全文获取类型
收费全文 | 515篇 |
免费 | 0篇 |
国内免费 | 4篇 |
专业分类
系统科学 | 3篇 |
教育与普及 | 4篇 |
现状及发展 | 109篇 |
研究方法 | 56篇 |
综合类 | 317篇 |
自然研究 | 30篇 |
出版年
2018年 | 2篇 |
2014年 | 2篇 |
2013年 | 3篇 |
2012年 | 21篇 |
2011年 | 64篇 |
2010年 | 8篇 |
2008年 | 27篇 |
2007年 | 23篇 |
2006年 | 19篇 |
2005年 | 23篇 |
2004年 | 9篇 |
2003年 | 11篇 |
2002年 | 17篇 |
2001年 | 18篇 |
2000年 | 15篇 |
1999年 | 9篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1992年 | 9篇 |
1991年 | 5篇 |
1990年 | 12篇 |
1989年 | 6篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1983年 | 9篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 12篇 |
1976年 | 17篇 |
1975年 | 3篇 |
1974年 | 7篇 |
1973年 | 5篇 |
1972年 | 15篇 |
1971年 | 20篇 |
1970年 | 14篇 |
1969年 | 4篇 |
1968年 | 11篇 |
1967年 | 3篇 |
1966年 | 8篇 |
1965年 | 8篇 |
1964年 | 1篇 |
1961年 | 1篇 |
1960年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有519条查询结果,搜索用时 15 毫秒
91.
High resolution studies on the crystal structure of glycogen phosphorylase b have identified the catalytic site to which the substrate glucose-1-phosphate binds strongly with some local conformational changes. The site is situated 8 A (phosphate-to-phosphate distance) from pyridoxal phosphate, an essential cofactor of all glycogen phosphorylases. The catalytic site is 33 A from the site in the N-terminal portion of the molecule to which adenine nucleotides bind. In contrast to phosphorylase a (the active form of the enzyme which is phosphorylated at Ser 14), the positions of the first 19 residues of phosphorylase b are not well defined. 相似文献
92.
Rieger syndrome, an autosomal dominant disorder, includes ocular, craniofacial and umbilical abnormalities. The pitx2 homeobox gene, which is mutated in Rieger syndrome, has been proposed to be the effector molecule interpreting left-right axial information from the early embryonic trunk to each organ. Here we have used gene targeting in mice to generate a loss-of-function allele that would be predicted to result in organ randomization or isomerization. Although pitx2-/- embryos had abnormal cardiac morphogenesis, mutant hearts looped in the normal direction. Pitx2-/- embryos had correctly oriented, but arrested, embryonic rotation and right pulmonary isomerism. They also had defective development of the mandibular and maxillary facial prominences, regression of the stomodeum and arrested tooth development. Fgf8 expression was absent, and Bmp4 expression was expanded in the branchial-arch ectoderm. These data reveal a critical role for pitx2 in left-right asymmetry but indicate that pitx2 may function at an intermediate step in cardiac morphogenesis and embryonic rotation. 相似文献
93.
94.
95.
96.
Optical imaging is a promising method to identify and locate 2D materials efficiently and non-invasively. By putting a 2D material on a substrate, the nanolayer will add to an optical path and create a contrast to the case when the nanolayer is absent. This optical contrast imaging can be used to identify the 2D material and its number of layers. To make the optical imaging process in the laboratories an effective tool, Fresnel Law as a model was used to simulate the optical imaging results of 2D materials(graphene, Mo S2 and MoSe_2) on top of different thickness of SiO_2 and Si wafer in the present investigation. The results provide the details of the optimal conditions(optimal light wavelength and optimal SiO_2 thickness) to identify and locate single to few 2D nanolayers, which can be used directly in laboratories. The optical contrasts of 1–5 layers of molybdenum disulfide(MoS_2) and molybdenum diselenide(MoSe_2) were simulated. To the best of our knowledge, it is the first time that the optical contrast results of MoSe_2 have been reported. In particular, this work highlights the sensitivity of the model on the accuracy of the refractive indices used. It is demonstrated that through computational modeling that optical contrast can allow effective determination of number of layers in few layer 2D materials. 相似文献
97.
随着计算机辅助设计、加工、制造等应用的迅速发展,设计相关信息与知识管理面临信息量迅猛增长的挑战与机遇。其中,本体应用成为描述设计信息与知识的一种可行、有效及丰富的建模手段。本文通过回顾本体应用于设计工程领域的相关研究,对设计工程中基于本体的应用进行了整体分类,阐明了设计工程领域中本体应用的核心学术研究问题,并为后续相关的5项研究热点及未来发展方向提出了建议。 相似文献
98.
Chao Li Nicholas Johnson Victor Ostanin Andrew Shevchuk Liming Ying Yuri Korchev David Klenerman 《自然科学进展(英文版)》2008,18(6):671-678
Microscopy is an essential technique for observation on living cells. There is currently great interest in applying scanning probe microscopy to image-living biological cells in their natural environment at the nanometer scale. Scanning ion conductance microscopy is a new form of scanning probe microscopy, which enables non-contact high-resolution imaging of living biological cells. Based on a scanned nanopipette in physiological buffer, the distance feedback control uses the ion current to control the distance between the pipette tip and the sample surface. However, this feedback control has difficulties over slopes on convoluted cell surfaces, which limits its resolution. In this study, we present an improved form of feedback control that removes the contribution of up to the third-order slope from the ion current signal, hence providing a more accurate signal for controlling the distance. We show that this allows faster and lower noise topographic high-resolution imaging. 相似文献
99.
The question of whether the plankton communities in low-nutrient regions of the ocean, comprising 80% of the global ocean surface area, are net producers or consumers of oxygen and fixed carbon is a key uncertainty in the global carbon cycle. Direct measurements in bottle experiments indicate net oxygen consumption in the sunlit zone, whereas geochemical evidence suggests that the upper ocean is a net source of oxygen. One possible resolution to this conflict is that primary production in the gyres is episodic and thus difficult to observe: in this model, oligotrophic regions would be net consumers of oxygen during most of the year, but strong, brief events with high primary production rates might produce enough fixed carbon and dissolved oxygen to yield net production as an average over the annual cycle. Here we examine the balance of oxygen production over three years at sites in the North and South Pacific subtropical gyres using the new technique of oxygen sensors deployed on profiling floats. We find that mixing events during early winter homogenize the upper water column and cause low oxygen concentrations. Oxygen then increases below the mixed layer at a nearly constant rate that is similar to independent measures of net community production. This continuous oxygen increase is consistent with an ecosystem that is a net producer of fixed carbon (net autotrophic) throughout the year, with episodic events not required to sustain positive oxygen production. 相似文献
100.
Fourier analysis of vertebral shape change due to growth and selection for large and small body size
P. O'Higgins D. R. Johnson T. J. McAndrew 《Cellular and molecular life sciences : CMLS》1988,44(9):776-777
Summary Fourier analysis of videodigitised outlines of mouse vertebrae from two stocks, a pseudo-longitudinal series of mice aged 25–60 days and one selected for large or small body size over many generations shows that the shape changes due to normal growth are not similar to those produced by selection for body size. 相似文献