共查询到20条相似文献，搜索用时 46 毫秒
1.
Protein translocation across biological membranes 总被引：1，自引：0，他引：1
Subcellular compartments have unique protein compositions, yet protein synthesis only occurs in the cytosol and in mitochondria and chloroplasts. How do proteins get where they need to go? The first steps are targeting to an organelle and efficient translocation across its limiting membrane. Given that most transport systems are exquisitely substrate specific, how are diverse protein sequences recognized for translocation? Are they translocated as linear polypeptide chains or after folding? During translocation, how are diverse amino acyl side chains accommodated? What are the proteins and the lipid environment that catalyze transport and couple it to energy? How is translocation coordinated with protein synthesis and folding, and how are partially translocated transmembrane proteins released into the lipid bilayer? We review here the marked progress of the past 35 years and salient questions for future work. Subcellular compartments have unique protein compositions, yet protein synthesis only occurs in the cytosol and in mitochondria and chloroplasts. How do proteins get where they need to go? The first steps are targeting to an organelle and efficient translocation across its limiting membrane. Given that most transport systems are exquisitely substrate specific, how are diverse protein sequences recognized for translocation? Are they translocated as linear polypeptide chains or after folding? During translocation, how are diverse amino acyl side chains accommodated? What are the proteins and the lipid environment that catalyze transport and couple it to energy? How is translocation coordinated with protein synthesis and folding, and how are partially translocated transmembrane proteins released into the lipid bilayer? We review here the marked progress of the past 35 years and salient questions for future work. 相似文献
2.
退化红壤恢复过程中灌木层主要种群的生态位 总被引：3，自引：1，他引：2
对闽西退化红壤采取种植乔木(P2)、牧草(P3)、灌木(P4)3种治理措施,以及对侵蚀对照地(P1)、次生林对照地(P5)的灌木层种群数量、生态位进行了研究.结果表明,3种治理措施不同程度提高了灌木层的物种丰富度.黄栀子、梅叶冬青、木蜡树、马尾松等Shannon-Wiener生态位宽度(B(sw)i)超过0.5;多数种群生态位宽度较小,其中60.38%的种群仅占据1个资源(B(sw)i=0),多数仅分布于次生林,表明侵蚀地与治理林地资源状态与次生林有较大差距,未能满足演替中后期植物生长的需要.种群Pianka生态位重叠值(NOik)与种对的生态位宽度、占据的资源位及相遇几率、重要值及其分布格局存在复杂的非线性关系.广生态位种对的重叠值普遍较高(NOik>0.5),狭生态位种对重叠值普遍较小(NOik<0.3),其余类型生态位种对重叠值为0-1. 相似文献
3.
稻鸭共作系统中主要捕食性天敌的生态位 总被引：2，自引：0，他引：2
【目的】探明稻鸭共作系统中主要捕食性天敌类群对资源的利用程度及在资源分配上的内在关系。【方法】在对稻鸭共作和常规稻作系统中主要捕食性天敌进行田间调查的基础上,研究主要捕食性天敌群落的结构和生态位,分析各类群在资源序列上分布的数量特征和资源利用状况。【结果】常规稻作区、稻鸭共作区中主要捕食性天敌的优势类群均为微蛛、园蛛、狼蛛和肖蛸科,以园蛛科的优势度指数为最高,分别为0.4999和0.6665。稻田养鸭对主要捕食性天敌类群的数量和分布格局有直接或间接的影响,使其时—空二维生态位宽度、生态位重叠度与生态位相似性比例均发生了不同程度的改变。其中微蛛、园蛛和肖蛸的生态位宽度值低于常规区,微蛛与球蛛、肖蛸与跳蛛、跳蛛与管巢蛛及管巢蛛与瓢虫的二维生态位重叠度比常规区增加了70.65%—75.57%。【结论】稻鸭共作使主要捕食性天敌在同一资源等级上的竞争加强,在时间上的同步性更为明显,尽管捕食性天敌的总个体数比常规区下降了19.56%,但天敌充分利用了时间资源序列中的较多等级,且分布更为均匀,发生时间长,可对害虫种群进行持续控制,因而控虫效果更为显著。 相似文献
4.
Langer JS 《Science (New York, N.Y.)》1989,243(4895):1150-1156
There has emerged recently a new theoretical picture of the way in which patterns are formed in dendritic crystal growth and in the closely analogous phenomenon of viscous fingering in fluids. Some interesting questions that arise in connection with this theory include: How broad is its range of validity? How do we understand dynamic stability in systems of this kind? What is the origin of sidebranches? Can weak noise, or even microscopic thermal fluctuations, play a role in determining the macroscopic features of these systems? 相似文献
5.
生态位理论及其测度研究进展 总被引：71，自引：3，他引：71
许多学者都试图给生态位下定义 ,但最具代表性的当推Grinnell、Elton和Hutchinson分别给出的“空间生态位”、“功能生态位”和“多维超体积生态位”定义 ;生态位测度包括生态位宽度、生态位重叠、生态位体积及生态位维数等 ,其中生态位宽度和生态位重叠是描述一个物种的生态位以及物种生态位间关系的重要指标 .该文列举并分析了每一个测度公式的优缺点 ,进而优选出适合森林群落的生态位计测公式 .指出利用性竞争的一个必要条件是生态位重叠 ,但重叠并不一定导致竞争 ,竞争是在资源供应不足且生态位重叠条件下形成的 . 相似文献
6.
In coming to understand the world-in learning concepts, acquiring language, and grasping causal relations-our minds make inferences that appear to go far beyond the data available. How do we do it? This review describes recent approaches to reverse-engineering human learning and cognitive development and, in parallel, engineering more humanlike machine learning systems. Computational models that perform probabilistic inference over hierarchies of flexibly structured representations can address some of the deepest questions about the nature and origins of human thought: How does abstract knowledge guide learning and reasoning from sparse data? What forms does our knowledge take, across different domains and tasks? And how is that abstract knowledge itself acquired? 相似文献
7.
澜沧江上游森林珍稀草本植物生态位研究 总被引：14，自引：1，他引：14
20 0 2年和 2 0 0 3年 7— 8月 ,笔者采用常规调查方法对三江源自然保护区内澜沧江上游物种进行了调查研究 .以不同群落类型和海拔高度作为资源位 ,分别以物种重要值和个体多度为生态位计测的状态指标 ,对澜沧江上游桃儿七、星叶草和角盘兰及选取的几种对照草本植物进行生态位的计测和分析 .结果表明 :桃儿七、星叶草、角盘兰生态位宽度值小 ,充分显示了它们的稀有性 ,在该地区应优先保护 .桃儿七种群在海拔 36 0 0～ 36 5 0m分布最多 ,并且种群数量随海拔高度的增加逐渐减少 ;星叶草种群在海拔 36 5 0～ 375 0m分布最多 ,但种群数量不随海拔高度变化呈现规律性变化 .草本植物物种的生态位相似性比例值大于 0 5的占 5 0 % ,桃儿七、星叶草、角盘兰的生态位相似性比例大 ,说明它们在该地区的生态位相似程度大 .草本植物的生态位重叠值较小 ,大于 0 0 1的占 2 1 4 3% .桃儿七、星叶草、角盘兰之间生态位重叠程度小 ,它们分布的群落类型相似程度小 . 相似文献
8.
Sternberg RJ 《Science (New York, N.Y.)》1985,230(4730):1111-1118
Theories of intelligence, and some of the research testing them, are designed to answer three basic questions about intelligence: (i) What is the relation of intelligence to the internal world of the individual? (ii) What is the relation of intelligence to the external world of the individual? (iii) What is the relation of intelligence to experience? Various models of the mind underlying the theories have been proposed; the strengths and limitations of these models are assessed. A theory that addresses all three questions simultaneously is the triarchic theory. 相似文献
9.
若尔盖高原不同退化程度草地植物种群生态位特征 总被引：1，自引：0，他引：1
为探索高寒脆弱生态区不同退化程度草地生态系统中各植物种群资源利用能力,通过草地群落学调查结合物种重要值、Shannon-Wiener生态位宽度指数、Pianka生态位重叠指数等对若尔盖高原轻度退化、中度退化、重度退化和极重度退化草地植物群落特征及主要种的生态位进行了研究。结果表明:①在轻度、中度退化群落中,建群种为莎草科Cyperaceae植物线叶嵩草Kobresia capillifolia,已侵入不可食性杂类草,且具有较高的重要值;在重度、极重度群落中,建群种更替为禾本科Poaceae植物垂穗披碱草Elymus nutans和赖草Leymus secalinus,群落中不可食性杂类草的重要值高于轻度和中度退化群落。②随着退化程度加剧,除建群种具有较高的生态位宽度以外,退化草地指示种也在群落中占较大的生态位宽度,增加了草地优良牧草的生长压力。③群落中主要种之间的所有生态位重叠指数平均值随退化程度加剧而降低,依次为轻度退化（0.59）,中度退退化（0.58）,重度退化（0.42）和极重度退化（0.40）,主要种群之间对资源的利用随退化程度加剧在整体上逐渐趋向于离散。轻度退化群落中的线叶嵩草、黄帚橐吾Ligularia virgaurea,中度退化中的线叶嵩草、乳白香青Anaphalis lactea,重度退化中的垂穗披碱草以及极重度退化中的赖草,与群落中的其他种群之间的生态位重叠值较高。综上,若尔盖高原草地在退化过程中,伴随具有不同资源利用策略的植物种群替代的过程。 相似文献
10.
突脉青冈群落乔木层优势种群生态位研究 总被引：2，自引：1，他引：1
采用统计学、数学方法对突脉青冈群落乔木层生态位状况进行定量分析,揭示突脉青冈群落优势种群生态位特征,以恢复其退化生态系统的树种结构和生态功能。结果表明,突脉青冈和米槠生态位较宽,其生态位值分别为0.96和0.83,群落内其它树种的生态位宽度在0.2-0.4之间。生态位相似比例和生态位重叠值表明生态位宽度较大的种,在群落中与其他种生态位相似比例和生态位重叠值较高。说明突脉青冈和米槠2个树种是群落中的优势种群,能更好地适应当地环境,充分利用群落中的资源。该群落继续向前发展,可能发展成为突脉青冈和米槠占优势的顶级群落。 相似文献
11.
春季杨树苗圃主要地下害虫群落生态位研究 总被引：3，自引：0，他引：3
分析了杨树苗圃春季主要地下害虫群落的空间生态位、时间生态位以及时空二维生态位。结果表明:6种主要地下害虫生态位各不相同:在空间维度上,小黄鳃金龟、华北大黑鳃金龟以及阔胫鳃金龟生态位宽度较大,黑绒鳃金龟和沟金针虫生态位宽度较小;小黄鳃金龟与沟金针虫生态位重叠值较大,毛黄脊鳃金龟与沟金针虫重叠值较小。在时间维度上,除毛黄脊鳃金龟生态位宽度较小外,其他地下害虫生态位宽度均较大。在时空维度上,阔胫鳃金龟生态位宽度最大,沟金针虫生态位宽度最小;阔胫鳃金龟与沟金针虫生态位重叠值较大,黑绒鳃金龟与沟金针虫重叠值较小。 相似文献
12.
Population experiments with Anolis lizard species demonstrate a relation between the amount of between-species competition and the degree of interspecific resource partitioning (the more the partitioning the less the competition). Specifically, the amount of resource partitioning between the two species (Anolis gingivinus and Anolis wattsi pogus) on the island of St. Maarten is less than that between the two species (Anolis bimaculatus and Anolis wattsi schwartzi) on the island of St. Eustatius. The presence of Anolis wattsi both lowers the growth rates and raises the perch heights of Anolis gingivinus individuals. In contrast, Anolis wattsi has no effect on Anolis bimaculatus. Thus, when there is less resource partitioning, Anolis wattsi has a greater competitive effect. This verifies, for these species, a central assumption of competition theory: the strength of between-species competition is inversely related to the amount of interspecific resource partitioning. 相似文献
13.
对合肥地区小麦穗期的三种蚜虫及其主要天敌的生态位宽度及重叠值进行了测定,分析了各种群在资源利用方面的种间关系。三种蚜虫蚜茧峰、瓢虫主要分布在麦株40cm以上的部位,每两种群之间的垂直生态位重叠值都达0.7以上;每种水平生态位宽度值都在0.6以上,每两种群之间水平生态位重叠值都在0.63以下。生态位随时间有一定的变化。麦蚜天敌的优势类群为蚜茧峰。 相似文献
14.
How stem cells are recruited to and maintained in their niches is crucial to understanding their regulation and use in regenerative medicine. Here, we demonstrate that DE-cadherin-mediated cell adhesion is required for anchoring germline stem cells (GSCs) in their niches in the Drosophila ovary. Two major components of this adhesion process, DE-cadherin and Armadillo/beta-catenin, accumulate at high levels in the junctions between GSCs and cap cells, one of the niche components. Removal of these proteins from GSCs results in stem cell loss. Furthermore, DE-cadherin is required for recruiting GSCs to their niche. Our study demonstrates that anchorage of GSCs in their niche by DE-cadherin-mediated adhesion is important for stem cell maintenance and function. 相似文献
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16.
克拉玛依农业综合开发区外围荒漠植被主要植物种的生态位分析 总被引：10，自引：6，他引：4
采用Shannon-Wiener指数和Pianka测度方法,分别从群落梯度和3个单一环境因子(土壤水分、盐分、酸碱度)对构成克拉玛依农业综合开发区外围荒漠植被的10个主要植物种的生态位分化进行了定量分析.结果表明:①白刺、梭梭和白梭梭的生态位较宽,分布较广,数量较多,利用资源充分,而盐穗木、盐节木和红砂的生态位较窄,生长范围狭小,其余主要物种介于两者之间;②根据生态宽度可以把10个主要植物种划分成4类;③所测度的物种在土壤水分维上的生态位重叠都较大,而在盐分和酸碱度维上则表现出了较大的分异. 相似文献
17.
Coulson T MacNulty DR Stahler DR vonHoldt B Wayne RK Smith DW 《Science (New York, N.Y.)》2011,334(6060):1275-1278
Environmental change has been observed to generate simultaneous responses in population dynamics, life history, gene frequencies, and morphology in a number of species. But how common are such eco-evolutionary responses to environmental change likely to be? Are they inevitable, or do they require a specific type of change? Can we accurately predict eco-evolutionary responses? We address these questions using theory and data from the study of Yellowstone wolves. We show that environmental change is expected to generate eco-evolutionary change, that changes in the average environment will affect wolves to a greater extent than changes in how variable it is, and that accurate prediction of the consequences of environmental change will probably prove elusive. 相似文献
18.
Farmer innovation diffusion (FID) in the developing world is not simply the adoption of an innovation made by farmers, but a process of communication and cooperation between farmers, governments, and other stakeholders. While increasing attention has been paid to farmer innovation, little is known about how farmers’ innovations are successfully diffused. To fill this gap, this paper aims to address the following questions: What conditions are necessary for farmers to participate in FID? How is a collaborative network built up between farmers and stakeholders for this purpose? And what roles can government play? The above questions are addressed through analysis of the diffusion of winter greenhouse technology in China. A framework for analyzing a FID system is developed, and the conclusion is drawn that building mutual trust and collaborative networks is crucial for the success of FID. Furthermore, this network building can be broken down into various levels with different scales, speeds and consequences for FID: informal networks among farmers themselves, farmer-led networks, and government-facilitated networks. The success of government intervention depends upon building and enhancing the collaborative networks in which farmer leadership is crucial. 相似文献
19.
英语词汇中的医学术语 总被引：1，自引：0，他引：1
本文目的在说明如何理解和掌握英语医学术语。主要从以下几方面来说明这个问题。一、医学术语和希腊文拉丁文的关系。二、医学术语的构成为什么要采用希腊文拉丁文的词素?三、医学术语是如何构成的?四、用什么方法去理解和记忆医学术语? 相似文献
20.
岷江干旱河谷主要灌木种群生态位研究 总被引：14，自引：0，他引：14
利用Levins生态位宽度指数和Schoener生态位重叠指数分析了岷江干旱河谷20种主要灌木种群在土壤水分、全N、速效P、速效K和pH 5个资源维上的生态位宽度和生态位重叠特征.结果表明:小花滇紫草、三花莸、白刺花、多花胡枝子、马鞍羊蹄甲具有较大生态位宽度,对环境有较强的适应能力,在维持岷江干旱河谷群落的物种多样性和相对稳定性中发挥重要作用;种群在各个资源维上的宽度值不同,说明对不同的资源利用能力不尽相同,在不同资源空间中的生态适应性亦不同.各个资源维上所有种群生态位宽度和的大小依次为pH全N土壤水分速效K速效P.由此看出,物种对速效P和速效K的利用能力低于对土壤水分的利用;同属种群间的重叠值较低.具有较高生态位宽度的物种一般具有较高的生态位重叠,但并非生态位宽度小的物种其重叠值也就小.灌木种群生态位的研究可用于指导岷江干旱河谷植被的保护和恢复. 相似文献