全文获取类型
收费全文 | 43148篇 |
免费 | 3862篇 |
国内免费 | 31篇 |
出版年
2022年 | 275篇 |
2021年 | 599篇 |
2020年 | 396篇 |
2019年 | 489篇 |
2018年 | 642篇 |
2017年 | 558篇 |
2016年 | 1046篇 |
2015年 | 1763篇 |
2014年 | 1881篇 |
2013年 | 2461篇 |
2012年 | 2987篇 |
2011年 | 3060篇 |
2010年 | 1948篇 |
2009年 | 1659篇 |
2008年 | 2519篇 |
2007年 | 2511篇 |
2006年 | 2279篇 |
2005年 | 2341篇 |
2004年 | 2293篇 |
2003年 | 2185篇 |
2002年 | 2133篇 |
2001年 | 507篇 |
2000年 | 407篇 |
1999年 | 512篇 |
1998年 | 608篇 |
1997年 | 462篇 |
1996年 | 412篇 |
1995年 | 394篇 |
1994年 | 356篇 |
1993年 | 347篇 |
1992年 | 385篇 |
1991年 | 327篇 |
1990年 | 270篇 |
1989年 | 273篇 |
1988年 | 274篇 |
1987年 | 251篇 |
1986年 | 234篇 |
1985年 | 302篇 |
1984年 | 322篇 |
1983年 | 258篇 |
1982年 | 351篇 |
1981年 | 275篇 |
1980年 | 256篇 |
1979年 | 190篇 |
1978年 | 222篇 |
1977年 | 208篇 |
1976年 | 178篇 |
1975年 | 163篇 |
1974年 | 189篇 |
1973年 | 195篇 |
排序方式: 共有10000条查询结果,搜索用时 344 毫秒
931.
Pilar Galan Helene Thibault Paul Preziosi Serge Hercberg 《Biological trace element research》1992,32(1-3):421-426
The relationship between iron status and capacity for IL-2 production by lymphocytes was assessed in 81 children from 6 mo
to 3 yr of age selected at random from a population with low socioeconomic status, undergoing free systematic examination
in four children's health centers in the Paris area. Iron deficiency was defined by the existence of at least two abnormal
values among the three indicators of iron status: serum ferritin level ≤12 μg/L, transferrin saturation <12%, and erythrocyte
protoporphyrin concentration >3 μg/g hemoglobin. According to this definition, 53 children were classified as iron deficient
and 28 as iron sufficient. No differences were observed between the iron-deficient and iron-sufficient groups in terms of
the IL-2 concentration without stimulation by PHA. IL-2 production by lymphocytes stimulated with PHA, as well as the stimulation
index (ratio of IL-2 concentration following stimulation by PHA to that of IL-2 concentration without stimulation by PHA)
were significantly lower in iron-deficient children. The reduction in IL-2 production by activated lymphocytes observed in
our study of iron-deficient children may be responsible for impairments in immunity found by other authors, particularly in
cell-mediated immunity. 相似文献
932.
933.
934.
935.
Agrobacterium and plant genetic engineering 总被引:27,自引:0,他引:27
Erratum
Agrobacterium and plant genetic engineering 相似文献936.
Summary Cellobiose oxidase from Phanerochaete chrysosporium was used for continuous monitoring of cellulase action on microcrystalline cellulose (Avicel). Two protocols are described, the parameter monitored being either the decline in electrode potential as ferricyanide is reduced or consumption of dioxygen. Most experiments used a commercial cellulase preparation from Trichoderma reesei and ferricyanide as acceptor. Within 1 min of an addition of cellulase, ferricyanide reduction reached a steady rate. This was converted into a rate of production of substrate for celobiose oxidase, in mol·min–1. Experiments were conducted either with a constant concentration of cellulase and increasing Avicel, or with constant Avicel and increasing cellulase. Kinetic analysis of the experiments with constant cellulase indicated a K
mof 4.8 ± 1.0 (g cellulose)·1–1, which was close to the value predicted from binding studies. The specific activity of the cellulase was measured as 375±25 mol·(g cellulase)–1·min–1 in experiments with a high cellulose concentration, but was less than half this value when the cellulose was saturated with cellulase. The maximal rate of cellulose degradation was 9.6±1.3 mol·(g cellulose)–1·min–1. 相似文献
937.
938.
Use of Bioluminescence Markers To Detect Pseudomonas spp. in the Rhizosphere 总被引:12,自引:7,他引:5
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Letty A. de Weger Paul Dunbar Walter F. Mahafee Ben J. J. Lugtenberg Gary S. Sayler 《Applied microbiology》1991,57(12):3641-3644
The use of bioluminescence as a sensitive marker for detection of Pseudomonas spp. in the rhizosphere was investigated. Continuous expression of the luxCDABE genes, required for bioluminescence, was not detectable in the rhizosphere. However, when either a naphthalene-inducible luxCDABE construct or a constitutive luxAB construct (coding only for the luciferase) was introduced into the Pseudomonas cells, light emission could be initiated just prior to measurement by the addition of naphthalene or the substrate for luciferase, n-decyl aldehyde, respectively. These Pseudomonas cells could successfully be detected in the rhizosphere by using autophotography or optical fiber light measurement techniques. Detection required the presence of 103 to 104 CFU/cm of root, showing that the bioluminescence technique is at least 1,000-fold more sensitive than β-galactosidase-based systems. 相似文献
939.
Structural investigation of bacteriorhodopsin and some of its photoproducts by polarized Fourier transform infrared spectroscopic methods-difference spectroscopy and photoselection 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《Biophysical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The direction of selected IR-transition moments of the retinal chromophore of bacteriorhodopsin (BR) and functional active amino acid residues are determined for light- and dark-adapted BR and for the intermediates K and L of the photocycle. Torsions around single bonds of the chromophore are found to be present in all the investigated BR states. The number of twisted single bonds and the magnitude of these torsions decreases in the order K, L, light-adapted BR, dark-adapted BR. In the last, only the C14—C15 single bond is twisted. The orientation of molecular planes and chemical bonds of such protein side chains, which are perturbed during the transition of light-adapted BR to the respective intermediates, are deduced and the results compared with the current three dimensional model of BR. Trp 86 and Trp 185 are found to form a rigid part of the protein, whereas Asp 96 and Asp 115 perform molecular rearrangements upon formation of the L-intermediate. 相似文献
940.
We develop a model of the undulatory locomotion of nematodes, in particular that of Caenorhabditis elegans, based on mechanics. The model takes into account the most important forces acting on a moving worm and allows the computer simulation of a creeping nematode. These forces are produced by the interior pressure in the liquid-filled body cavity, the elasticity of the cuticle, the excitation of certain sets of muscles and the friction between the body and its support.
We propose that muscle excitation patterns can be generated by stretch receptor control. By solving numerically the equations of motion of the model of the nematode, we demonstrate that these muscle excitation patterns are suitable for the propulsion of the animal.
相似文献