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991.
992.
Respiration as the main determinant of carbon balance in European forests 总被引:106,自引:0,他引:106
Valentini R Matteucci G Dolman AJ Schulze ED Rebmann C Moors EJ Granier A Gross P Jensen NO Pilegaard K Lindroth A Grelle A Bernhofer C Grünwald T Aubinet M Ceulemans R Kowalski AS Vesala T Rannik U Berbigier P Loustau D Gudmundsson J Thorgeirsson H Ibrom A Morgenstern K Clement R 《Nature》2000,404(6780):861-865
Carbon exchange between the terrestrial biosphere and the atmosphere is one of the key processes that need to be assessed in the context of the Kyoto Protocol. Several studies suggest that the terrestrial biosphere is gaining carbon, but these estimates are obtained primarily by indirect methods, and the factors that control terrestrial carbon exchange, its magnitude and primary locations, are under debate. Here we present data of net ecosystem carbon exchange, collected between 1996 and 1998 from 15 European forests, which confirm that many European forest ecosystems act as carbon sinks. The annual carbon balances range from an uptake of 6.6 tonnes of carbon per hectare per year to a release of nearly 1 t C ha(-1) yr(-1), with a large variability between forests. The data show a significant increase of carbon uptake with decreasing latitude, whereas the gross primary production seems to be largely independent of latitude. Our observations indicate that, in general, ecosystem respiration determines net ecosystem carbon exchange. Also, for an accurate assessment of the carbon balance in a particular forest ecosystem, remote sensing of the normalized difference vegetation index or estimates based on forest inventories may not be sufficient. 相似文献
993.
Functional cartography of complex metabolic networks 总被引:16,自引:0,他引:16
High-throughput techniques are leading to an explosive growth in the size of biological databases and creating the opportunity to revolutionize our understanding of life and disease. Interpretation of these data remains, however, a major scientific challenge. Here, we propose a methodology that enables us to extract and display information contained in complex networks. Specifically, we demonstrate that we can find functional modules in complex networks, and classify nodes into universal roles according to their pattern of intra- and inter-module connections. The method thus yields a 'cartographic representation' of complex networks. Metabolic networks are among the most challenging biological networks and, arguably, the ones with most potential for immediate applicability. We use our method to analyse the metabolic networks of twelve organisms from three different superkingdoms. We find that, typically, 80% of the nodes are only connected to other nodes within their respective modules, and that nodes with different roles are affected by different evolutionary constraints and pressures. Remarkably, we find that metabolites that participate in only a few reactions but that connect different modules are more conserved than hubs whose links are mostly within a single module. 相似文献
994.
鱼尼丁受体类杀虫剂的研究进展 总被引:2,自引:0,他引:2
介绍了鱼尼丁受体类新型杀虫剂的创制现状、作用机制、生物活性、毒性及合成方法,由于此类杀虫剂的结构独特、作用方式新颖、对鳞翅目害虫效果好、杀虫广谱,该类药剂对各种益虫和天敌安全,对现用的杀虫剂无交互抗性,具有良好的发展前景. 相似文献
995.
根据实现差动保护功能所需信息类型的不同,提出分相电流差动和综合电流差动的新分类方法,并列举已有的综合电流差动保护方案.在对已有方案的性能评估和分析基础上,给出负序电流与正序电流故障分量相配合的综合电流差动新原理.理论分析和EMTP仿真表明,该方案可以对各种类型的区内故障灵敏动作. 相似文献
996.
Most recently produced wind generators use variable-speed systems for high-efficiency operation and low mechanical stress.In this study of a variable-speed standalone system that uses a synchronous machine,a power converter for the system was developed and tested.The maximum power points were approximated in a line according to the wind speed changes,and the linear approximation MPPT algorithm was used to determine the duty ratio that corresponds to the intersection point of the observed power and linearized line using the power generated from the generator at a point.With the fabricated converter,it was verified that the voltage,current and theta were normal with the varied load current. 相似文献
997.
Mechanism of closure of the aortic valve 总被引:5,自引:0,他引:5
998.
Dunham A Matthews LH Burton J Ashurst JL Howe KL Ashcroft KJ Beare DM Burford DC Hunt SE Griffiths-Jones S Jones MC Keenan SJ Oliver K Scott CE Ainscough R Almeida JP Ambrose KD Andrews DT Ashwell RI Babbage AK Bagguley CL Bailey J Bannerjee R Barlow KF Bates K Beasley H Bird CP Bray-Allen S Brown AJ Brown JY Burrill W Carder C Carter NP Chapman JC Clamp ME Clark SY Clarke G Clee CM Clegg SC Cobley V Collins JE Corby N Coville GJ Deloukas P Dhami P Dunham I Dunn M Earthrowl ME Ellington AG 《Nature》2004,428(6982):522-528
Chromosome 13 is the largest acrocentric human chromosome. It carries genes involved in cancer including the breast cancer type 2 (BRCA2) and retinoblastoma (RB1) genes, is frequently rearranged in B-cell chronic lymphocytic leukaemia, and contains the DAOA locus associated with bipolar disorder and schizophrenia. We describe completion and analysis of 95.5 megabases (Mb) of sequence from chromosome 13, which contains 633 genes and 296 pseudogenes. We estimate that more than 95.4% of the protein-coding genes of this chromosome have been identified, on the basis of comparison with other vertebrate genome sequences. Additionally, 105 putative non-coding RNA genes were found. Chromosome 13 has one of the lowest gene densities (6.5 genes per Mb) among human chromosomes, and contains a central region of 38 Mb where the gene density drops to only 3.1 genes per Mb. 相似文献
999.
Catastrophic shifts in ecosystems 总被引:140,自引:0,他引:140
All ecosystems are exposed to gradual changes in climate, nutrient loading, habitat fragmentation or biotic exploitation. Nature is usually assumed to respond to gradual change in a smooth way. However, studies on lakes, coral reefs, oceans, forests and arid lands have shown that smooth change can be interrupted by sudden drastic switches to a contrasting state. Although diverse events can trigger such shifts, recent studies show that a loss of resilience usually paves the way for a switch to an alternative state. This suggests that strategies for sustainable management of such ecosystems should focus on maintaining resilience. 相似文献
1000.
Homology of proteasome subunits to a major histocompatibility complex-linked LMP gene. 总被引:6,自引:0,他引:6
The class II region of the major histocompatibility complex (MHC) contains genes encoding at least two subunits of a large, intracellular protein complex (the low molecular mass polypeptide, or LMP, complex). This complex is biochemically similar to the proteasome, an abundant and well conserved protein complex having multiple proteolytic activities. Here we report the isolation of a complementary DNA corresponding to one of the subunits of the LMP complex, LMP-2. The protein predicted from this cDNA sequence closely matches the amino-terminal peptide sequence of a rat proteasome subunit, confirming that the proteasome and the LMP complex share polypeptide subunits. The LMP-2 gene is tightly linked to HAM1, a gene thought to be required for translocating peptide fragments of endogenous antigens into the endoplasmic reticulum for association with MHC class I molecules. These observations suggest that the LMP complex may be responsible for generating peptides from cytoplasmic antigen during antigen processing. 相似文献