全文获取类型
收费全文 | 8084篇 |
免费 | 737篇 |
国内免费 | 8篇 |
专业分类
8829篇 |
出版年
2023年 | 50篇 |
2022年 | 120篇 |
2021年 | 289篇 |
2020年 | 132篇 |
2019年 | 183篇 |
2018年 | 182篇 |
2017年 | 163篇 |
2016年 | 282篇 |
2015年 | 463篇 |
2014年 | 464篇 |
2013年 | 491篇 |
2012年 | 653篇 |
2011年 | 647篇 |
2010年 | 383篇 |
2009年 | 321篇 |
2008年 | 497篇 |
2007年 | 385篇 |
2006年 | 383篇 |
2005年 | 351篇 |
2004年 | 308篇 |
2003年 | 312篇 |
2002年 | 255篇 |
2001年 | 111篇 |
2000年 | 96篇 |
1999年 | 100篇 |
1998年 | 65篇 |
1997年 | 41篇 |
1996年 | 52篇 |
1995年 | 38篇 |
1994年 | 28篇 |
1993年 | 41篇 |
1992年 | 55篇 |
1991年 | 61篇 |
1990年 | 47篇 |
1989年 | 37篇 |
1988年 | 57篇 |
1987年 | 48篇 |
1986年 | 42篇 |
1985年 | 39篇 |
1984年 | 31篇 |
1983年 | 37篇 |
1982年 | 29篇 |
1981年 | 31篇 |
1978年 | 25篇 |
1977年 | 27篇 |
1976年 | 29篇 |
1975年 | 26篇 |
1974年 | 38篇 |
1972年 | 29篇 |
1969年 | 21篇 |
排序方式: 共有8829条查询结果,搜索用时 15 毫秒
1.
I Djajanegara R Holtzapffel P M Finnegan M H Hoefnagel D A Berthold J T Wiskich D A Day 《FEBS letters》1999,454(3):220-224
The alternative oxidase is a quinol oxidase of the respiratory chain of plants and some fungi and protists. Its activity is regulated by redox-sensitive disulphide bond formation between neighbouring subunits and direct interaction with certain alpha-ketoacids. To investigate these regulatory mechanisms, we undertook site-directed mutagenesis of soybean and Arabidopsis alternative oxidase cDNAs, and expressed them in tobacco plants and Escherichia coli, respectively. The homologous C99 and C127 residues of GmAOX3 and AtAOX1a, respectively, were changed to serine. In the plant system, this substitution prevented oxidative inactivation of alternative oxidase and rendered the protein insensitive to pyruvate activation, in agreement with the recent results from other laboratories [Rhoads et al. (1998) J. Biol. Chem. 273, 30750-30756; Vanlerberghe et al. (1998) Plant Cell 10, 1551-1560]. However, the mutated protein is instead activated specifically by succinate. Measurements of AtAOX1a activity in bacterial membranes lacking succinate dehydrogenase confirmed that the stimulation of the mutant protein's activity by succinate did not involve its metabolism. Examples of alternative oxidase proteins with the C to S substitution occur in nature and these oxidases are expected to be activated under most conditions in vivo, with implications for the efficiency of respiration in the tissues which express them. 相似文献
2.
3.
4.
Joshua R Edwards Evangelos A Diamantakos Jacob D Peuler Peter C Lamar Walter C Prozialeck 《BMC physiology》2007,7(1):1
Background
Ethidium homodimer is a cell-membrane impermeant nuclear fluorochrome that has been widely used to identify necrotic cells in culture. Here, we describe a novel technique for evaluating necrosis of epithelial cells in the proximal tubule that involves perfusing ethidium homodimer through the intact rat kidney. As a positive control for inducing necrosis, rats were treated with 3.5, 1.75, 0.87 and 0.43 mg/kg mercuric chloride (Hg2+, intraperitoneal), treatments which have previously been shown to rapidly cause dose-dependent necrosis of the proximal tubule. Twenty-four h after the administration of Hg2+, ethidium homodimer (5 μM) was perfused through the intact left kidney while the animal was anesthetized. The kidney was then removed, placed in embedding medium, frozen and cryosectioned at a thickness of 5 μm. Sections were permeabilized with -20°C methanol and then stained with 4',6-diamidino-2-phenylindole (DAPI) to label total nuclei. Total cell number was determined from the DAPI staining in random microscopic fields and the number of necrotic cells in the same field was determined by ethidium homodimer labeling. 相似文献5.
Summary Spontaneous cell-to-cell transformation between naturally competent bacteria on selective media resulted in an overestimation of the transferability of genetic information. EDTA effectively prevented transformation on selective media whereas DNaseI did not reliably inhibit cell-to-cell transformation. An improved method to estimate gene transfer frequencies is described. 相似文献
6.
7.
8.
9.
Parathyroid hormone-related protein (PTHrP) can be secreted from cells and interact with its receptor, the Type 1 PTH/PTHrP Receptor (PTHR1) in an autocrine, paracrine or endocrine fashion. PTHrP can also remain inside cells and be transported into the nucleus, where its functions are unclear, although recent experiments suggest that it may broadly regulate cell survival and senescence. Disruption of either the PTHrP or PTHR1 gene results in many abnormalities including a failure of embryonic mammary gland development in mice and in humans. In order to examine the potential functions of nuclear PTHrP in the breast, we examined mammary gland development in PTHrP (1–84) knock-in mice, which express a mutant form of PTHrP that lacks the C-terminus and nuclear localization signals and which can be secreted but cannot enter the nucleus. Interestingly, we found that PTHrP (1–84) knock-in mice had defects in mammary mesenchyme differentiation and mammary duct outgrowth that were nearly identical to those previously described in PTHrP−/− and PTHR1−/− mice. However, the mammary buds in PTHrP (1–84) knock-in mice had severe reductions in mutant PTHrP mRNA levels, suggesting that the developmental defects were due to insufficient production of PTHrP by mammary epithelial cells and not loss of PTHrP nuclear function. Examination of the effects of nuclear PTHrP in the mammary gland in vivo will require the development of alternative animal models. 相似文献
10.
We conducted an experiment to assess the change in foraging efficiency resulting from diet-induced morphological and behavioural plasticity in a species of freshwater, threespine stickleback (Gasterosteus sp.). Different degrees of morphological and behavioural change were induced using two prey items commonly found in the diet of this species, allowing us to estimate the relative importance of each type of plasticity. The purpose of the experiment was twofold. First, earlier work had suggested that diet variability might be an important factor in the evolution of trophic morphological plasticity in sticklebacks. The present results extend this work by revealing the adaptive significance of morphological plasticity. The current experiment also qualitatively assessed the compatibility of the time scale of morphological change with that of the natural resource variability experienced by this species. The results indicate that diet-induced plasticity improves foraging efficiency continuously for up to 72 days of prey exposure. This is probably due in part to plasticity of the external trophic morphology but our results also suggest a complex interplay between morphology and behaviour. The time scale appears to be matched to that of natural diet variability although it is possible that some traits exhibit non-labile plasticity. Our discussion highlights the important distinction between conditions favouring the evolution of labile versus non-labile plasticity. The second objective of the experiment was to determine the relative importance of morphological and behavioural plasticity. Few studies have attempted to quantify the adaptive significance of morphological plasticity and no study to our knowledge has separated the effects of morphological and behavioural plasticity. Our experiment reveals that both behavioural and morphological plasticity are important and it also suggests a dichotomy between the two: behavioural plasticity predominately affects searching efficiency whereas morphological plasticity predominately affects handling efficiency. 相似文献