevent ammonia-induced
death of mice. These results strongly support the idea that acute ammonia toxicity is mediated by activation of NMDA receptors. 相似文献
33.
Verónica Lipperheide Guillermo Quindós Yolanda Jiménez José Pontón José Vicente Bagán-Sebastián José Manuel Aguirre 《Mycopathologia》1996,134(2):75-82
Prevalence of yeasts in 35 leukoplakia and 34 oral lichen planus patients was compared with that observed in persons without oral diseases. Serotype and morphotype were determined on Candida albicans isolates. Yeasts were isolated from the oral cavity specimens of 43.7% of the patients. C. albicans (serotype A) was the predominant species (76% in leukoplakia, 88.2% in lichen planus and 60.8% in healthy persons). Sixteen morphotypes were encountered on malt extract agar, being 732, 733, 734, 753 and 754 the most frequently found. Morphotypes SP1N and SP1Y were the most common on Sabouraud-trypheniltetrazolium agar (68.4% of the isolates from leukoplakia and 73.3% from lichen planus, but only 46.6% of the isolates from healthy oral mucosa showed SP1N morphotype). Presence of oral lesions was associated with a marked reduction in the yeast species and C. albicans biotypes, suggesting that C. albicans and particularly some of its biotypes, show a high potential of adaptation to the changes associated with the development of oral leukoplakia and lichen planus. 相似文献
34.
Francisco M. Pinto Yves Chupeau Vicente M. Cabrera 《In vitro cellular & developmental biology. Plant》1995,31(2):96-100
An efficient and easy method for genetic characterization of plant somatic hybrids is proposed. In a first qualitative approach,
four somatic hybrids and their parental species (Nicotiana tabacum andN. plumbaginifolia) were characterized by DNA fingerprinting and Random Amplification of Polymorphic DNA (RAPD). After this, a quantitative
estimation of the degree of parental contribution to the hybrids was carried out by means of a slot-blot analysis. Both qualitative
methods, showed one hybrid identical toN. tabacum, two almost identical toN. plumbaginifolia, and a fourth similar to this parental species, but with someN. tabacum admixture. The quantitative method, for the same hybrids, gave 83%, 7%, 7%, and 37%N. tabacum DNA contribution, respectively. 相似文献
35.
Contamination studies by ochratoxin A on pork kidney and chicken liver has been carried out in Catalonia (Spain). 73% of the pork kidney samples analyzed did not contain an amount of ochratoxin A over our detection limit (0.5 ng/g) whereas only 7% had contamination higher than 1 ng/g. None of the chicken samples analyzed were contaminated by this toxin above the detection limit. All contamination levels found are below the maximum levels accepted by several countries for this kind of material. A confirmative test is necessary before discarding false positive samples. 相似文献
36.
Lacker (1981) and Lacker & Akin (1988) developed a mathematical model of follicular maturation and ovulation; this model of only four parameters accounts for a large number of results obtained over the past decade or more on the control of follicular growth and ovulation in mammals. It establishes a single law of maturation for each follicle which describes the interactions between growing follicles. The function put forward is sufficient to explain the constancy of the number of ovulations or large follicles in a female as well as the variability of this number among strains or species and for either induced or spontaneous ovulators. According to the model, the number of ovulations or large follicles lies between two limits that are themselves simple functions of two parameters of the model. Moreover, Lacker's model exhibits interesting characteristics in agreement with results obtained by physiologists: in particular, it predicts that the number of ovulating or large follicles is independent of:
- the total number of maturing follicles,
- the process of recruitment of newly maturing follicles towards the terminal maturation (Poisson or other),
- the form of the LH or FSH secretion curves as functions of the systemic level of oestradiol. The model further predicts that
- selection and dominance of follicles result from the feedback between the ovary and the hypophysis through the interactions between follicles; these interactions are expressed by the maturation function of the model.
- recovery from atresia is possible for a follicle: from decreasing, the rate of secretion of oestradiol may increase.
- the revised model suggests a renewal of follicles during the sexual cycle, as “waves of follicular growth”.
37.
Timothy A. McCaffrey Domenick J. Falcone Diane Vicente Baoheng Du Seth Consigli Wolfgang Borth 《Journal of cellular physiology》1994,159(1):51-59
The transforming growth factor-β (TGF-β) family of proteins exert diverse and potent effects on proliferation, differentiation, and extracellular matrix synthesis. However, relatively little is known about the stability or processing of endogenous TGF-β activity in vitro or in vivo. Our previous work indicated that (1) TGF-β1 has strong heparin-binding properties that were not previously recognized because of neutralization by iodination, and (2) heparin, and certain other polyanions, could block the binding of TGF-β1 to α2-macroglobulin (α2-M). The present studies investigated the influence of heparin-like molecules on the stability of the TGF-β1 signal in the pericellular environment. The results indicate that heparin and fucoidan, a naturally occurring sulfated L-fucose polymer, suppress the formation of an initial non-covalent interaction between 125I-TGF-β1 and activated α2-M. Electrophoresis of 125I-TGF-β1 showed that fucoidan protects TGF-β1 from proteolytic degradation by plasmin and trypsin. While plasmin caused little, if any, activation of latent TGF-β derived from vascular smooth muscle cells (SMC), plasmin degraded acid-activated TGF-β, and purified TGF-β1, and this degradation was inhibited by fucoidan. In vitro, heparin and fucoidan tripled the half-life of 125I-TGF-β1 and doubled the amount of cell-associated 125I-TGF-β1. Consistent with this protective effect, heparin- and fucoidan-treated SMC demonstrated elevated levels of active, but not latent, TGF-β activity. © 1994 wiley-Liss, Inc. 相似文献
38.
Elena Kosenko Yuri Kaminsky Eugenio Grau María-Dolores Miñana Goizane Marcaida Santiago Grisolía Vicente Felipo 《Journal of neurochemistry》1994,63(6):2172-2178
Abstract: Injection of large doses of ammonia into rats leads to depletion of brain ATP. However, the molecular mechanism leading to ATP depletion is not clear. The aim of the present work was to assess whether ammonium-induced depletion of ATP is mediated by activation of the NMDA receptor. It is shown that injection of MK-801, an antagonist of the NMDA receptor, prevented ammonia-induced ATP depletion but did not prevent changes in glutamine, glutamate, glycogen, glucose, and ketone bodies. Ammonia injection increased Na+ ,K+ -ATPase activity by 76%. This increase was also prevented by previous injection of MK-801. The molecular mechanism leading to activation of the ATPase was further studied. Na+ ,K+ -ATPase activity in samples from ammonia-injected rats was normalized by "in vitro" incubation with phorbol 12-myristate 13-acetate, an activator of protein kinase C. The results obtained suggest that ammonia-induced ATP depletion is mediated by activation of the NMDA receptor, which results in decreased protein kinase C-mediated phosphorylation of Na+ ,K+ -ATPase and, therefore, increased activity of the ATPase and increased consumption of ATP. 相似文献
39.
We have analysed the viability of cellular clones induced by mitotic recombination in Drosophila melanogaster/D. simulans hybrid females during larval growth. These clones contain a portion of either melanogaster or simulans genomes in homozygosity. Analysis has been carried out for the X and the second chromosomes, as well as for the 3L chromosome arm. Clones were not found in certain structures, and in others they appeared in a very low frequency. Only in abdominal tergites was a significant number of clones observed, although their frequency was lower than in melanogaster abdomens. The bigger the portion of the genome that is homozygous, the less viable is the recombinant melano-gaster/simulans hybrid clone. The few clones that appeared may represent cases in which mitotic recombination took place in distal chromosome intervals, so that the clones contained a small portion of either melanogaster or simulans chromosomes in homozygosity. Moreover, Lhr, a gene of D. simulans that suppresses the lethality of male and female melanogaster/simulans hybrids, does not suppress the lethality of the recombinant melanogaster/simulans clones. Thus, it appears that there is not just a single gene, but at least one per tested chromosome arm (and maybe more) that cause hybrid lethality. Therefore, the two species, D. melanogaster and D. simulans, have diverged to such a degree that the absence of part of the genome of one species cannot be substituted by the corresponding part of the genome of the other, probably due to the formation of co-adapted gene complexes in both species following their divergent evolution after speciation. The disruption of those coadapted gene complexes would cause the lethality of the recombinant hybrid clones. 相似文献
40.
Mariana Vlad E. Bordas Rodica Tomus Doina Sava Ecaterina Farkas G. Uza 《Biological trace element research》1993,38(1):47-54
Serum copper concentration increases significantly (p<0.01) in rats with experimental atherosclerosis compared to a control group. The serum zinc, the zinc, and copper concentration
in abdominal aorta and in liver decreases significantly (p<0.05) compared to the control group. Administration of copper sulfate for 100 d in these animals induces a significant increase
of serum copper (p<0.01), decrease of serum cholesterol (p<0.05) and increase of liver copper concentration as compared with the group fed only a high cholesterol diet. In the aorta
of these animals the copper concentration increases and edema and lipid infiltration are considerably less than in the group
of animals fed only a high lipid diet. 相似文献