全文获取类型
收费全文 | 597篇 |
免费 | 91篇 |
国内免费 | 37篇 |
出版年
2024年 | 3篇 |
2023年 | 3篇 |
2022年 | 5篇 |
2021年 | 11篇 |
2020年 | 30篇 |
2019年 | 29篇 |
2018年 | 36篇 |
2017年 | 30篇 |
2016年 | 20篇 |
2015年 | 24篇 |
2014年 | 50篇 |
2013年 | 41篇 |
2012年 | 20篇 |
2011年 | 24篇 |
2010年 | 23篇 |
2009年 | 31篇 |
2008年 | 26篇 |
2007年 | 40篇 |
2006年 | 22篇 |
2005年 | 30篇 |
2004年 | 15篇 |
2003年 | 22篇 |
2002年 | 19篇 |
2001年 | 14篇 |
2000年 | 14篇 |
1999年 | 16篇 |
1998年 | 12篇 |
1997年 | 9篇 |
1996年 | 10篇 |
1995年 | 7篇 |
1994年 | 7篇 |
1993年 | 10篇 |
1992年 | 7篇 |
1991年 | 6篇 |
1990年 | 14篇 |
1989年 | 12篇 |
1988年 | 3篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 4篇 |
1978年 | 1篇 |
1977年 | 2篇 |
排序方式: 共有725条查询结果,搜索用时 15 毫秒
61.
Hole-burned absorption and line-narrowed fluorescence spectra are studied at 5 K in wild type and mutant LH1 and LH2 antenna preparations from the photosynthetic purple bacterium Rhodobacter sphaeroides. Evidence was found in all samples, even in intact membranes, of the presence of a broad distribution of bacteriochlorophyll species that are unable to communicate energy between each other and to the exciton states of functional antenna complexes. The distribution maximum of these localized species determined by zero phonon hole action spectroscopy is at 783.5 nm in purified LH1 complexes and at 786.8 nm in B850-only mutant LH2 complexes. A well-resolved peak at 807 nm in LH1 complexes is assigned to the exciton band structure of functional core antenna complexes. Similar structure in LH2 complexes overlaps with the distribution of localized species. Off-diagonal (structural) disorder may be responsible for this exciton band structure. Our data also imply that pair-wise inter-chlorophyll couplings determine the resonance fluorescence lineshape of excitonic polarons. 相似文献
62.
The ubiquity and importance of photosynthetic organisms in nature has made the molecular mechanisms of photosynthesis a widely studied subject at both structural and functional levels. A current challenge is to understand the supramolecular assembly of the proteins involved in photosynthesis in native membranes. We have used atomic force microscopy to study the architecture of the photosynthetic apparatus and analyze the structure of single molecules in chromatophores of Phaeospirillum molischianum. Core complexes are formed by the reaction center enclosed by an elliptical light harvesting complex 1. LH2 are octameric rings, assembled either with cores or in hexagonally packed LH2 antenna domains. The symmetry mismatch caused by octameric LH2 packing in a hexagonal lattice, that could be avoided in a square lattice, suggests lipophobic effects rather than specific inter-molecular interactions drive protein organization. The core and LH2 complexes are organized to form a supramolecular assembly reminiscent to that found in Rhodospirillum photometricum, and very different from that observed in Rhodobacter sphaeroides, Rb. blasticus, and Blastochloris viridis. 相似文献
63.
Vincent Calcagno Vincent Bonhomme Yan Thomas Michael C. Singer Denis Bourguet 《Proceedings. Biological sciences / The Royal Society》2010,277(1694):2703-2709
Divergent adaptation to host plant species may be the major mechanism driving speciation and adaptive radiations in phytophagous insects. Host plants can differ intrinsically in a number of attributes, but the role of natural enemies in host plant specialization is often underappreciated. Here, we report behavioural divergence between the European corn borer (ECB, Ostrinia nubilalis) and its sibling species Ostrinia scapulalis, in relation to a major enemy: humans. Harvesting maize imposes selective mortality on Ostrinia larvae: those located above the cut-off line of the stalk face almost certain death. We show that ECB larvae diapause closer to the ground than those of O. scapulalis, which is sympatric but feeds mainly on weeds. The difference in diapause height results from genetically determined differences in geotactic behaviour. ECB larvae descend towards the ground specifically at harvest time, increasing their chances of surviving harvesting by about 50 per cent over O. scapulalis larvae. Natural enemies appear as a major driver of host-plant specialization in this example, stressing the need to consider ‘tri-trophic’ ecological niches to understand insect diversification. Our results also strongly suggest that geotaxis evolved as a singular instance of behavioural resistance in a major agricultural pest. 相似文献
64.
65.
66.
Understory Vegetation Dynamics of North American Boreal Forests 总被引:2,自引:0,他引:2
Understory vegetation is the most diverse and least understood component of North American boreal forests. Understory communities are important as they act as drivers of overstory succession and nutrient cycling. The objective of this review was to examine how understory vegetation abundance, composition, and diversity change with stand development after a major stand replacing disturbance. Understory vegetation abundance and diversity increase rapidly after fire, in response to abundant resources and an influx of disturbance adapted species. The highest diversity occurs within the first 40 years following fire, and declines indefinitely thereafter as a result of decreasing productivity and increased dominance of a small number of late successional feather mosses and woody plant species. Vascular plant and bryophyte/lichen communities undergo very different successional changes. Vascular plant communities are dynamic and change more dramatically with time after fire, whereas bryophyte and lichen communities are much slower to establish and change over time. Considerable variations in these processes exist depending on canopy composition, site condition, regional climate, and frequently occurring non-stand-replacing disturbances. Forest management practices represent a unique disturbance process and can result in different understory vegetation communities from those observed for natural processes, with potential implications for overstory succession and long-term productivity. Because of the importance of understory vegetation on nutrient cycling and overstory composition, post-harvest treatments emulating stand-replacing fire are required to maintain understory diversity, composition, and promote stand productivity in boreal forests. 相似文献
67.
The utility of shallow water bodies in urban environments is frequently compromised either by dense beds of submerged plants
or cyanobacterial blooms associated with nutrient enrichment. Although submerged plants are often harvested to facilitate
recreational uses, this activity may alter the phytoplankton community, which in turn, also may restrict the use of the lake.
We tested whether (i) plant harvesting reduced the abundance of flagellate algae and increased the abundance of cyanobacteria,
and (ii) whether increasing levels of nutrient enrichment caused shifts in the dominance of heterocytous cyanobacteria, non-heterocytous
cyanobacteria and Chlorophyta, in a shallow urban lake in Southern Australia as has been observed for shallow Danish lakes
in previous studies. These predictions were tested with large (3000 l), replicated mesocosms in a warm, highly productive,
shallow lake densely colonised by the submerged angiosperm, Vallisnaria americana Michaux. The heterokont algae, Chlorophyta, Cyanobacteria and Cryptophyta were the most numerous algal divisions in the lake.
The Euglenophyta, although uncommon in early summer, became more abundant towards the end of summer. The Dinophyta and Charophyta
were rare. The abundance of the heterokont algae and Euglenophyta was significantly reduced by plant harvesting even after
plants had partially re-established 18 weeks after initial harvesting. The decline in the Euglenophyta in response to plant
harvesting is consistent with earlier findings, that the relative abundance of flagellate algae tends to be greater in the
presence of submerged plants. Contrary to our prediction, we found that the Cyanobacteria did not increase in response to
plant harvesting, however the response may be altered under higher nutrient levels. Algal responses to nutrient enrichment
in the presence of dense V. americana plants generally followed the patterns observed in shallow Danish lakes despite the large differences in climatic conditions.
Both studies found that the abundance of heterocytous cyanobacteria declined at higher levels of nutrient enrichment, whereas
non-heterocytous cyanobacteria and chlorophytes increased. 相似文献
68.
Leaves and chloroplasts from Arabidopsis plants with increased amounts of PsbS protein showed the same percentage increase in nonphotochemical quenching in comparison to the wild type both in the presence and absence of zeaxanthin. The absorption change at 525-535 nm was also more pronounced in both cases. It is suggested that PsbS alone can cause the quenching, supporting the model in which zeaxanthin acts as an allosteric activator and is not the primary cause of the process. It is proposed that PsbS acts as a trigger of the conformational change that leads to the establishment of nonphotochemical quenching. 相似文献
69.
Many human influences on the world's ecosystems have their largest direct impacts at either the top or the bottom of the food web. To predict their ecosystem-wide consequences we must understand how these impacts propagate. A long-standing, but so far elusive, problem in this endeavour is how to reduce food web complexity to a mathematically tractable, but empirically relevant system. Simplification to main energy channels linking primary producers to top consumers has been recently advocated. Following this approach, we propose a general framework for the analysis of bottom-up and top-down forcing of ecosystems by reducing food webs to two energy pathways originating from a limiting resource shared by competing guilds of primary producers (e.g. edible vs. defended plants). Exploring dynamical models of such webs we find that their equilibrium responses to nutrient enrichment and top consumer harvesting are determined by only two easily measurable topological properties: the lengths of the component food chains (odd-odd, odd-even, or even-even) and presence vs. absence of a generalist top consumer reconnecting the two pathways (yielding looped vs. branched webs). Many results generalise to other looped or branched web structures and the model can be easily adapted to include a detrital pathway. 相似文献
70.
采用RT-PCR和RACE技术,从一年生毛竹(Phyllostachys edulis)新鲜叶片中分离到3个捕光色素结合蛋白基因cab-PhE2、cab-PhE3和cab-PhE6,GenBank登记号分别为EF207230、EF405877和EF628209.序列分析表明,3个基因分别与其它植物如水稻、玉米、大麦等PSⅡ的Ihcb2、Ihcb1、Ihcb3基因有着较高的一致性,其编码的蛋白属于大量捕光天线.蛋白结构预测表明,3个基因编码的蛋白均由导肽和成熟蛋白组成,其中成熟蛋白的二级结构均包含4个α螺旋结构、3个类胡萝卜素结合位点,以及相应的叶绿素a、b结合位点.组织特异性表达检测表明,3个基因在叶片、叶鞘和幼茎中均有表达,且在叶片中最高,而在根中未检测到表达.在强光照射( 1500μmol·m-2·s-1)条件下,3个基因的表达量均呈下调趋势,不同基因间存在着一定的差异,其中cab-PhE3和cab-PhE2的表达丰度在光照4h内下降较快,至6 h cab-PhE2接近于零,而cab-PhE6经光照4h表达丰度仍为对照的70%以上,但之后2h后很快降至对照的5%以下. 相似文献