全文获取类型
收费全文 | 19382篇 |
免费 | 1997篇 |
国内免费 | 2807篇 |
出版年
2024年 | 85篇 |
2023年 | 354篇 |
2022年 | 840篇 |
2021年 | 1265篇 |
2020年 | 935篇 |
2019年 | 1144篇 |
2018年 | 1008篇 |
2017年 | 758篇 |
2016年 | 973篇 |
2015年 | 1379篇 |
2014年 | 1657篇 |
2013年 | 1596篇 |
2012年 | 1875篇 |
2011年 | 1690篇 |
2010年 | 1072篇 |
2009年 | 920篇 |
2008年 | 992篇 |
2007年 | 802篇 |
2006年 | 703篇 |
2005年 | 634篇 |
2004年 | 619篇 |
2003年 | 547篇 |
2002年 | 471篇 |
2001年 | 285篇 |
2000年 | 252篇 |
1999年 | 208篇 |
1998年 | 131篇 |
1997年 | 104篇 |
1996年 | 77篇 |
1995年 | 79篇 |
1994年 | 66篇 |
1993年 | 67篇 |
1992年 | 80篇 |
1991年 | 59篇 |
1990年 | 65篇 |
1989年 | 63篇 |
1988年 | 37篇 |
1987年 | 39篇 |
1986年 | 33篇 |
1985年 | 32篇 |
1984年 | 27篇 |
1983年 | 21篇 |
1982年 | 16篇 |
1981年 | 10篇 |
1980年 | 16篇 |
1977年 | 9篇 |
1972年 | 11篇 |
1971年 | 9篇 |
1970年 | 9篇 |
1969年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
It is well-known that Escherichia coli grows more slowly on gluconeogenic carbon sources than on glucose. This phenomenon has been attributed to either energy or monomer limitation. To investigate this problem further, we varied the expression levels of pck, encoding phosphoenolpyruvate carboxykinase (Pck), and pps, encoding phosphoenolpyruvate synthase (Pps). We found that the growth rates of E. coli in minimal medium supplemented with succinate and with pyruvate are limited by the levels of Pck and Pps, respectively. Optimal overexpression of pck or pps increases the unrestricted growth rates on succinate and on pyruvate, respectively, to the same level attained by the wild-type growth rate on glycerol. Since Pps is needed to supply precursors for biosyntheses, we conclude that E. coli growing on pyruvate is limited by monomer supply. However, because pck is required both for biosyntheses and catabolism for cells growing on succinate, it is possible that growth on succinate is limited by both monomer and energy supplies. The growth yield with respect to oxygen remains approximately constant, even though the overproduction of these enzymes enhances gluconeogenic growth. It appears that the constant yield for oxygen is characteristic of efficient growth on a particular substrate and that the yield is already optimal for wild-type strains. Further increases in either Pck or Pps above the optimal levels become growth inhibitory, and the growth yield for oxygen is reduced, indicating less efficient growth. 相似文献
92.
离子转运蛋白在维持细胞内pH稳态、离子动态平衡等方面发挥着重要作用。钠离子转运体和钾离子转运体在嗜盐耐盐微生物中广泛存在,其"保钾排钠"机制是微生物抗盐胁迫的两大策略之一。近年来,嗜盐耐盐微生物中许多新型钠、钾离子转运体被陆续发现,如RDD蛋白、UPF0118蛋白、DUF蛋白和KimA蛋白等;Fe3+、Mg2+等其他金属离子的转运蛋白也被证实可通过影响微生物胞内相容性溶质的合成起到渗透调节的作用。本文综述了嗜盐耐盐微生物中抗盐胁迫相关的各类离子转运蛋白,分析其分子结构和工作机理,并对这些蛋白在农业方面的应用进行了展望。继续发现新的离子转运蛋白,探究抗盐胁迫相关离子转运蛋白的结构和机理,解析各转运系统的协同作用及分子调控机制,将进一步加深对嗜盐耐盐微生物抗盐胁迫调控的认识,并为盐碱地农作物的改良等提供新的思路。 相似文献
93.
本文应用数量遗传学方法,结合概率运算,从理论上分析了由X连锁基因控制的性状在雌雄群体中的变异情况。表明雄(男)性群体的这类性状遗传变异范围有大于雌(女)性群体的倾向。对这一问题的探讨,将有助于对X连锁基因及所决定性状的遗传特性、进化特点及基因定位等的研究提供新的思路。 相似文献
94.
The conformation of native and denaturedPhaseolus coccineus var. rubronanus lectin was studied by circular dichroism (CD) and correlated to the hemagglutinating activity. The far-UV CD spectrum at 25°C showed a broad, negative band around 223 nm and a positive one at 196 nm. CD data analysis of the lectin indicated a -sheet-rich protein. At high temperatures, the spectrum was blue-shifted with increasing magnitude; these changes correlated well with the loss of the activity. The conformation of lectin betweenpH 2 and 10 remained essentially unchanged. AtpH 13 the CD spectrum resembled that of unordered form with a negative band near 200 nm and the activity was completely lost. The denatured lectin in 6 M guanidine hydrochloride would be renatured upon diluting the denaturant to 0.75 M; the changes in CD spectrum again correlated well with the loss of the activity. The effect of sodium dodecyl sulfate on the lectin was drastic; it sharply increased thea-helix at the expense of the -sheet and reduced the activity; the changes reached a plateau above 20 mM surfactant. 相似文献
95.
咖啡因具有抑制一些酶的活性,可减弱或消除生物体自身对激光引起的生物效应的修复作用。试验证明咖啡因对番茄种子萌发有强烈的抑制作用,咖啡因和激光复合处理生物效应显著,可降低番茄种子的发芽,并促进番茄果实增大和生长前期速率加快。 相似文献
96.
Jörg Müller Oliver Mitesser Marc W. Cadotte Fons van der Plas Akira S. Mori Christian Ammer Anne Chao Michael Scherer-Lorenzen Petr Baldrian Claus Bässler Peter Biedermann Simone Cesarz Alice Claßen Benjamin M. Delory Heike Feldhaar Andreas Fichtner Torsten Hothorn Claudia Kuenzer Marcell K. Peters Kerstin Pierick Thomas Schmitt Bernhard Schuldt Dominik Seidel Diana Six Ingolf Steffan-Dewenter Simon Thorn Goddert von Oheimb Martin Wegmann Wolfgang W. Weisser Nico Eisenhauer 《Global Change Biology》2023,29(6):1437-1450
Intensification of land use by humans has led to a homogenization of landscapes and decreasing resilience of ecosystems globally due to a loss of biodiversity, including the majority of forests. Biodiversity–ecosystem functioning (BEF) research has provided compelling evidence for a positive effect of biodiversity on ecosystem functions and services at the local (α-diversity) scale, but we largely lack empirical evidence on how the loss of between-patch β-diversity affects biodiversity and multifunctionality at the landscape scale (γ-diversity). Here, we present a novel concept and experimental framework for elucidating BEF patterns at α-, β-, and γ-scales in real landscapes at a forest management-relevant scale. We examine this framework using 22 temperate broadleaf production forests, dominated by Fagus sylvatica. In 11 of these forests, we manipulated the structure between forest patches by increasing variation in canopy cover and deadwood. We hypothesized that an increase in landscape heterogeneity would enhance the β-diversity of different trophic levels, as well as the β-functionality of various ecosystem functions. We will develop a new statistical framework for BEF studies extending across scales and incorporating biodiversity measures from taxonomic to functional to phylogenetic diversity using Hill numbers. We will further expand the Hill number concept to multifunctionality allowing the decomposition of γ-multifunctionality into α- and β-components. Combining this analytic framework with our experimental data will allow us to test how an increase in between patch heterogeneity affects biodiversity and multifunctionality across spatial scales and trophic levels to help inform and improve forest resilience under climate change. Such an integrative concept for biodiversity and functionality, including spatial scales and multiple aspects of diversity and multifunctionality as well as physical and environmental structure in forests, will go far beyond the current widely applied approach in forestry to increase resilience of future forests through the manipulation of tree species composition. 相似文献
97.
频繁的人类生产活动使植被遭到破坏,造成基岩裸露的石漠化现象,严重制约了喀斯特地区自然和社会经济的发展,随着生态修复工程的大力开展,微生物群落的结构和功能在生态修复中逐渐受到重视,因为微生物作为喀斯特生态系统的重要组成部分,不仅在物质循环过程中起着重要作用,也在喀斯特生态系统修复中占有十分重要的地位。所以微生物群落结构和功能的研究可以作为衡量生态系统稳定性的重要指标。就中国喀斯特地区的典型植被恢复的不同阶段、成土过程、不同的土地利用方式、矿山修复过程以及不同水域中的微生物群落结构及功能等方面的研究状况进行系统梳理,结合实例综合论述喀斯特地区生态修复过程中微生物作用的研究进展,以期为喀斯特地区生态修复提供参考。 相似文献
98.
福州地区桑白蚧发生动态和药剂防治试验 总被引:1,自引:0,他引:1
桑白蚧在福州地区一年发生4代.以雌成虫在寄主枝干上越冬.越冬代(第4代)一雌虫产卵量多的达278粒.少的36粒,平均171粒,比第2代产卵量多2.6倍.比第3代多4.5倍.药剂防治试验结果,在2龄幼蚧高峰期,用25%扑虱灵可温性粉剂1500倍液,40%氧化乐果乳油800-1000倍液和95%机油乳剂50—100倍液喷雾.防治效果可达90%左右.用25%扑虱是可湿性粉剂1000-1500倍液.喷酒幼蚕触杀试验和喷洒桑叶喂蚕胃毒试验结果.对幼蚕安全.用扑虱灵防治桑树上的桑白蚧,对养蚕业无不良影响。 相似文献
99.
100.
Miao Yan Xin Li Jinbao Gu Guojun Gao Ziyu Wu Peng Xue 《Journal of biochemical and molecular toxicology》2023,37(3):e23265
Prostate cancer (PCa) is a challenging issue for men's health worldwide due to its uncontrolled proliferation and high metastatic potential. Increasing evidence has supported plant extracts and natural plant derivatives as promising antitumor therapy with less toxic side effects. Yuanhuacine is an active component isolated from Daphne genkwa and can effectively suppress the tumorigenesis of several cancers. However, its role in PCa remains unclear. In this study, yuanhuacine dose-dependently inhibited the proliferation and induced apoptosis of PCa cells. Moreover, yuanhuacine also restrained the invasion and migration of PCa cells. Mechanically, yuanhuacine decreased the ubiquitination and degradation of p53 protein, and ultimately increased p53 levels, which was regulated by inhibiting the phosphorylation and total protein levels of mouse double minute 2 (MDM2). Moreover, elevation of MDM2 reversed the suppressive efficacy of yuanhuacine in PCa cell viability, invasion, and migration. The network pharmacologic and bioinformatics analysis confirmed that MDM2 might be a common target of D. genkwa and LINC00665. Furthermore, yuanhuacine inhibited LINC00665 expression. Upregulation of LINC00665 reversed yuanhuacine-mediated inhibition in MDM2 protein expression and suppressed p53 levels by enhancing its ubiquitination in yuanhuacine-treated cells. Importantly, the inhibitory effects of yuanhuacine on cell viability and metastatic potential were offset after LINC00665 elevation. Together, the current findings highlight that yuanhuacine may possess tumor-suppressive efficacy by inhibiting LINC00665-mediated MDM2/p53 ubiquitination signaling. Therefore, this study indicates that yuanhuacine may be a promising candidate for the treatment of PCa. 相似文献