全文获取类型
收费全文 | 154018篇 |
免费 | 24102篇 |
国内免费 | 11142篇 |
专业分类
189262篇 |
出版年
2024年 | 288篇 |
2023年 | 1682篇 |
2022年 | 3911篇 |
2021年 | 6877篇 |
2020年 | 6280篇 |
2019年 | 8704篇 |
2018年 | 8549篇 |
2017年 | 7549篇 |
2016年 | 9153篇 |
2015年 | 11524篇 |
2014年 | 12696篇 |
2013年 | 13668篇 |
2012年 | 13602篇 |
2011年 | 12079篇 |
2010年 | 9472篇 |
2009年 | 7605篇 |
2008年 | 7577篇 |
2007年 | 6476篇 |
2006年 | 5689篇 |
2005年 | 4657篇 |
2004年 | 4039篇 |
2003年 | 3735篇 |
2002年 | 3245篇 |
2001年 | 2729篇 |
2000年 | 2369篇 |
1999年 | 2294篇 |
1998年 | 1330篇 |
1997年 | 1313篇 |
1996年 | 1228篇 |
1995年 | 1101篇 |
1994年 | 1052篇 |
1993年 | 782篇 |
1992年 | 1045篇 |
1991年 | 790篇 |
1990年 | 618篇 |
1989年 | 583篇 |
1988年 | 440篇 |
1987年 | 444篇 |
1986年 | 328篇 |
1985年 | 340篇 |
1984年 | 204篇 |
1983年 | 199篇 |
1982年 | 142篇 |
1981年 | 112篇 |
1980年 | 74篇 |
1979年 | 88篇 |
1977年 | 63篇 |
1975年 | 58篇 |
1974年 | 54篇 |
1973年 | 56篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Plant Molecular Biology - 相似文献
2.
Juan Viruel Pilar Catalán José Gabriel Segarra‐Moragues 《Journal of Biogeography》2012,39(6):1073-1085
Aim The Chilean endemic Dioscorea biloba (Dioscoreaceae) is a dioecious geophyte that shows a remarkable 600 km north–south disjunction in the peripheral arid area of the Atacama Desert. Its restricted present‐day distribution and probable Neogene origin indicate that its populations have a history linked to that of the Atacama Desert, making this an ideal model species with which to investigate the biogeography of the region. Location Chile, Atacama Desert and peripheral arid area. Methods Two hundred and seventy‐five individuals from nine populations were genotyped for seven nuclear microsatellite loci, and plastid trnL–F and trnT–L sequences were obtained for a representative subset of these. Analyses included the estimation of genetic diversity and population structure through clustering, Bayesian and analysis of molecular variance analyses, and statistical parsimony networks of chloroplast haplotypes. Isolation by distance was tested against alternative dispersal hypotheses. Results Microsatellite markers revealed moderate to high levels of genetic diversity within populations, with those from the southern Limarí Valley showing the highest values and northern populations showing less exclusive alleles. Bayesian analysis of microsatellite data identified three genetic groups that corresponded to geographical ranges. Chloroplast phylogeography revealed no haplotypes shared between northern and southern ranges, and little haplotype sharing between the two neighbouring southern valleys. Dispersal models suggested the presence of extinct hypothetical populations between the southern and northern ranges. Main conclusions Our results are consistent with prolonged isolation of the northern and southern groups, mediated by the life‐history traits of the species. Significant isolation was revealed at both large and moderate distances as gene flow was not evident even between neighbouring valleys. Bayesian analyses of microsatellite and chloroplast haplotype diversity identified the southern area of Limarí as the probable area of origin of the species. Our data do not support recent dispersal of D. biloba from the southern range into Antofagasta, but indicate the fragmentation of an earlier wider range, concomitant with the Pliocene–Pleistocene climatic oscillations, with subsequent extinctions of the Atacama Desert populations and the divergence of the peripheral ones as a consequence of genetic drift. 相似文献
3.
4.
5.
6.
7.
8.
Minjuan Shen Mingli Lin Mengqi Zhu Wenxin Zhang Danyang Lu Huanhuan Liu Jingjing Deng Kehua Que Xu Zhang 《Biochimica et Biophysica Acta (BBA)/General Subjects》2019,1863(1):167-181
Since their discovery, matrix vesicles (MVs) containing minerals have received considerable attention for their role in the mineralization of bone, dentin and calcified cartilage. Additionally, MVs' association with collagen fibrils, which serve as the scaffold for calcification in the organic matrix, has been repeatedly highlighted. The primary purpose of the present study was to establish a MVs–mimicking model (PEG-S-ACP/micelle) in vitro for studying the exact mechanism of MVs-mediated extra/intra fibrillar mineralization of collagen in vivo. In this study, high-concentration serine was used to stabilize the amorphous calcium phosphate (S-ACP), which was subsequently mixed with polyethylene glycol (PEG) to form PEG-S-ACP nanoparticles. The nanoparticles were loaded in the polysorbate 80 micelle through a micelle self-assembly process in an aqueous environment. This MVs–mimicking model is referred to as the PEG-S-ACP/micelle model. By adjusting the pH and surface tension of the PEG-S-ACP/micelle, two forms of minerals (crystalline mineral nodules and ACP nanoparticles) were released to achieve the extrafibrillar and intrafibrillar mineralization, respectively. This in vitro mineralization process reproduced the mineral nodules mediating in vivo extrafibrillar mineralization and provided key insights into a possible mechanism of biomineralization by which in vivo intrafibrillar mineralization could be induced by ACP nanoparticles released from MVs. Also, the PEG-S-ACP/micelle model provides a promising methodology to prepare mineralized collagen scaffolds for repairing bone defects in bone tissue engineering. 相似文献
9.
Characterization of phosphotyrosyl-protein phosphatase activity associated with calcineurin 总被引:3,自引:0,他引:3
J Chernoff M A Sells H C Li 《Biochemical and biophysical research communications》1984,121(1):141-148
Calcineurin purified from bovine brain is shown to possess phosphotyrosyl -protein phosphatase activity towards proteins phosphorylated by the epidermal growth factor receptor/kinase. The phosphatase activity is augmented by Ca2+/calmodulin or divalent cation (Ni2+ greater than Mn2+ greater than Mg2+ greater than Co2+). In the simultaneous presence of all three effectors, the enzymatic activity is synergistically increased. Ca2+/calmodulin activates the Mg2+-supported activity by decreasing the Km value for phosphotyrosyl -casein from 2.2 to 0.6 microM, and increasing the Vmax from 0.4 to 4.6 nmol/min/mg. These results represent the first demonstration that calcineurin can dephosphorylate phosphotyrosyl -proteins and suggest a novel mechanism of activation of this enzyme. 相似文献
10.
E Giannazzo F Licata G Li Volsi G Mio F Santangelo M Scivoli 《Bollettino della Società italiana di biologia sperimentale》1992,68(11):677-683
A computer-assisted analysis of the spatial distribution of neurons having homogeneous characteristics is described in this paper. The camera lucida drawings of sections of a brain nucleus and the points representing the neurons labeled on the basis of a specific behavior of discharge rates were digitized on a personal computer Amiga 2000 or IBM compatible. Our software provided: a) the computerized, stereotaxically oriented reconstruction of the stored sections and of the plotted neurons; b) the identification within each section of the mass center (MC) of the units sharing a given behavior and of the area where the density of such neurons was maximal (MDA). The routine was tested on the spatial distribution of neuronal responses to serotonin in the lateral vestibular nucleus. 相似文献