全文获取类型
收费全文 | 2317篇 |
免费 | 225篇 |
国内免费 | 1篇 |
专业分类
2543篇 |
出版年
2021年 | 31篇 |
2020年 | 24篇 |
2019年 | 29篇 |
2018年 | 32篇 |
2017年 | 16篇 |
2016年 | 43篇 |
2015年 | 87篇 |
2014年 | 106篇 |
2013年 | 134篇 |
2012年 | 115篇 |
2011年 | 128篇 |
2010年 | 86篇 |
2009年 | 65篇 |
2008年 | 108篇 |
2007年 | 115篇 |
2006年 | 100篇 |
2005年 | 116篇 |
2004年 | 102篇 |
2003年 | 85篇 |
2002年 | 83篇 |
2001年 | 42篇 |
2000年 | 36篇 |
1999年 | 42篇 |
1998年 | 32篇 |
1997年 | 18篇 |
1996年 | 24篇 |
1995年 | 20篇 |
1994年 | 17篇 |
1993年 | 27篇 |
1992年 | 37篇 |
1991年 | 22篇 |
1990年 | 18篇 |
1989年 | 26篇 |
1988年 | 23篇 |
1987年 | 15篇 |
1985年 | 21篇 |
1984年 | 19篇 |
1983年 | 23篇 |
1982年 | 28篇 |
1981年 | 24篇 |
1980年 | 28篇 |
1979年 | 22篇 |
1978年 | 22篇 |
1977年 | 20篇 |
1976年 | 15篇 |
1975年 | 15篇 |
1974年 | 18篇 |
1973年 | 18篇 |
1971年 | 25篇 |
1970年 | 14篇 |
排序方式: 共有2543条查询结果,搜索用时 15 毫秒
911.
Paredes-Gamero EJ Leon CM Borojevic R Oshiro ME Ferreira AT 《The Journal of biological chemistry》2008,283(46):31909-31919
The role of intracellular Ca2+ (Ca2+i) on hematopoiesis was investigated in long term bone marrow cultures using cytokines and agonists of P2 receptors. Cytokines interleukin 3 and granulocyte/macrophage colony stimulator factor promoted a modest increase in Ca2+i concentration ([Ca2+]i) with activation of phospholipase Cgamma, MEK1/2, and Ca2+/calmodulin kinase II. Involvement of protein kinase C was restricted to stimulation with interleukin 3. In addition, these cytokines promoted proliferation (20 times) and an increase in the Gr-1(-)Mac-1+ population with participation of gap junctions (GJ). Nevertheless ATP, ADP, and UTP promoted a large increase in [Ca2+]i, moderate proliferation (6 times), a reduction in the primitive Gr-1(-)Mac-1(-)c-Kit+ population, and differentiation into macrophages without participation of GJ. It is likely that Ca2+i participates as a regulator of hematopoietic signaling: moderate increases in [Ca2+]i would be related to cytokine-dependent proliferation with participation of GJ, whereas high increases in [Ca2+]i would be related to macrophage differentiation without maintenance of the primitive population. 相似文献
912.
Farhy LS Du Z Zeng Q Veldhuis PP Johnson ML Brayman KL McCall AL 《American journal of physiology. Endocrinology and metabolism》2008,295(3):E575-E585
Glucagon counterregulation (GCR) is a key protection against hypoglycemia that is compromised in diabetes via an unknown mechanism. To test the hypothesis that alpha-cell-inhibiting signals that are switched off during hypoglycemia amplify GCR, we studied streptozotocin (STZ)-treated male Wistar rats and estimated the effect on GCR of intrapancreatic infusion and termination during hypoglycemia of saline, insulin, and somatostatin. Times 10 min before and 45 min after the switch-off were analyzed. Insulin and somatostatin, but not saline, switch-off significantly increased the glucagon levels (P = 0.03), and the fold increases relative to baseline were significantly higher (P < 0.05) in the insulin and somatostatin groups vs. the saline group. The peak concentrations were also higher in the insulin (368 pg/ml) and somatostatin (228 pg/ml) groups vs. the saline (114 pg/ml) group (P < 0.05). GCR was pulsatile in most animals, indicating a feedback regulation. After the switch-off, the number of secretory events and the total pulsatile production were lower in the saline group vs. the insulin and somatostatin groups (P < 0.05), indicating enhancement of glucagon pulsatile activity by insulin and somatostatin compared with saline. Network modeling analysis demonstrates that reciprocal interactions between alpha- and delta-cells can explain the amplification by interpreting the GCR as a rebound response to the switch-off. The model justifies experimental designs to further study the intrapancreatic network in relation to the switch-off phenomenon. The results of this proof-of-concept interdisciplinary study support the hypothesis that GCR develops as a rebound pulsatile response of the intrapancreatic endocrine feedback network to switch-off of alpha-cell-inhibiting islet signals. 相似文献
913.
Habitat structural complexity mediates food web dynamics in a freshwater macrophyte community 总被引:2,自引:0,他引:2
A considerable amount of research has investigated the influence of habitat structure on predator success, yet few studies have explored the implications for community structure and food-web dynamics. The relative importance of macrophyte structure and fish predation on the composition of the macroinvertebrate and periphyton communities in a lowland river was investigated using a multifactorial caging experiment. We hypothesised that: (1) fish predators are less effective in a more structurally complex macrophyte analogue; (2) strong direct and indirect effects of fish predators (e.g. trophic cascades) are less likely to occur in a structurally complex habitat; and (3) the strength of these patterns is influenced by the composition of the prevailing community assemblage. We measured the abundance and composition of the macroinvertebrate and periphyton communities associated with three different-shaped macrophyte analogues, under different fish predator treatments and at different times. Macrophyte analogue architecture had strong, consistent effects on both the macroinvertebrate and periphyton communities; both were most abundant and diverse on the most structurally complex plant analogue. In contrast, the fish predators affected only a subset of the macroinvertebrate community and there was a suggestion of minor indirect effects on periphyton community composition. Contrary to expectations, the fish predators had their strongest effects in the most structurally complex macrophyte analogue. We conclude that in this system, macrophyte shape strongly regulates the associated freshwater assemblage, resulting in a diverse community structure less likely to exhibit strong effects of fish predation. 相似文献
914.
Pérez-Vich B Leon AJ Grondona M Velasco L Fernández-Martínez JM 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》2006,112(5):867-875
Increasing the stearic acid content to improve sunflower (Helianthus annuus L.) oil quality is a desirable breeding objective for food-processing applications. CAS-14 is a sunflower mutant line with
a high stearic acid content in its seed oil (>35% vs. <6% in currently grown sunflower hybrids), which is controlled by the
Es3 gene. However, the expression of the high stearic acid character in CAS-14 is strongly influenced by temperature during
seed maturation and it is not uniform along the seed. The objectives of this study were (1) to identify PCR-based molecular
markers linked to the Es3 gene from CAS-14, (2) to map this gene on the sunflower genetic map, and (3) to characterize the
interaction between CAS-14 and CAS-3, a sunflower high stearic acid (about 26%) mutant line with the Es1 and Es2 genes determining
this trait. Two F2 mapping populations were developed from crosses between CAS-14 and P21, a nuclear male sterile line with the Ms11 gene controlling this character, and between CAS-14 and CAS-3. One hundred and thirty-three individuals from P21×CAS-14,
and 164 individuals from CAS-3×CAS-14 were phenotyped in F2 and F3 seed generations for fatty acid composition using gas–liquid chromatography, and they were then genotyped with microsatellite
[simple sequence repeat (SSR)] and insertion–deletion (INDEL) markers. Bulk segregant analysis in the P21×CAS-14 population
identified two markers on LG 8 putatively linked to Es3. A large linkage group was identified using additional markers mapping
to LG 8. Es3 mapped to the distal half of LG 8 and was flanked by the SSR markers ORS243 and ORS1161 at genetic distances
of 0.5, and 3.9 cM, respectively. The Ms11 gene was also mapped to LG 8 and genetic distance between this gene and Es3 was found to be 7.4 cM. In the CAS-3×CAS-14 population,
two QTLs were identified on LG 1 and LG 8, which underlie the Es1 gene from CAS-3 and the Es3 gene from CAS-14, respectively.
A significant epistatic interaction between these two QTLs was found. Results from this study provided a basis for determining
CAS-14 efficient breeding strategies. 相似文献
915.
Krüger C Hultberg A Marcotte H Hermans P Bezemer S Frenken LG Hammarström L 《Applied microbiology and biotechnology》2006,72(4):732-737
Streptococcus mutans is the main cause of dental caries. We evaluated the therapeutic effect of variable regions of a llama heavy chain antibody fragments directed against S. mutans named S36-VHH (S for Streptococcus) alone or fused with glucose oxidase (GOx) from Aspergillus niger. Western blot analysis and ELISA revealed binding of the S36-VHH to the streptococcal antigen I/II adhesin molecule of S. mutans serotype C. In a rat-desalivated caries model, daily administration of S36-VHH significantly reduced the development of smooth surface caries. No additional therapeutic effect of GOx was observed. Our results suggest that llama VHH antibodies may be a potential benefit as prophylaxis against dental caries. 相似文献
916.
The Physiology, Genetics and Molecular Biology of Plant Aluminum Resistance and Toxicity 总被引:27,自引:0,他引:27
Aluminum (Al) toxicity is the primary factor limiting crop production on acidic soils (pH values of 5 or below), and because
50% of the world’s potentially arable lands are acidic, Al toxicity is a very important limitation to worldwide crop production.
This review examines our current understanding of mechanisms of Al toxicity, as well as the physiological, genetic and molecular
basis for Al resistance. Al resistance can be achieved by mechanisms that facilitate Al exclusion from the root apex (Al exclusion)
and/or by mechanisms that confer the ability of plants to tolerate Al in the plant symplasm (Al tolerance). Compelling evidence
has been presented in the literature for a resistance mechanism based on exclusion of Al due to Al-activated carboxylate release
from the growing root tip. More recently, researchers have provided support for an additional Al-resistance mechanism involving
internal detoxification of Al with carboxylate ligands (deprotonated organic acids) and the sequestration of the Al-carboxylate
complexes in the vacuole. This is a field that is entering a phase of new discovery, as researchers are on the verge of identifying
some of the genes that contribute to Al resistance in plants. The identification and characterization of Al resistance genes
will not only greatly advance our understanding of Al-resistance mechanisms, but more importantly, will be the source of new
molecular resources that researchers will use to develop improved crops better suited for cultivation on acid soils. 相似文献
917.
Jimenez-Candil J Morinigo JL Ledesma C Leon V Martín-Luengo C 《Indian pacing and electrophysiology journal》2008,8(3):158-171
Background
In atrioventricular nodal re-entrant tachycardias (AVNRT), the achievement of Junctional Rhythms (JR) during Radiofrequency Ablation (RF) is a sensitive but non-specific marker of success. Our aim is to analyze prospectively the predictors of non-inducibility of AVNRT, focusing on the characteristics of the JR.Methods
We included 75 patients with reproducibly inducible AVNRT. Ablation was performed following an electro-anatomical approach. After each application, the induction protocol was repeated.Results
A total of 341 applications were performed. Although the achievement of ≥1 JR was necessary to obtain the non-inducibility, and the cumulative number of junctional beats (CJB) was higher in effective applications, no CJB cut-off was associated with a success rate higher than 75%. After the observation of a significant correlation between the sinus cycle length (CL) pre-RF and the CL of the JR (JR-CL) (c=0.52; p<0.001), the sinus CL pre-RF/JR-CL ratio (CL-ratio) adequately differentiated the successful vs. unsuccessful applications: 1.41±0.23 vs. 1.17±0.2 (p<0.001). In a multivariate analysis, a CBJ 11 (p<0.001) and a CL-ratio 1.25 (p<0.001) were found to be the only independent predictors of success. The combination of ≥ 11 of CJB with a CL ratio ≥ 1.25 achieved non-inducibility in 97% of our patients.Conclusion
1) The specificity of the occurrence of JR as a marker of the successful ablation of AVNRT is increased by the CL-ratio. 2) The achievement of ≥ 11 of CJB with a CL ratio ≥ 1.25 predicts non-inducibility in almost all patients. 相似文献918.
A previous study of the relationships amongst three subgroups of the Austral Asplenium ferns found conflicting signal between the two chloroplast loci investigated. Because organelle genomes like those of chloroplasts and mitochondria are thought to be non-recombining, with a single evolutionary history, we sequenced four additional chloroplast loci with the expectation that this would resolve these relationships. Instead, the conflict was only magnified. Although tree-building analyses favoured one of the three possible trees, one of the alternative trees actually had one more supporting site (six versus five) and received greater support in spectral and neighbor-net analyses. Simulations suggested that chance alone was unlikely to produce strong support for two of the possible trees and none for the third. Likelihood permutation tests indicated that the concatenated chloroplast sequence data appeared to have experienced recombination. However, recombination between the chloroplast genomes of different species would be highly atypical, and corollary supporting observations, like chloroplast heteroplasmy, are lacking. Wider taxon sampling clarified the composition of the Austral group, but the conflicting signal meant analyses (e.g., morphological evolution, biogeographic) conditional on a well-supported phylogeny could not be performed. 相似文献
919.
920.
Investigating heavy-metal hyperaccumulation using Thlaspi caerulescens as a model system 总被引:1,自引:0,他引:1