全文获取类型
收费全文 | 4046篇 |
免费 | 367篇 |
出版年
2023年 | 21篇 |
2022年 | 29篇 |
2021年 | 75篇 |
2020年 | 46篇 |
2019年 | 90篇 |
2018年 | 91篇 |
2017年 | 76篇 |
2016年 | 156篇 |
2015年 | 189篇 |
2014年 | 208篇 |
2013年 | 261篇 |
2012年 | 257篇 |
2011年 | 232篇 |
2010年 | 164篇 |
2009年 | 172篇 |
2008年 | 176篇 |
2007年 | 174篇 |
2006年 | 185篇 |
2005年 | 178篇 |
2004年 | 167篇 |
2003年 | 152篇 |
2002年 | 140篇 |
2001年 | 114篇 |
2000年 | 124篇 |
1999年 | 83篇 |
1998年 | 62篇 |
1997年 | 59篇 |
1996年 | 39篇 |
1995年 | 35篇 |
1994年 | 37篇 |
1993年 | 42篇 |
1992年 | 68篇 |
1991年 | 51篇 |
1990年 | 57篇 |
1989年 | 46篇 |
1988年 | 47篇 |
1987年 | 44篇 |
1986年 | 29篇 |
1985年 | 32篇 |
1984年 | 29篇 |
1983年 | 16篇 |
1982年 | 21篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1979年 | 14篇 |
1978年 | 14篇 |
1977年 | 10篇 |
1976年 | 12篇 |
1975年 | 12篇 |
1974年 | 6篇 |
排序方式: 共有4413条查询结果,搜索用时 15 毫秒
941.
M nica M. Mu oz de Toro Enrique H. Luque 《The Journal of steroid biochemistry and molecular biology》1997,60(5-6):277-284
Estrogen, through estrogen receptors (ERs), may regulate the synthesis of progesterone receptors (PRs) and of a heat shock estrogen receptor-associated protein (hsp27). In female breast carcinoma (FBC) both proteins serve as surrogate indicators for the presence of functional ERs. In addition, the expression of these proteins was related to other prognostic indicators of value in female breast tumours. Endocrine disorders, hormone therapy and altered estrogen metabolism have been associated with the development of male breast cancer (MBC), suggesting that evaluation of the expression of ER, PR and hsp27 might improve our understanding of the biology of this tumour. ER and PR status and hsp27 expression were evaluated by immunohistochemistry in 16 primary MBC patients. The interrelationships between these parameters were established and compared with the clinicopathological data on the tumours. Ten (56%) MBC patients were ER-positive, 69% were PR-positive and all samples were hsp27-positive. Our series of MBC patients showed a positive correlation between ERs and PRs, however there was a lack of correlation between hsp27 and ERs or PRs. MBCs did not exhibit any correlation between the biomarkers studied and known prognostic indicators for females (e.g. Scarff-Bloom-Richardson (SBR) or modified SBR (MSBR) grade, T stage, lymph node status). This is the first published series reporting the incidence of hsp27 in MBC. The lack of association between the expression of ERs and hsp27 found in MBC differs from the results reported for FBC, moreover the expression of ERs, PRs or hsp27 did not correlate with the clinicopathological parameters that have prognostic value in females. Although the data were obtained from a relatively small sample population, our findings suggest that MBC and FBC are biologically different tumours with respect to the expression of the studied proteins. 相似文献
942.
M. Castaeda-Agull Luz M. del Castillo J. R. Whitaker A. L. Tappel 《The Journal of general physiology》1961,44(6):1103-1120
The kinetic effects resulting from changes in the medium ionic strength on reactions involving trypsin or α-chymotrypsin are different. The reaction rate increases continuously as the ionic strength increases with α-chymotrypsin. With trypsin, the rate increases at low ionic strengths but as the ionic strength further increases a gradual inhibitory effect is observed. The effects produced by different salts of various valence types (from uni-univalent to uni-trivalent or tri-univalent) are essentially the same, and they are a function of the square root of the ionic strength. The quantitative differences among the various salts may be accounted for on the basis of individual properties of the ions, such as the size of the hydrated ion, "association," etc. The effects of salts on the enzymic reactions described herein are amenable to the same electrostatic treatment applicable to non-enzymatic reactions. By applying Brönsted's basic kinetic concepts and the Debye-Hückel law of electrolyte activity, it appears that the salt effects are mainly due to changes in the dissociation of ionizable groups. This appears to be a general method for analyzing the effect of inorganic ions on enzymic reactions. 相似文献
943.
M. R. De Felipe J. M. Pozuelo M. M. Lucas F. Fernández del Campo 《Physiologia plantarum》1986,66(3):563-568
The effect of isoproturon [N,N-dimethyl N'4-(1-methyl-ethyl) phenyl urea] on the germination, growth of root and ultrastructure of the photosynthetic apparatus was investigated in two wheat cultivars ( Triticum sativum L. cvs Castan and Esquilache) and a weed ( Lolium rigidum Gaud.). No inhibition of germination was apparent in any of the three plants studied. The inhibition of root growth was considerably significant, especially in cv. Esquilache. After 48 h treatment with isoproturon (340 μ M ), the chloroplasts of the two wheat cultivars were found to be similar, showing a higher number of grana with fewer thylakoids per granum (lower stacking degree), broader grana stacks than in the control and an absence of starch. However, after 60 h of treatment, swelling of the thylakoids occurred in the Esquilache cultivar.
The rye grass was the most affected by the herbicide; at low concentration (3.4 μ M ) the number and size of the grana decreased considerably and the stroma thylakoids were destroyed. At higher concentration (17 μ M ), there was a strong accumulation in certain areas of the stroma of a substance with a lipid-like appearance, probably derived from the disintegrating thylakoids. At the same concentrations some chloroplast envelopes were completely desintegrated.
The Hill reaction of the three plants exhibited a similar sensitivity towards the herbicide. 相似文献
The rye grass was the most affected by the herbicide; at low concentration (3.4 μ M ) the number and size of the grana decreased considerably and the stroma thylakoids were destroyed. At higher concentration (17 μ M ), there was a strong accumulation in certain areas of the stroma of a substance with a lipid-like appearance, probably derived from the disintegrating thylakoids. At the same concentrations some chloroplast envelopes were completely desintegrated.
The Hill reaction of the three plants exhibited a similar sensitivity towards the herbicide. 相似文献
944.
Britaldo Soares-Filho † Ane Alencar‡ Daniel Nepstad‡§ Gustavo Cerqueira Maria del Carmen Vera Diaz‡ Sérgio Rivero‡ Luis Solórzano§ Eliane Voll 《Global Change Biology》2004,10(5):745-764
The spatial distribution of human activities in forest frontier regions is strongly influenced by transportation infrastructure. With the planned paving of 6000 km of highway in the Amazon Basin, agricultural frontier expansion will follow, triggering potentially large changes in the location and rate of deforestation. We developed a land‐cover change simulation model that is responsive to road paving and policy intervention scenarios for the BR‐163 highway in central Amazonia. This corridor links the cities of Cuiabá, in central Brazil, and Santarém, on the southern margin of the Amazon River. It connects important soybean production regions and burgeoning population centers in Mato Grosso State with the international port of Santarém, but 1000 km of this road are still not paved. It is within this context that the Brazilian government has prioritized the paving of this road to turn it into a major soybean exportation facility. The model assesses the impacts of this road paving within four scenarios: two population scenarios (high and moderate growth) and two policy intervention scenarios. In the ‘business‐as‐usual’ policy scenario, the responses of deforestation and land abandonment to road paving are estimated based on historical rates of Amazon regions that had a major road paved. In the ‘governance’ scenario, several plausible improvements in the enforcement of environmental regulations, support for sustainable land‐use systems, and local institutional capacity are invoked to modify the historical rates. Model inputs include data collected during expeditions and through participatory mapping exercises conducted with agents from four major frontier types along the road. The model has two components. A scenario‐generating submodel is coupled to a landscape dynamics simulator, ‘DINAMICA’, which spatially allocates the land‐cover transitions using a GIS database. The model was run for 30 years, divided into annual time steps. It predicted more than twice as much deforestation along the corridor in business‐as‐usual vs. governance scenarios. The model demonstrates how field data gathered along a 1000 km corridor can be used to develop plausible scenarios of future land‐cover change trajectories that are relevant to both global change science and the decision‐making process of governments and civil society in an important rainforest region. 相似文献
945.
Ariana Barberá Noraylis Lorenzo Peter van Kooten Joel van Roon Wilco de Jager Dinorah Prada Jorge Gómez Gabriel Padrón Willem van Eden Femke Broere María del Carmen Domínguez 《Cell stress & chaperones》2016,21(4):735-744
Rheumatoid arthritis (RA) is a systemic autoimmune disease characterized by a chronic relapsing-remitting joint inflammation. Perturbations in the balance between CD4?+?T cells producing IL-17 and CD4?+?CD25highFoxP3?+?Tregs correlate with irreversible bone and cartilage destruction in RA. APL1 is an altered peptide ligand derived from a CD4+ T-cell epitope of human HSP60, an autoantigen expressed in the inflamed synovium, which increases the frequency of CD4?+?CD25highFoxP3+ Tregs in peripheral blood mononuclear cells from RA patients. The aim of this study was to evaluate the suppressive capacity of Tregs induced by APL1 on proliferation of effector CD4+ T cells using co-culture experiments. Enhanced Treg-mediated suppression was observed in APL1-treated cultures compared with cells cultured only with media. Subsequent analyses using autologous cross-over experiments showed that the enhanced Treg suppression in APL1-treated cultures could reflect increased suppressive function of Tregs against APL1-responsive T cells. On the other hand, APL1-treatment had a significant effect reducing IL-17 levels produced by effector CD4+ T cells. Hence, this peptide has the ability to increase the frequency of Tregs and their suppressive properties whereas effector T cells produce less IL-17. Thus, we propose that APL1 therapy could help to ameliorate the pathogenic Th17/Treg balance in RA patients. 相似文献
946.
B. Pérez-Vich L. del Moral L. Velasco B. S. Bushman S. J. Knapp A. Leon J. M. Fernández-Martínez S. T. Berry 《Molecular breeding : new strategies in plant improvement》2016,36(4):43
Sunflower (Helianthus annuus L.) seed oil with high palmitic acid content has enhanced thermo-oxidative stability, which makes it well suited to high-temperature uses. CAS-5 is a sunflower mutant line that accumulates over 25 % palmitic acid in its seed oil, compared to 5–8 % in conventional cultivars. The objective of this study was to investigate the molecular basis of the high-palmitic acid trait in CAS-5 through both candidate gene and QTL mapping approaches. An F2 population derived from the cross between CAS-5 and the conventional line HA-89 was developed. A 3-ketoacyl-ACP synthase II (KASII) locus on a telomeric region of linkage group (LG) 9 of the sunflower genetic map was found to co-segregate with palmitic acid content in this population. The KASII locus explained the vast majority of the phenotypic variation (98 %) of the trait. Two minor QTL affecting palmitic acid content were also found on the lower half of LG 9 and on LG 17. Additionally, QTL associated with other major fatty acids (stearic, oleic, and linoleic acid) were identified on LG 1, 6, and 10. This result may reflect untapped genetic variation that could exist among sunflower cultivars for genes determining fatty acid composition. In addition to demonstrating the major role of a KASII locus in the accumulation of high levels of palmitic acid in CAS-5 seeds, this study stressed the importance of characterizing genes with minor effects on fatty acid profile in order to establish optimal breeding strategies for modifying fatty acid composition in sunflower seed oil. 相似文献
947.
948.
949.
950.