全文获取类型
收费全文 | 5608篇 |
免费 | 448篇 |
国内免费 | 3篇 |
专业分类
6059篇 |
出版年
2023年 | 42篇 |
2022年 | 114篇 |
2021年 | 186篇 |
2020年 | 80篇 |
2019年 | 106篇 |
2018年 | 142篇 |
2017年 | 106篇 |
2016年 | 200篇 |
2015年 | 294篇 |
2014年 | 331篇 |
2013年 | 405篇 |
2012年 | 467篇 |
2011年 | 482篇 |
2010年 | 285篇 |
2009年 | 263篇 |
2008年 | 291篇 |
2007年 | 310篇 |
2006年 | 284篇 |
2005年 | 256篇 |
2004年 | 231篇 |
2003年 | 205篇 |
2002年 | 187篇 |
2001年 | 47篇 |
2000年 | 42篇 |
1999年 | 49篇 |
1998年 | 55篇 |
1997年 | 40篇 |
1996年 | 29篇 |
1995年 | 33篇 |
1994年 | 25篇 |
1993年 | 29篇 |
1992年 | 26篇 |
1991年 | 25篇 |
1990年 | 27篇 |
1989年 | 25篇 |
1988年 | 24篇 |
1987年 | 12篇 |
1986年 | 12篇 |
1985年 | 25篇 |
1984年 | 22篇 |
1983年 | 21篇 |
1982年 | 23篇 |
1981年 | 24篇 |
1980年 | 25篇 |
1979年 | 23篇 |
1978年 | 25篇 |
1977年 | 13篇 |
1974年 | 13篇 |
1970年 | 9篇 |
1968年 | 10篇 |
排序方式: 共有6059条查询结果,搜索用时 0 毫秒
81.
Atsushi Nishida Dorina Cadar Man K. Xu Timothy Croudace Peter B. Jones Diana Kuh Marcus Richards MRC National Survey of Health Development scientific data collection team 《PloS one》2016,11(1)
Variations in markers of adolescent self-organization predict a range of economic and health-related outcomes in general population studies. Using a population-based birth cohort study we investigated associations between adolescent self-organization and two common factors over adulthood influencing health, smoking and alcohol consumption. The MRC National Survey of Health and Development (the British 1946 birth cohort) was used to test associations between a dimensional measure of adolescent self-organization derived from teacher ratings, and summary longitudinal measures of smoking and alcohol consumption over the ensuing five decades. Multinomial regression models were adjusted for sex, adolescent emotional and conduct problems, occupational social class of origin, childhood cognition, educational attainment and adult occupational social class. With all covariates adjusted, higher adolescent self-organization was associated with fewer smoking pack years, although not with quitting; there was no association with alcohol consumption across adulthood (none or heavy compared with light to moderate). Adolescent self-organization appears to be protective against smoking, but not against heavy alcohol consumption. Interpretation of this differential effect should be embedded in an understanding of the social and sociodemographic context in which these health behaviours occur over time. 相似文献
82.
Diana E. Bowler Mikkel A. J. Kvasnes Hans C. Pedersen Brett K. Sandercock Erlend B. Nilsen 《Proceedings. Biological sciences / The Royal Society》2020,287(1941)
According to classic theory, species'' population dynamics and distributions are less influenced by species interactions under harsh climatic conditions compared to under more benign climatic conditions. In alpine and boreal ecosystems in Fennoscandia, the cyclic dynamics of rodents strongly affect many other species, including ground-nesting birds such as ptarmigan. According to the ‘alternative prey hypothesis’ (APH), the densities of ground-nesting birds and rodents are positively associated due to predator–prey dynamics and prey-switching. However, it remains unclear how the strength of these predator-mediated interactions change along a climatic harshness gradient in comparison with the effects of climatic variation. We built a hierarchical Bayesian model to estimate the sensitivity of ptarmigan populations to interannual variation in climate and rodent occurrence across Norway during 2007–2017. Ptarmigan abundance was positively linked with rodent occurrence, consistent with the APH. Moreover, we found that the link between ptarmigan abundance and rodent dynamics was strongest in colder regions. Our study highlights how species interactions play an important role in population dynamics of species at high latitudes and suggests that they can become even more important in the most climatically harsh regions. 相似文献
83.
Hernandez-Prieto MA Tibiletti T Abasova L Kirilovsky D Vass I Funk C 《Biochimica et biophysica acta》2011,1807(9):1143-1151
The five small CAB-like proteins (ScpA-E) of the cyanobacterium Synechocystis sp. PCC 6803 belong to the family of stress-induced light-harvesting-like proteins, but are constitutively expressed in a mutant deficient of Photosystem I (PSI). Using absorption, fluorescence and thermoluminescence measurements this PSI-less strain was compared with a mutant, in which all SCPs were additionally deleted. Depletion of SCPs led to structural rearrangements in Photosystem II (PSII): less photosystems were assembled; and in these, the Q(B) site was modified. Despite the lower amount of PSII, the SCP-deficient cells contained the same amount of phycobilisomes (PBS) as the control. Although the excess PBS were functionally disconnected, their fluorescence was quenched under high irradiance by the activated Orange Carotenoid Protein (OCP). Additionally the amount of OCP, but not of the iron-stress induced protein (isiA), was higher in this SCP-depleted mutant compared with the control. As previously described, the lack of SCPs affects the chlorophyll biosynthesis (Vavilin, D., Brune, D. C., Vermaas, W. (2005) Biochim Biophys Acta 1708, 91-101). We demonstrate that chlorophyll synthesis is required for efficient PSII repair and that it is partly impaired in the absence of SCPs. At the same time, the amount of chlorophyll also seems to influence the expression of ScpC and ScpD. 相似文献
84.
Hoffmann T Bös M Stadler H Schnider P Hunkeler W Godel T Galley G Ballard TM Higgins GA Poli SM Sleight AJ 《Bioorganic & medicinal chemistry letters》2006,16(5):1362-1365
The discovery of a novel, achiral pyridine class of potent and orally active neurokinin-1 (NK(1)) receptor antagonists is described. The evaluation of this class is briefly outlined, leading to the identification of netupitant 21 and befetupitant 29, two new proprietary chemical entities with high affinity and excellent CNS penetration. 相似文献
85.
Clara E. Cho Diana Sánchez-Hernández Sandra A. Reza-López Pedro S.P. Huot Young-In Kim G. Harvey Anderson 《Epigenetics》2013,8(7):710-719
Our aim was to comprehensively analyze promoter hypermethylation of a panel of novel and known methylation markers for thyroid neoplasms and to establish their relationship with BRAF mutation and clinicopathologic parameters of thyroid cancer. A cohort of thyroid tumors, consisting of 44 cancers and 44 benign thyroid lesions, as well as 15 samples of adjacent normal thyroid tissue, was evaluated for BRAF mutation and promoter hypermethylation. Genes for quantitative methylation specific PCR (QMSP) were selected by a candidate gene approach. Twenty-two genes were tested: TSHR, RASSF1A, RARβ2, DAPK, hMLH1, ATM, S100, p16, CTNNB1, GSTP1, CALCA, TIMP3, TGFßR2, THBS1, MINT1, CTNNB1, MT1G, PAK3, NISCH, DCC, AIM1 and KIF1A. The PCR-based “mutector assay” was used to detect BRAF mutation. All p values reported are two sided. Considerable overlap was seen in the methylation markers among the different tissue groups. Significantly higher methylation frequency and level were observed for KIF1A and RARß2 in cancer samples compared with benign tumors. A negative correlation between BRAF mutation and RASSF1A methylation, and a positive correlation with RARß2 methylation were observed in accordance with previous results. In addition, positive correlation with TIMP3 and a marginal correlation with DCC methylation were observed. The present study constitutes a comprehensive promoter methylation profile of thyroid neoplasia and shows that results must be analyzed in a tissue-specific manner to identify clinically useful methylation markers. Integration of genetic and epigenetic changes in thyroid cancer will help identify relevant biologic pathways that drive its development. 相似文献
86.
PCOS (polycystic ovary syndrome) is a heterogeneous disease characterized by hyperandrogenaemia, hirsutism, oligo- or amenorrhea, insulin resistance and anovulation. The aim of the present study was to evaluate if the balance between the ovarian expression of Bax (proapoptotic protein) and Bcl-2 (antiapoptotic protein) is altered in a PCOS model developed in rats by DHEA (dehydroepiandrosterone) administration. In addition, the ovarian morphology and the circulating progesterone levels were evaluated. Histological studies confirmed the presence of follicular cysts, atretic follicles and the absence of corpora lutea in the ovaries from the PCOS group and a significant decrease in circulating progesterone levels. Immunohistochemical studies showed that the expression of Bcl-2 and Bax were mainly localized in granulosa cells of AFs (antral follicles) in both groups. Bax expression was greater in preantral and AFs from PCOS ovarian sections than in the controls. In contrast, intense Bcl-2 immunostaining was observed in the control AFs, while Bcl-2 protein was either absent in PFs (preantral follicles) or weakly expressed in AFs from PCOS rats. These results were partially confirmed by Western studies. Data revealed that the ovarian level of Bcl-2 protein was lower in PCOS than in the control and that there were no differences in Bax ovarian levels between groups. However, Bax/Bcl-2 ratio was significantly higher in PCOS group than in the control group. In conclusion, an increase in ovarian apoptosis through an imbalance among the Bcl-2 family members may be involved in the transformation of growing follicles in cystic follicles in the ovaries from DHEA-induced PCOS rats. 相似文献
87.
MDR-like ABC transporter AtPGP4 is involved in auxin-mediated lateral root and root hair development 总被引:17,自引:0,他引:17
Santelia D Vincenzetti V Azzarello E Bovet L Fukao Y Düchtig P Mancuso S Martinoia E Geisler M 《FEBS letters》2005,579(24):5399-5406
Previous data have suggested an involvement of MDR/PGP-like ABC transporters in transport of the plant hormone auxin and, recently, AtPGP1 has been demonstrated to catalyze the primary active export of auxin. Here we show that related isoform AtPGP4 is expressed predominantly during early root development. AtPGP4 loss-of-function plants reveal enhanced lateral root initiation and root hair lengths both known to be under the control of auxin. Further, atpgp4 plants show altered sensitivities toward auxin and the auxin transport inhibitor, NPA. Finally, mutant roots reveal elevated free auxin levels and reduced auxin transport capacities. These results together with yeast growth assays suggest a direct involvement of AtPGP4 in auxin transport processes controlling lateral root and root hair development. 相似文献
88.
E. Poli A. Bergmann F. J. Casson W. A. Hornsby A. G. Peeters M. Siccinio D. Zarzoso 《Plasma Physics Reports》2016,42(5):450-464
The role of the bootstrap and polarization currents for the stability of neoclassical tearing modes is investigated employing both a drift kinetic and a gyrokinetic approach. The adiabatic response of the ions around the island separatrix implies, for island widths below or around the ion thermal banana width, density flattening for islands rotating at the ion diamagnetic frequency, while for islands rotating at the electron diamagnetic frequency the density is unperturbed and the only contribution to the neoclassical drive arises from electron temperature flattening. As for the polarization current, the full inclusion of finite orbit width effects in the calculation of the potential developing in a rotating island leads to a smoothing of the discontinuous derivatives exhibited by the analytic potential on which the polarization term used in the modeling is based. This leads to a reduction of the polarization-current contribution with respect to the analytic estimate, in line with other studies. Other contributions to the perpendicular ion current, related to the response of the particles around the island separatrix, are found to compete or even dominate the polarization-current term for realistic island rotation frequencies. 相似文献
89.
90.