全文获取类型
收费全文 | 2939篇 |
免费 | 219篇 |
国内免费 | 285篇 |
专业分类
3443篇 |
出版年
2024年 | 6篇 |
2023年 | 25篇 |
2022年 | 39篇 |
2021年 | 52篇 |
2020年 | 52篇 |
2019年 | 74篇 |
2018年 | 91篇 |
2017年 | 61篇 |
2016年 | 80篇 |
2015年 | 77篇 |
2014年 | 92篇 |
2013年 | 193篇 |
2012年 | 68篇 |
2011年 | 111篇 |
2010年 | 98篇 |
2009年 | 171篇 |
2008年 | 176篇 |
2007年 | 207篇 |
2006年 | 183篇 |
2005年 | 135篇 |
2004年 | 133篇 |
2003年 | 119篇 |
2002年 | 99篇 |
2001年 | 69篇 |
2000年 | 69篇 |
1999年 | 78篇 |
1998年 | 59篇 |
1997年 | 55篇 |
1996年 | 66篇 |
1995年 | 46篇 |
1994年 | 57篇 |
1993年 | 53篇 |
1992年 | 64篇 |
1991年 | 40篇 |
1990年 | 37篇 |
1989年 | 37篇 |
1988年 | 41篇 |
1987年 | 34篇 |
1986年 | 31篇 |
1985年 | 36篇 |
1984年 | 49篇 |
1983年 | 26篇 |
1982年 | 30篇 |
1981年 | 28篇 |
1980年 | 35篇 |
1979年 | 23篇 |
1978年 | 11篇 |
1977年 | 8篇 |
1974年 | 3篇 |
1973年 | 7篇 |
排序方式: 共有3443条查询结果,搜索用时 15 毫秒
41.
A simple and reliable method is described which combines ultrafiltration technique with atomic absorption spectrophotometry
to determine the Zn fractions in human blood plasma and seminal plasma. Ultrafiltrable, loosely bound, and firmly bound Zn
can be measured using this method in the presence or absence of ethylene-diaminetetraacetic acid (EDTA). The YMT membranes
for the ultrafiltration must be rinsed thoroughly before use. In contrast to Zn in blood plasma, a large part of Zn in the
seminal plasma was found to be ultrafiltrabe. This method can be applied to study the physiologically active part of Zn in
body fluids related to various disease states. 相似文献
42.
Jose-Emilio Mesonero María-Carmen Rodríguez Yoldi María-Jesús Rodríguez Yoldi 《Biological trace element research》1996,51(2):149-159
The element Cd is considered to have no biological function and is highly toxic to humans and animals. Toxic effects of this
metal upon cell membrane structure and function have been shown. On the other hand, Ca is an essential element in a wide variety
of cellular activities. The present study was initiated to research whether the interaction between Ca and Cd could affect
D-galactose absorption across the rabbit jejunum in vitro. In media with Ca2+, when CdCl2 was present at 0.5 or 1 mM, Cd was found to significantly reduce the sugar absorption. In Ca2+-free media, where CaCl2, was omitted and replaced isotonically with choline chloride, the sugar transport was not modified by Cd, but when CaCl2 was replaced isotonically with MgCl2, the inhibition is observed. Verapamil at 10−6
M (blocking mainly Ca2+ transport) did not modify the inhibitory effect of cadmium on D-galactose transport. When 10−6
M of A 23187 (Ca2+ specific ionophore) was added in media with/without Ca2+; CdCl2 produced no change in D-galactose transport. These results suggest that Ca and Cd could have affinity for the same chemical
groups of enterocyte membrane, which would be related with the intestinal absorption of D-galactose. 相似文献
43.
Antonio Cupane Eugenio Vitrano Pal Ormos G.Ulrich Nienhaus 《Biophysical chemistry》1996,60(3):111-117
We have measured the Soret band of the photoproduct obtained by complete photolysis of sperm whale carbonmonoxymyoglobin at 10 K. The experimental spectrum has been modeled with an analytical expression that takes into account the homogeneous bandwidth, the coupling of the electronic transition with both high and low frequency vibrational modes, and the effects of static conformational heterogeneity. The comparison with deoxymyoglobin at low temperature reveals three main differences. In the photoproduct, the Soret band is shifted to red. The band is less asymmetric, and an enhanced coupling to the heme vibrational mode at 674 cm−1 is observed. These differences reflect incomplete relaxation of the active site after ligand dissociation. The smaller band asymmetry of the photoproduct can be explained by a smaller displacement of the iron atom from the mean porphyrin plane, in quantitative agreement with the X-ray structure analysis. The enhanced vibrational coupling is attributed to a subtle heme distortion from the planar geometry that is barely detectable in the X-ray structure. 相似文献
44.
45.
Chang-Hwei Chen Laurence G. Roth Robert MacColl Donald S. Berns 《Biophysical chemistry》1994,50(3):313-321
The structural stability of the protein, phycocyanin isolated from two strains of cyanophyta, Synechococcus lividus (thermophile) and Phormidium luridum (mesophile), are investigated by comparative thermal and denaturant unfolding, using differential scanning calorimetry, visible absorption spectrophotometry, and circular dichroism. The thermophilic protein exhibits a much higher temperature and enthalpy of unfolding from the native to the denatured state. The concentration of urea at half-completion of thermal unfolding is essentially the same between the thermophilic and mesophilic proteins; in contrast, the corresponding temperature and the enthalpy of thermal unfolding are much higher for the thermophilic protein. In addition, the concentration of urea at which the non-thermal (denaturant) unfolding of protein is half-completed, as detected by either circular dichroism or absorption spectroscopy, is significantly higher in the thermophilic protein, while the apparent free energy of unfolding only shows a moderate difference between the two proteins. The distinct differences in the enthalpy of thermal unfolding and the free energy of denaturant unfolding are interpreted in terms of a significant entropy change associated with the unfolding of these proteins. This entropy contribution is much higher in the thermophilic protein, and may be derived from its more rigid overall structure that possesses higher internal hydrophobicity and stronger internal packing. 相似文献
46.
The dynamic pressure method (DPM) is used for measurement of k(L)a in a 1-m(3) pilot scale fermentor in coalescing (distilled water) and noncoalescing (0.3 M Na(2)SO(4) aqueous solution) batches. The method consists in recording oxygen concentration in a batch after a small pressure change (20 kPa) in the fermentor. The upward pressure change is brought about by temporary closing and subsequent throttling of outlet gas stream and the downward change by full reopening of the gas outlet. Absorption of pure oxygen yields the same k(L)a values as absorption of air. In noncoalescing batch, the downward k(L)a values are always higher than the upward values owing to spontaneous nucleation of bubbles. The experiments performed in a stirred cell confirm this behavior. Thus, only upward pressure change should be used for measurement. The correlation of k(L)a data measured in small (18-L) and large (1000-L) vessels based on power dissipated and superficial gas velocity are in a good agreement. Unlike the DPM, the classical dynamic methods yield, under the same conditions, excessively low values of k(L)a (the dynamic startup method) or fail to produce data at all (the dynamic method with interchange of air for N(2)). (c) 1994 John Wiley & Sons, Inc. 相似文献
47.
Modelling root architecture: are there tradeoffs between efficiency and potential of resource acquisition? 总被引:5,自引:1,他引:4
G. M. BERNTSON 《The New phytologist》1994,127(3):483-493
48.
49.
J. Sklenar G. G. Fox B. C. Loughman A. D. B. Pannifer R. G. Ratcliffe 《Plant and Soil》1994,167(1):57-62
Although the sensitivity of the plasma membrane H+-ATPase to vanadate is well known, the metabolic response of plant cells to vanadate is less well characterised in vivo and its use as an inhibitor in whole plant experiments has had mixed success. Experiments with maize (Zea mays, L.) roots and with purified plasma membrane fractions from the same tissues showed that exposure to vanadate caused: (i) a reduction in the capacity for phosphate uptake; (ii) a reduction in the extractable ATPase activity from the tissue; and (iii) a significant increase in the ATP level. The measurements on the extractable ATPase activity and the ATP level showed that the effect of vanadate developed slowly, apparently reflecting the slow accumulation of intracellular vanadate. The marked effect of vanadate on the ATP level-exposure to 500 M vanadate for 5 h doubled the ATP content of the roots tips-indicates that there is no stringent control over the ATP level in the roots and that the plasma membrane H+-ATPase activity is likely to have a significant role in determining the ATP level under normal conditions. 相似文献
50.
Fine root growth and demographic responses to nutrient patches in four old-field plant species 总被引:10,自引:0,他引:10
Proliferation of roots in a nutrient patch can occur either as a result of an increase in root length (morphological response) or by a change in root birth or death rates (demographic responses). In this study we attempted to distinguish between these two mechanisms of response to nutrient patches and to compare the responses of four old-field plant species (two annuals, two perennials). For all four species combined, there were significant increases in root numbers and root length in fertilized patches. Root proliferation in fertilized patches was largely due to increased birth (=branching) rates of new roots. However, there was also a significant increase in root death rates in the fertilized patches which reduced the magnitude of the increase in net root numbers. Plots for individual species suggested they differed in the magnitude and timing of root proliferation in fertilized patches due to differences in root birth and death rates. However, because of the limited sample size in this study, there was only a marginally significant difference among species in root birth rates, and no difference in death rates. Further studies are currently underway to better quantify species differences in the demographic mechanism, as well as magnitude, of response to nutrient patches and if this would affect the ability to exploit small-scale heterogeneity in soil resources. 相似文献