全文获取类型
收费全文 | 579篇 |
免费 | 49篇 |
国内免费 | 16篇 |
专业分类
644篇 |
出版年
2023年 | 13篇 |
2022年 | 6篇 |
2021年 | 12篇 |
2020年 | 25篇 |
2019年 | 27篇 |
2018年 | 23篇 |
2017年 | 19篇 |
2016年 | 21篇 |
2015年 | 19篇 |
2014年 | 22篇 |
2013年 | 48篇 |
2012年 | 23篇 |
2011年 | 31篇 |
2010年 | 16篇 |
2009年 | 20篇 |
2008年 | 21篇 |
2007年 | 31篇 |
2006年 | 28篇 |
2005年 | 25篇 |
2004年 | 21篇 |
2003年 | 15篇 |
2002年 | 16篇 |
2001年 | 22篇 |
2000年 | 10篇 |
1999年 | 12篇 |
1998年 | 12篇 |
1997年 | 10篇 |
1996年 | 11篇 |
1995年 | 6篇 |
1994年 | 4篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 5篇 |
1990年 | 9篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 8篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1975年 | 3篇 |
1974年 | 1篇 |
1972年 | 2篇 |
排序方式: 共有644条查询结果,搜索用时 14 毫秒
101.
102.
High-latitude environments, such as the Antarctic McMurdo Dry Valley lakes, are subject to seasonally segregated light–dark cycles, which have important consequences for microbial diversity and function on an annual basis. Owing largely to the logistical difficulties of sampling polar environments during the darkness of winter, little is known about planktonic microbial community responses to the cessation of photosynthetic primary production during the austral sunset, which lingers from approximately February to April. Here, we hypothesized that changes in bacterial, archaeal and eukaryotic community structure, particularly shifts in favor of chemolithotrophs and mixotrophs, would manifest during the transition to polar night. Our work represents the first concurrent molecular characterization, using 454 pyrosequencing of hypervariable regions of the small-subunit ribosomal RNA gene, of bacterial, archaeal and eukaryotic communities in permanently ice-covered lakes Fryxell and Bonney, before and during the polar night transition. We found vertically stratified populations that varied at the community and/or operational taxonomic unit-level between lakes and seasons. Network analysis based on operational taxonomic unit level interactions revealed nonrandomly structured microbial communities organized into modules (groups of taxa) containing key metabolic potential capacities, including photoheterotrophy, mixotrophy and chemolithotrophy, which are likely to be differentially favored during the transition to polar night. 相似文献
103.
104.
生长素极性运输研究进展 总被引:25,自引:0,他引:25
高等植物的生长发育受激素的广泛调控,其中生长素的作用尤为独特,因为生长素在植物组织内的浓度梯度是由其极性运输维持的,而正是激素在植物组织的相对含量决定了该组织的发育命运。高等植物体内存在可运输的化学信使的概念首先由Darwin父子提出。通过对金丝鸟木亡草(Phalarisca nariensis)幼苗的向光性的研究,他们认为植物的向光性受到一种可运输的物质的调控[1]。后来发现这一物质是生长素,在自然界中主要存在的形式是IAA。到本世纪 30年代,禾谷类植物中的生长素的极性运输得到证实,后来发现所有… 相似文献
105.
本文研究人参总皂甙在海马齿状回颗粒细胞层诱发LTP效应和促进大鼠记忆保持能力时,对海马神经元核仁组织者区和苔藓纤维末梢出芽的影响。给人参总皂甙第7天可显著提高群峰电位(PS)幅度,缩短PS起始和峰潜伏期,并可显著提高大鼠记忆保持能力。此时人参总皂甙可使海马CA3区锥体细胞和齿状回颗粒细胞Ag-NOR数较盐水组大鼠的平均提高66.17±2.32%和72.07±0.93%(P<0.01);同时还可使大鼠的海马苔藓纤维末梢出芽级分平均较生理盐水组大鼠的平均堤高1-3倍(P<0.01)。提示人参总皂甙可提高神经元内rDNA的转录活性促进蛋白质合成,同时还可促进海马苔藓纤维末梢出芽以提高海马突触的传递功效。 相似文献
106.
人参皂苷单体定向转化的生物催化及应用进展 总被引:2,自引:0,他引:2
人参是我国传统中药,药效显著、应用广泛。通过定向修饰与转化人参皂苷糖基可产生高抗癌活性稀有人参皂苷。传统化学法由于制备工艺极其复杂、成本过高,不能应用于临床,微生物及其酶系转化成为解决该瓶颈问题的最可行手段。有关全细胞催化、糖苷酶重组表达、固定化及其催化分子识别机制和溶剂工程的生物转化已有大量综述报道,但尚无在人参皂苷转化应用中的系统研究。文中通过对人参皂苷单体生物转化理论和应用研究最新进展的回顾,结合目前广泛采用的生物催化方法的讨论,系统梳理归纳了能够改善产物专一性、提高催化效率,且具有工业应用前景的人参皂苷单体定向转化方法。基于酶分子设计以及离子液体溶剂工程,对人参皂苷单体抗癌药物和食品、保健品市场的开发、规模化制备进行了展望。 相似文献
107.
Tei J Kani S Hanai K Miyaguchi T Yamamoto K 《Development, growth & differentiation》2004,46(5):439-447
Immature starfish oocytes, which are arrested at the first meiotic prophase and contain a large nucleus called the germinal vesicle (GV), are known to accept multiple sperm on insemination. We found that if these polyspermic starfish oocytes are induced to mature, they often form small protrusion(s) adjacent to the first polar body emitted shortly earlier. We refer to these protrusion(s) as 'polar-body-like structures (PLS).' Fluorescent staining of PLS indicated that they were not merely cytoplasmic protrusions, but contained some chromatin. Maturing process of these polyspermic oocytes was examined by immnofluorescent staining, which showed that: (i) numerous sperm asters were observed after the onset of GV breakdown; (ii) before the first polar body (PB1) emission, a complex microtubular structure resembling a multipolar spindle was formed; and (iii) several isolated asters were observed after PB1 emission. These results indicate that PLS formation may be induced by interaction of meiosis-I spindle with paternal centrosomes incorporated at GV stage. 相似文献
108.
Influences of salinity on the physiology and distribution of the Arctic coralline algae,Lithothamnion glaciale (Corallinales,Rhodophyta)
下载免费PDF全文
![点击此处可从《Journal of phycology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Kathryn M. Schoenrock Marion Bacquet Danni Pearce Brice R. Rea J. Edward Schofield James Lea Doug Mair Nicholas Kamenos 《Journal of phycology》2018,54(5):690-702
In Greenland, free‐living red coralline algae contribute to and dominate marine habitats along the coastline. Lithothamnion glaciale dominates coralline algae beds in many regions of the Arctic, but never in Godthåbsfjord, Greenland, where Clathromorphum sp. is dominant. To investigate environmental impacts on coralline algae distribution, calcification and primary productivity were measured in situ during summers of 2015 and 2016, and annual patterns of productivity in L. glaciale were monitored in laboratory‐based mesocosm experiments where temperature and salinity were manipulated to mimic high glacial melt. The results of field and cold‐room measurements indicate that both L. glaciale and Clathromorphum sp. had low calcification and photosynthetic rates during the Greenland summer (2015 and 2016), with maximum of 1.225 ± 0.17 or 0.002 ± 0.023 μmol CaCO 3 · g?1 · h?1 and ?0.007 ±0.003 or ?0.004 ± 0.001 mg O2 · L?1 · h?1 in each species respectively. Mesocosm experiments indicate L. glaciale is a seasonal responder; photosynthetic and calcification rates increase with annual light cycles. Furthermore, metabolic processes in L. glaciale were negatively influenced by low salinity; positive growth rates only occurred in marine treatments where individuals accumulated an average of 1.85 ± 1.73 mg · d?1 of biomass through summer. These results indicate high freshwater input to the Godthåbsfjord region may drive the low abundance of L . glaciale , and could decrease species distribution as climate change increases freshwater input to the Arctic marine system via enhanced ice sheet runoff and glacier calving. 相似文献
109.
Annette Gulik Vittorio Luzzati Mario De Rosa Agata Gambacorta 《Journal of molecular biology》1985,182(1):131-149
We describe in this work the structure and polymorphism of a variety of lipids extracted from Sulfolobus solfataricus, an extreme thermoacidophilic archaebacterium growing at about 85 °C and pH 2. These lipids are quite different from the usual fatty acid lipids of eukaryotes and prokaryotes: each molecule consists of two C40 ω-ω′ biphytanyl residues (with 0 to 4 cyclopentane groups per residue), ether linked at both ends to two (variably substituted) glycerol or nonitol groups. Four lipid preparations were studied; the total and the polar lipid extracts, and two hydrolytic fractions, the symmetric glycerol dialkyl glycerol tetraether and the asymmetric glycerol dialkyl nonitol tetraether, as a function of water content and temperature, using X-ray scattering techniques. The main conclusions from the study of the four lipid preparations can be summarized as follows. (1) As with other lipids, a remarkable number and variety of phases are observed over a temperature-concentration range close to “physiological” conditions. The possibility is discussed that this polymorphism reflects a fundamental property of lipids, closely related to their physiological rôle. (2) As in other lipids, two types of chain conformations are observed: a disordered one (type α) at high temperature; at lower temperature, a more ordered packing of stiff chains, all parallel to each other (type β′). At temperatures and degrees of hydration approaching the conditions prevailing in the living cell, the conformation is of type α. (3) In all the phases with chains in the α conformation, the unsubstituted glycerol headgroups, whose concentration is high in these lipids, segregate in the hydrocarbon matrix, away from the other polar groups. This property may have interesting biological consequences: for example, the chains of a fraction of the bipolar lipid molecules can span hydrocarbon gaps as wide as 75 Å. (4) Two cubic phases are observed in the total and the polar lipid extracts, which display a remarkable degree of metastability, most unusual in lipid phase transitions involving structures with chains in the α conformation. This phenomenon can be explained by the interplay of the physical structure of the cubic phases (the two contain two intertwined and unconnected three-dimensional networks of rods) and the chemical structure of the lipid molecules: the two headgroups of most molecules being anchored on each of the two networks of rods, the migration of the lipid molecules is hindered by the two independent diffusion processes and by the entanglement of the chains. The possibility is discussed that this phenomenon may reflect an evolutionary response to a challenge of the natural habitat of these archaebacteria. 相似文献
110.
Seven-day-old leek seedlings actively synthesize lipids in vivo from [1-14C]acetate, both in the light and in the dark. In the dark, phospholipid synthesis is more effective than galactolipid synthesis. Whatever the time of acetate incorporation by the etiolated seedlings, very long chain fatty acids having from 20 to 26 carbon atoms are found in all the polar lipids, including the acyl-CoAs. All of the labelled very long chain fatty acids incorporated into the polar lipids are saturated. On the other hand, the labelled C18-fatty acids are unsaturated in phospholipids and galactolipids and almost no label is found in the saturated or unsaturated C18-fatty acids of the acyl-CoAs. 相似文献