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→‎Phase transitions in biological systems 生物系统中的相变

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'''渗流percolation'''具有临界指数，能够发生相变。最简单的例子是发生在二维方格中的渗流。其中每一个格子以概率<math>p</math>标记。对于较小的<math>p</math>值，格子就形成较小的'''团簇clusters'''。但是当<math>p</math>达到某个阈值pc时就会出现一个巨大的团簇，此时发生二阶相变。pc附近的P∞行为是P∞~(p-pc)β——其中{{mvar|β}}是一个临界指数。

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=== ~~Phase transitions in biological systems~~ 生物系统中的相变 ===

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===生物系统中的相变 ===

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Phase transitions play many important roles in biological systems. Examples include the [[lipid bilayer]] formation, the [[Coil–globule transition|coil-globule transition]] in the process of [[protein folding]] and [[DNA melting]], liquid crystal-like transitions in the process of [[DNA condensation]], and cooperative ligand binding to DNA and proteins with the character of phase transition.

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Phase transitions play many important roles in biological systems. Examples include the lipid bilayer formation, the coil-globule transition in the process of protein folding and DNA melting, liquid crystal-like transitions in the process of DNA condensation, and cooperative ligand binding to DNA and proteins with the character of phase transition.

相变在生物系统中也具有重要的作用。比如''' 脂质双层lipid bilayer'''的形成，''' 蛋白质折叠protein folding'''和''' DNA解链DNA melting'''过程中的''' 坍塌转变coil–globule transition'''，''' DNA缩合DNA condensation'''过程中的液晶转变，以及具有相变特征的配体与DNA和蛋白质的结合。

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In ''biological membranes'', gel to liquid crystalline phase transitions play a critical role in physiological functioning of biomembranes. In gel phase, due to low fluidity of membrane lipid fatty-acyl chains, membrane proteins have restricted movement and thus are restrained in exercise of their physiological role. Plants depend critically on photosynthesis by chloroplast thylakoid membranes which are exposed cold environmental temperatures. Thylakoid membranes retain innate fluidity even at relatively low temperatures because of high degree of fatty-acyl disorder allowed by their high content of linolenic acid, 18-carbon chain with 3-double bonds. Gel-to-liquid crystalline phase transition temperature of biological membranes can be determined by many techniques including calorimetry, fluorescence, [[spin label]] [[electron paramagnetic resonance]] and [[NMR]] by recording measurements of the concerned parameter by at series of sample temperatures. A simple method for its determination from 13-C NMR line intensities has also been proposed.

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In biological membranes, gel to liquid crystalline phase transitions play a critical role in physiological functioning of biomembranes. In gel phase, due to low fluidity of membrane lipid fatty-acyl chains, membrane proteins have restricted movement and thus are restrained in exercise of their physiological role. Plants depend critically on photosynthesis by chloroplast thylakoid membranes which are exposed cold environmental temperatures. Thylakoid membranes retain innate fluidity even at relatively low temperatures because of high degree of fatty-acyl disorder allowed by their high content of linolenic acid, 18-carbon chain with 3-double bonds. Gel-to-liquid crystalline phase transition temperature of biological membranes can be determined by many techniques including calorimetry, fluorescence, spin label electron paramagnetic resonance and NMR by recording measurements of the concerned parameter by at series of sample temperatures. A simple method for its determination from 13-C NMR line intensities has also been proposed.

在''' 生物膜biological membranes'''中，从'''凝胶gel'''到液晶的相变是其维持生理机能的关键。在凝胶相中，由于'''膜脂质脂肪酰基链membrane lipid fatty-acyl chains'''的流动性低，'''膜蛋白membrane proteins'''的运动受到限制，因此其生理机能的发挥受到限制。植物非常依赖暴露于低温环境下'''叶绿体类囊体膜chloroplast thylakoid membranes'''的'''光合作用photosynthesis'''。由于其高含量的'''亚麻酸linolenic acid'''——带有3个双键的18碳链允许高度的脂肪酰基紊乱，类囊体膜即使在相对较低的温度下也能保持固有的流动性。运用众多技术，包括'''量热法calorimetry'''，'''荧光法fluorescence'''，'''自旋标记电子顺磁共振spin label electron paramagnetic resonance'''和NMR，通过记录各种实验温度下有关参数的测量值来确定生物膜凝胶到液晶相变的温度。人们还提出了一种用13-C NMR谱线强度来测定的简单方法。

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It has been proposed that some biological systems might lie near critical points. Examples include [[neural network]]s in the salamander retina, bird flocks gene expression networks in Drosophila, and protein folding. However, it is not clear whether or not alternative reasons could explain some of the phenomena supporting arguments for criticality. It has also been suggested that biological organisms share two key properties of phase transitions: the change of macroscopic behavior and the coherence of a system at a critical point.

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It has been proposed that some biological systems might lie near critical points. Examples include neural networks in the salamander retina, bird flocks gene expression networks in Drosophila, and protein folding. However, it is not clear whether or not alternative reasons could explain some of the phenomena supporting arguments for criticality. It has also been suggested that biological organisms share two key properties of phase transitions: the change of macroscopic behavior and the coherence of a system at a critical point.

曾经有观点认为生物系统可能位于临界点附近。例如蝾螈视网膜中的神经网络，果蝇中的鸟群基因表达网络和'''蛋白质折叠protein folding'''。但是，尚不清楚替代原因是否可以解释某些现象来支持关键性论证。另一个观点认为，生物有机体具有两个重要的相变特性：宏观行为的变化和系统在临界点的一致性。

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The characteristic feature of second order phase transitions is the appearance of fractals in some scale-free properties. It has long been known that protein globules are shaped by interactions with water. There are 20 amino acids that form side groups on protein peptide chains range from hydrophilic to hydrophobic, causing the former to lie near the globular surface, while the latter lie closer to the globular center. Twenty fractals were discovered in solvent associated surface areas of > 5000 protein segments [39]. The existence of these fractals proves that proteins function near critical points of second-order phase transitions.

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The characteristic feature of second order phase transitions is the appearance of fractals in some scale-free properties. It has long been known that protein globules are shaped by interactions with water. There are 20 amino acids that form side groups on protein peptide chains range from hydrophilic to hydrophilic, causing the former to lie near the globular surface, while the latter lie closer to the globular center. Twenty fractals were discovered in solvent associated surface areas of > 5000 protein segments [39]. The existence of these fractals proves that proteins function near critical points of second-order phase transitions.

二阶相变的特征是在某些'''无标度特性scale-free properties'''中出现了'''分形fractals'''。众所周知，蛋白质球是通过与水的相互作用形成的。'''蛋白质肽链protein peptide chains'''上形成侧基的'''氨基酸amino acids'''有20种（从'''亲水性hydrophilic'''到'''疏水性hydrophilic'''都有）使亲水性的氨基酸位于球状表面附近，疏水性的氨基酸更靠近球状中心。实验人员在一个在与溶剂相关的且表面积大于5000个蛋白质片段的区域中发现了二十个分形。这些分形的存在证明了蛋白质在二阶相变的临界点附近起作用。

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In groups of organisms in stress (when approaching critical transitions), correlations tend to increase, while at the same time, fluctuations also increase. This effect is supported by many experiments and observations of groups of people, mice, trees, and grassy plants.

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In groups of organisms in stress (when approaching critical transitions), correlations tend to increase, while at the same time, fluctuations also increase. This effect is supported by many experiments and observations of groups of people, mice, trees, and grassy plants.

在处于压力下的生物群中（接近临界转变时），相关性会增强，波动也会增加。许多以人、小鼠、树木和草类植物为研究对象的实验都得出了支持性的结果。

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