全文获取类型
收费全文 | 1524篇 |
免费 | 177篇 |
国内免费 | 213篇 |
专业分类
1914篇 |
出版年
2024年 | 6篇 |
2023年 | 38篇 |
2022年 | 44篇 |
2021年 | 82篇 |
2020年 | 64篇 |
2019年 | 76篇 |
2018年 | 69篇 |
2017年 | 72篇 |
2016年 | 81篇 |
2015年 | 65篇 |
2014年 | 83篇 |
2013年 | 103篇 |
2012年 | 76篇 |
2011年 | 64篇 |
2010年 | 59篇 |
2009年 | 79篇 |
2008年 | 90篇 |
2007年 | 80篇 |
2006年 | 67篇 |
2005年 | 61篇 |
2004年 | 43篇 |
2003年 | 50篇 |
2002年 | 46篇 |
2001年 | 34篇 |
2000年 | 34篇 |
1999年 | 27篇 |
1998年 | 22篇 |
1997年 | 24篇 |
1996年 | 15篇 |
1995年 | 19篇 |
1994年 | 26篇 |
1993年 | 18篇 |
1992年 | 26篇 |
1991年 | 20篇 |
1990年 | 16篇 |
1989年 | 9篇 |
1988年 | 13篇 |
1987年 | 9篇 |
1986年 | 12篇 |
1985年 | 11篇 |
1984年 | 15篇 |
1983年 | 10篇 |
1982年 | 16篇 |
1981年 | 12篇 |
1980年 | 12篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1974年 | 2篇 |
排序方式: 共有1914条查询结果,搜索用时 15 毫秒
1.
2.
Georg Steinhauser Johannes H. Sterba Karin Poljanc Max Bichler Karl Buchtela 《Journal of trace elements in medicine and biology》2006,20(3):119-153
In this study, 18 partly commercially available samples of rock salt from Austria, Germany, Pakistan, Poland, Switzerland, and Ukraine were investigated with respect to their content of trace elements using instrumental neutron activation analysis. Elements detected were Al, Ba, Br, Ca, Ce, Cl, Co, Cr, Cs, Eu, Fe, Hf, La, Mn, Na, Rb, Sb, Sc, Sm, Sr, Ta, Tb, Th, and Zn, some of them only in individual cases. An estimation of the bioavailability of these trace elements was performed by dissolving an equivalent of the sodium chloride samples in diluted hydrochloric acid (simulating stomach acid), filtering off the insoluble components, and analyzing the evaporated filtrate. It could be shown that in most cases bioactive trace elements like Fe can be found in rock salt in the form of almost insoluble compounds and are therefore not significantly bioavailable, whereas thorium, for example, was partly bioavailable in two cases. A significant contribution to the recommended daily intake of metal trace elements by using rock salt for nutrition can be excluded. 相似文献
3.
4.
J. G. H. Roebroek D. G. Stavenga 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1990,166(4):537-543
Summary The spectral characteristics of the pupil mechanism in blowfly photoreceptors and their dependence on light intensity have been investigated together with the intensity dependence of the receptor potential. The threshold for the pupil response as measured by reflectance is found at an intensity at which the peak of the receptor potential is about half maximal and the plateau potential starts to saturate. The reflectance saturates at about 3 log-units above threshold. The reflectance spectrum peaks near 620 nm, and its shape is independent of adaptation intensity. The absorbance change, measured by transmission, is extreme in the blue, at about 470 nm. The shape of the absorbance spectrum is slightly intensity dependent, presumably due to optical waveguide effects. The dynamic ranges of the light-induced reflectance and absorbance changes do not coincide. The reflectance change shows saturation at least 1 to 1.5 log units before the absorbance change saturates. 相似文献
5.
6.
7.
VIRGINIA SEYMOUR 《Plant, cell & environment》1985,8(9):631-638
Abstract The leaves of Berberis aquifolium (Pursh.) exhibit either diffuse or specular (shiny) reflection, depending on the variety, but in no case are the leaves obviously glaucous. The dull-surfaced leaves were less wettable than the glossy ones. Using scanning electron microscopy it was determined that the diffuse reflection was due to tubular crystals of wax 250 nm in diameter. The crystals were primarily composed of 19-nonacosanol, a 29-carbon secondary alcohol, as determined by gas chromatography-mass spectrometry. The chemical constituents of the wax underlying the tubes appeared to be the same as those of the wax from glossy leaves, with 29-carbon and 31-carbon n-alkanes and n-heptacosanol as major constituents. The reflection spectra of dull-surfaced (diffuse reflection) or glossy (specular reflection) leaves were the same, as were those of leaves with different amounts of epicuticular wax. Removing the epicuticular wax with chloroform did not change the spectrum. 相似文献
8.
Bisulfite reversibly inhibits the growth of a variety of microorganisms and has been used as a preservative in foods and beverages for that reason. We have now measured macromolecule synthesis in Escherichia coli K12 after bisulfite treatment. RNA synthesis, the synthesis of total protein, and of an inducible enzyme, beta-galactosidase, stopped almost immediately upon addition of 2 mM (or higher concentrations) of bisulfite. These functions resumed after a lag whose duration depended on the concentration of bisulfite added. The synthesis of DNA was slowed upon bisulfite addition, but did not stop entirely. The inhibition of RNA synthesis by bisulfite took place in both stringent and relaxed strains of E. coli and was not relieved upon addition of chloramphenicol. Stringent control was therefore not involved in this effect. No effect on protein synthesis was observed in the cell-free system of E. coli (using poly(U) or MS2 RNA as messenger) at bisulfite concentrations up to 10 mM. Protein synthesis inhibition in vivo was apparently not due to a reaction of bisulfite with a component of this system. In additional experiments, RNA polymerase was not impaired by bisulfite, and the growth inhibition effect was shown to proceed in the presence of inhibitors of free radical chain reactions. 相似文献
9.
Photoinhibition of photosynthesis in the brown alga, Dictyota dichotoma, was studied with a PAM fluorometer (Walz, Effeltrich, Germany) and a homemade oxygen measuring device. As a measure of fluorescence, Fv/Fm, and for the photosynthetic yield, ΔF/Fm', were used. Oxygen measurements show clearly that the observed degree, as well as the time course, of photoinhibition depends on the fluence rate of the light used to measure changes of the production rate. After photoinhibition of photosynthesis the depression of oxygen production caused by non-saturating fluence rates was generally much more pronounced than that caused by saturating or nearly saturating fluence rates. At minimal photoinhibition the initial slope and the convexity of the fluence rate-response curve of oxygen evolution decrease, whereas the level of light saturation decreases only after strong photoinhibition. Nevertheless, at different degrees of photoinhibition, changes in the degree of the upper bending of the fluence rate-response curve of oxygen production are also linearly correlated to changes in the fluorescence ratios (Fv/Fm and ΔF/Fm'). The action spectrum of photoinhibition, calculated on the basis of changes of Fv/Fm, indicates that the reaction center of PS I is not involved in photoinhibition. The lower effectiveness of blue light in comparison to effects of green and red light may be due to chloroplast displacement, as in the so-called strong light position, the light absorbed by the thalli in vivo is decreased. 相似文献
10.