Autopoiesis——自创生理论：
生命系统的组织
The Organization of living systems
（张江翻译：
John Minger:《Selfproducing systems-- Implications and Applications of Autopoiesis》一书第二章）
注：近年来，Autopoiesis在西方有关复杂系统、人工生命等领域的文献中频繁出现。该理论虽然不算新，但是对生命的认识可谓独到。它把生命看作一个自我创生的系统，即自己生产自己的一个自指系统，思想简洁而深刻。John Minger的Self-producingsystems一书虽然不是Autopoiesis理论的开山之作，但是叙述清晰、逻辑性很强，因此本人把该书的第二章描述Autopoiesis理论这部分摘出来反映，希望能够引起更多学者的注意。
2.1自创生理论的基本思想
Maturana和Varela（自创生理论Autopoiesis的两位创始人，智利的生物学家）提出的主要问题是：究竟什么是我们区分生命系统和非生命系统的本质原因？要知道，非生命系统也可能与生命系统同等复杂，那么我们究竟为什么可以判断它不是活的？正如Martian所问：什么是我们区别马和汽车的本质原因？这也就是Monod曾经用来说明自然系统和人造系统的本质区别的著名问题。
对于生物学家来说，这个问题由来已久，而且回答各式各样。首先，活力说认为在生命中，有某种未观察到的物质或者力量、规律可以说明生命的各种奇特的现象。其次，系统科学的发展以及一系列概念的提出包括反馈、动态平衡、开放系统等等为生命体那复杂的、具有很强目的性行为提供了机械论式的解释。虽然这种方法是一个大的进步，但是它无法解释为什么类似的机制和特征也可以被非生命的简单机器进行很好的构建。
第三种方法，也就是近来最普遍的方法，就是给生命组织提供一个基本的特征列表，——例如自我繁殖能力、信息处理能力、碳基化学基础以及核酸结构等等。这种方法的第一个困难是它完全是一种描述而并不是真正意义上的解释。它的确很有效，尤其当我们对没有争议的生命系统进行观察并关注它们的共性特征的时候。然而，这种技巧恰恰用“生命与非生命的区别”为理由对其自身进行了解释，也就是它仅仅是一种同意反复。因为这种方法并没有定义生命系统所独有的特征，也没有对这种特征如何生成我们所观察到的现象作任何解释。第二，人们不可避免地会对列表的内容缺乏共识。任意两个列表可能包含了不同的特征，并且我们很难验证列表中的每一种特征都是实际必须的，而且也很难验证列表是真正完全的。
Maturana和Varela的工作是基于对生命系统的自然现象的一系列观察之上作出的，我们将在这里进行简单的介绍，更详细的讨论放在后续的章节。
为了揭示生命系统的自然属性，Maturana和Varela两个人把目光集中到了一个基本的实例上：活体细胞。简要地说，一个细胞是由细胞膜包围而成的各种结构组成，包括核酸、线粒体、溶酶体当然还有它内部产生的各种各样的（通常也是复杂的）分子。这些结构发生着持续的化学相互作用。而对于细胞膜来说，这种相互作用就是它与外部介质的关系。所以细胞实际上是一种动态的、包含着惊人复杂性的化学网络。
那么究竟什么是类似细胞这样的主动、动态、鲜活的系统的整体特征呢？什么区别了一般的机器和生命呢？例如一个化学工厂也包含了大量复杂的部件，并且也通过相互作用的生产来形成一个组织整体，它们与生命的区别是什么呢？我们不能解释说：因为细胞正在执行一个更大的多细胞体生物的某种特定功能或者目的，所以该细胞才是活的。其原因是单细胞生命也可以通过自己的方式谋生而无论是否为更大的多细胞生命体的一部分。我们也不能用还原论的方法来通过某一个特定的细胞结构或者部件例如核酸或者DNA/RNA来解释。也就是说生命的活性必然产生于各种部分组织到一起的方式而不是某一个部件。为了理解Maturana和Varela的回答，我们首先要考察两个相关的问题：细胞在做什么，也就是细胞在生产什么？和什么生产了细胞？在这里，生产的意思是针对一个细胞自己而言的，而不是细胞的繁殖结果。
细胞在做什么呢？我们将在2.3节对这个问题进行详细讨论。然而，从原则上讲，它无非是在产生很多细胞体内的复杂的或者简单的物质（或者成为细胞膜），这些物质还能进一步参加新的生产细胞内部组成物质的过程。当然还有一些分子作为废料通过细胞膜从细胞中排泄了出去。那么，什么产生了细胞中的各种部件呢？现在可以回答，通过外部媒介输入的基本化学材料的帮助，细胞生产了它自己的构成。也就是说，一个细胞生产了它自身的部件，即这些部件的生产过程成为了一个闭圈（Closure）、一个正在进行的自我生产的过程（如图）。
2.2Autopoiesis理论的正式描述
目前，我已经对整个思想有了一个大致的描述，这一节开始更加详细的描述Maturana和Varela的理论细节。
我们观察到：所有的描述和解释都是来源于观察者，他（她）把一个实体或者一种现象从背景中区分出来。这种描述往往部分依赖于观察者的选择和观察过程，而且或多或少都会对被观察对象的实际行为造成影响。
Maturana把一个能被观察者从背景中区分出来的整体称为一个实体。例如，当我们称某物为汽车的时候，一些属性或者预定义的特征就被指定上去了（如它是可以移动的，可以载人，可驾驶的等等）。观察者还可以进一步把一个实体分解成多个单元和单元之间的关系，这种方法虽然不同但是仍然可以非常有效的来指定实体。它的结果就是把一个由多个部件构成的实体描述为一个整体的组织。
Maturana和Varela还特别强调区分实体的组织和实体的结构：
[组织]是指一种发生于部件之间的抽象关系。通过指定一个区域，组织在其中发生相互作用，并被赋予了不可分解的整体属性。组织这种关系定义了一个包含了某种特定类别组件的实体系统，并且决定了该实体的抽象属性。
[结构]是指实际的构成部件以及它们之间的实际关系。这些部件和关系必须满足于他们所属的合成实体的组织抽象关系，并且决定了一个发生部件之间的相互作用而扰动的空间区域，但是结构没有决定它作为一个实体应该具有哪些属性。
（译者注：与我们的理解正相反，组织相当于抽象的类，而结构是指该类的一种特定实例）
组织包含了部件之间的关系以及为了刻画实体术语某种特定类别部件所必需具备的性质。这就决定了它的属性是作为一个整体的。我们可以用正方形的概念来说明组织。一个正方形用空间上部件之间的相互关系定义，正方形就是一个有四个相等边、并且用直角连接在一起的图形。这里正方形就是组织，而任何一个实现正方形的物理方式都是一种能够实现这种抽象组织的特定结构。另外一个例子就是飞机，它可以被定义为若干部件，如翅膀、引擎、控制器、刹车器、座椅而且这些部件之间的关系使得飞机可以飞起来。如果一个实体拥有这样的组织，那么他就会被定义为一个飞机，因为它可以产生我们对于飞机这个整体所期望的属性。另一方面，飞机的结构描述了任意一个包含实际的部件和实际的关系的实现，例如机场跑道上的波音757。
的确，这种使用术语“结构”的方法与通常有很大不同。一般的，在对系统的描述中，结构是与那些不发生缓慢变化的过程相对比的概念，结构和组织几乎是可以互换的。而这里，结构既指静态的也指动态的元素。结构和组织的差别仅仅是具体事例和所有实例背后的抽象普遍性相比较而言的。这使我们想起了经典的结构主义哲学，它们把表面上的经验事件“结构”与一个不可观察到的可以生成这种表面结构的深层结构（“组织”）相关起来。
因此，任何一个存在的合成实体都同时具有结构和组织。有很多种不同的结构可以实现同样的组织，并且每一种结构都会有很多属性和相互作用是不能被组织来指定的，也是与组织不相关的。例如，一个飞机的形状、颜色、尺寸和材质。进一步，结构可以在没有组织更改的条件下改变或被改变。例如，对于经历了很长时间的飞机来说，它有新组装的部件、涂了新漆，但它还是一个飞机因为它的底层组织没有变化。然而，有一些改变就会破坏组织的存在，例如，一次撞击事故把飞机变成了残骸。
结构和组织的本质区别就是整体与部分的区别。飞机只有在作为一个整体的时候才能够飞起来。这是它作为一个整体组成的属性，是它的组织。然而，它的部分，可以依赖于它的所有属性在它自身的领域中与其他部分进行相互作用，然而，它们仅仅作为一个个体的部件来这样完成。与一只小鸟相撞会使得引擎停止；一个线路短路可以毁掉整个控制。这些都是对于结构的干扰，也许这种扰动可以导致整个结构的破坏和消失，也可能是可以弥补的，以至于飞机仍然能正常飞行。
在这种背景下，我们可以很好的理解Maturana和Varela对Autopoiesis理论的定义了。一个实体是通过定义实体隶属于某个类别的组织来刻画的，并且组织决定了该实体所能产生的外在表现。Maturana和Varela把生命系统的本质看作是动态的和自主的，并且认为自我生产是产生这些特性的本质原因。因此，生命的组织就是一种自我产生的系统—Autopoeisis。这种组织当然可以被无穷多的结构来实现。
对自创生系统的一个更加明确的定义是：一个动态的系统，它是一个由部件生成的网络合成的实体，该实体满足：
a) 通过相互作用迭代的再生产产生它们自己的网络
b) 用一个空间上的实体来实现这个网络。该实体要能产生出与它所在的相互作用的背景分离的边界。
这段引文的第一部分详细叙述了自我生产系统的一般思想，第二部分则特别指定了系统必须被实际认出是一个整体，并且它的边界也是系统自己产生的。关于第二点产生边界很重要，尤其是当人们把自创生理论应用到其他非生命领域例如社会的时候。虽然对于细胞来说是该定义是非常清晰的，但是这个定义本身并没有限定系统的实现一定是物理的实体。这就使得该思想是开放的并且可以讨论一些非常抽象的自创生系统，例如一组概念，一个细胞自动机，或者是一个通讯的过程。对于这类抽象的系统来说什么是它们的边界呢？我们真的能把这类系统称为是“活的”么？事实上，这个主题是非常有争议的，详细请看第3.3.2节。
当我们考虑自然中的组织的时候，这个有些空乏的概念还可以得到进一步地扩展。特别是，组织将会涉及到部件之间的关系。根据Maturana和Varela的理论，在物理系统中，这种关系必然是三个种类之一：构造（Constitution），特化（Specification）和顺序(Order)。构造关系是一种系统上的物理学拓扑关系，也就是它的三维几何形体（比如一个细胞）。例如，他们都有细胞膜，各个部件都保持着特定的相对距离，它们都具有需要的尺寸和形状。特化关系决定了被各种构建过程生产的部件必须保持自创生的连续性。最后，顺序关系与过程的动态有关。例如，各种分子的适当配量是在正确的时间以正确的频率生产出来的。关于这些关系的具体细节将在后面论述，然而我们清楚的是，“何处”、“什么”、“何时”细胞中发生了各种复杂的生产过程已经被这三种关系粗糙的说明了。
对于这类“自创生”必需要关系的描述似乎有那么一些目的论的意味。事实上并不是这样的，Maturana和Varela非常强烈的反对这种解释。的确，如果这样的部件和关系发生了，那么它们就会使得电化学过程自己生产自己，并且在正确的频率下，生产正确的类型构造出一个自创生的系统。但事实上，这并不是必需的。因为这样简单的组合也许发生也许不发生，这就像植物生长也许依靠水、光和营养的正确组合，也许并不依靠一样。
为了使早期的这些抽象特征更具有可操作性，Varela在1974年的时候开发了一个自创生的细胞自动机模型，并提出了辨别自创生系统的六个关键点，它们是：
i) 通过相互作用决定实体是否具有一个可区分的边界。如果可以区分出边界，则继续第2个步骤。如果没有，那么实体就是不可描述的，因此我们也没有什么可说的。
ii) 如果实体有组成的元素，也就是实体的部件，这些部件是可以描述的，那么就进入第3步，否则实体是一个不可分析的整体因此也不是一个自创生的系统。
iii) 判断实体是否为一个机械的系统，也就是系统的属性要能满足一定的关系，该关系决定了在实体中部件的交互和转换。如果这种情况满足，则跳到第4步，否则该实体不是一个自创生的系统。
iv)判断实体边界的组成元素和它们之间的关系是通过“近邻偏好相互作用”以及该实体所在的空间的属性来决定的。（译者注：这里的所谓近邻偏好相互作用preferential neighborhoodinteraction是指部件仅仅与它附近的局部元素发生相互作用而不是某种整体宏观的相互作用，就好比所有的人工生命模型一样，每个生命仅仅考虑它的局部环境而不失全局），如果不是这样，那么你没有得到一个自创生的实体，因为系统的边界不是由它自身决定的，而是由外界得到的。如果实体满足条件4，那么就跳到条件5
v) 判断实体的边界是否由实体内的相互作用产生的，或者通过产生的部件转换构成了边界，或者是那些进入组织边界的元素通过耦合转化构成的。如果这些情况都不是，那么这就不是一个自创生实体。如果满足跳到下一步
vi)如果实体内的其他部件也是如第5步中的那样由部件之间的相互作用产生的，同时，虽然有些部件并不是由实体内部组件通过相互作用产生的，但是却参与了其他构成部件的必要而持久的生产过程，那么你就在部件存在的空间中得到了一个自创生的实体。如果不满足这些情况，也就是系统内的部件并不是像5那样由系统内部的其他部件生成的，那么你并没有得到一个自创生的实体。
前三个标准是非常普遍的，它强调用一个清晰的边界定义了一个可分辨的整体，并且这个实体可以被分解成若干部件，而且实体操作这些组件是一种机械的过程，也就是说这些过程都是由部件的属性和关系决定的。自创生思想的核心是最后的三条。这些标准描述了一个交互过程和生产过程的动态网络(vi)，它产生了边界(v)，并且这个边界被组件之间的“近邻偏好相互作用”所维护。关键的一个概念就是部件的生产，而且生产的这些部件构成了边界。尤其是当我们把自创生理论扩展到非物理系统的时候显得尤为重要。
我们将把这些评价标准应用到细胞身上。在这里我们将简要的描述自创生关系的实现，以及通过应用分子生物学中描述的细胞的构成，正如Alberts或Freifelder，Raven和Johnson等人介绍的。
2.3细胞中的自创生过程说明
这一节将简单描述一个细胞中的化学物质自创生关系。Alberts等人曾经作过很好的有关分子生物学的介绍。
2.3.1应用六个标准
Zeleny和Hufford曾经对一个典型的细胞从六个关键点进行了分析，两种细胞的示意图如下图所示，其中一个是真核细胞，也就是细胞内有细胞核的，另一个是原核细胞，其中没有细胞核。
2.3.2构造、特化和顺序的自创生关系
除了六个标准以外，自创生理论还定义了三种必要的关系类型，下面针对一个典型的细胞来说明这些关系。
2.3.2.1构造关系
构造关系决定了细胞的三维形状和结构以便生产的其他关系可以被维护。这种关系是通过那些具备立体化学特性的分子并能够使得其他过程持续发生的分子来实现的。
一个明显的例子就是细胞膜或者细胞边界的构建。在动物细胞中，细胞膜是围绕着线粒体的，就像是围绕着细胞自己，它维护了细胞内的物质并且通过扩散而调节反应的速率。各种各样的反应分子沿着细胞膜按照一种特定的顺序分布着，以便能量生产能够有序而高效的完成。在植物细胞中，除了离子构成的细胞膜以外，还有一个包含了纤维素的细胞壁。这种纤维素是由葡萄糖物质长链包裹在一起而形成的钢硬的线，这就形成了植物的刚性。
第二个例子是蛋白质酶的活动位穴。这些活动位穴对于大部分反应来说起着催化剂的作用，它们能改变特定的物质而使得化学反应更容易发生。一般的，活动位穴存在于酶分子的特定位置。在这些位置上，氨基酸又是通过“催化剂”或者“合作酶”的帮助利用它结构的配置来适应特定的物质。物质分子和活动位穴相互锁定，只有当它们不再适应的时候才会改变。
2.3.2.2特化关系
在化学特性上，这些关系决定了细胞组件的唯一性。这种关系促使细胞组件通过它们之间的相互作用参与了整个细胞的构建。在DNA和RNA以及组建产生的介于酶和酶催化的物质之间的蛋白质中有两种主要的结构对应。
蛋白质合成是很复杂的过程，因为每一种蛋白的形成都需要连接20种不同的氨基酸以形成特定的组合，总计大概需要300或更多的单元。这就需要RNA模板分子，就好像每个蛋白质的裁缝一样，它按照顺序包含了对每个氨基酸以及酶和t-RNA的特定空间。
正如已经论述的，酶对于细胞中的大多数反应都是必要的，另外，每一种特定的反应都需要特定的酶以及相应的物质。因此细胞需要上百种的酶，每一种都是由细胞自己生产的。
2.3.2.3顺序关系
顺序关系涉及了细胞生产过程的动态。各种化学反应形成复杂的反馈闭环以保证各种产物的生产能够以正确的速率和顺序继续。例如，氧化反应的能量生产是在线粒体中被磷酸盐和ADP控制的。同时，化学反应消耗能量来生产ADP和磷酸盐，这样就可以使得整个过程自动化。并且，能量的高效利用就可以使得这些必要物质的生产更加高效。
2.3.3其他可能的自创生系统
一个非常有趣的问题是，自创生系统是否有其他的实例呢？例如，多细胞生命是不是也是自创生系统呢？Maturana关于这个问题的判断是模棱两可的，他怀疑类似动物和植物这样的生物是二级自创生系统。也就是它们的组件不是细胞中的那些分子而是细胞本身。另外，他也建议一些细胞系统并不是真正的自创生系统，而仅仅是群落。那么关于那些存在着闭圈的组织关系系统，但人们并不认为是生命的系统是否为自创生的呢？最后，非物理系统例如自创生自动机以及一群思想或者一个社会是否为自创生系统呢？
2.4．自创生理论在生物和化学中的应用
读者一定认为正如自创生理论所声名的，它应该在生物学领域起到关键作用。而事实上，生物学界接受这个观点却用了很多年的时间。在1979年的时候，我曾经给英国的著名生物学家StevenRose教授写信寻问自创生理论的情况。他回信说虽然Maturana是一个值得尊敬的生物学家，但是他不想对这个理论作出评价。生物学界有一个例外是Lynn Margulis，她提出了原核生命是由很多更简单的生命共生而形成的，这个理论本身也是备受争议的。
然而，随着生命的起源以及能够展现自创生特征的非生命化学系统这两个领域的增长，自创生理论在近年来受到了普遍的关注。人们还就自创生理论与Prigogine的耗散结构论进行了比较。Varela还曾经研究过免疫系统，免疫系统可以被认为是组织闭合的但却不是自创生的系统。由于这个问题比较专业，我们在这里就不讨论了。
2.4.1最小的细胞以及生命的起源
关于地球上的生命起源研究有两个方法主线。第一种方法是基于酶和基因的，它认为生命是用分子的特征以及基因的功能和结构来刻画的。第二种方法是认为生命是用细胞的新陈代谢特征来刻画的。然而，这两种方法都没有提供一个标准的生命模型能够解决所有的重要问题。尤其是在什么样的条件下原生化学系统能够形成获得生命系统呢？我们如何认识非陆地生命系统呢？它们从结构上究竟与我们有什么区别？
Fleischaker提出自创生以及最小细胞的概念一起可以给第二种方法提供一个健全的理论框架。自创生理论的目标自然是给生命提供了一个有用的可操作的定义，虽然Fleischaker争论说自创生系统的定义需要做一定修改。这种修改就是要把“生命”系统局限在物理的自创生系统而不是非物理的可能的生命系统。关于这个问题我们将在第3.3.2节讨论。
当我们把自创生理论作为生命的定义的时候，下一个步骤就是考虑如何构造一个基本的自创生的系统。注意，自创生理论既提供了所有的东西也等于什么都没说。一个自我构建的系统要么存在产生了它自己要么根本就不存在。根本没有两种之间的情况。这就导致了给定了地球原始的条件是否能够涌现出理论上的最小细胞出来。实际上，Fleischaker考虑了三种不同的最小细胞的特征：最小的细胞应该能够代表我们熟悉的生命形式；最小细胞应该能够抓住地球生命同样也是非地球生命的特征。
关于最后一点，没有什么是超过应用于物理空间自创生系统的六点特征的，如果过于具体则会添加不必要的约束。另一方面，我们可以针对现代的细胞详细论述。这样的细胞可以看作是“拥有DNA环状结构、ATP驱动的、具备能量转换功能、由蛋白质细胞膜的包裹的以酶为介质的细胞质溶液集体”。这种概化的说明显然可以涵盖原核生物（如细菌）和真核生物（如海藻、真菌、动物、植物细胞）甚至它们可能存在显著不同。
最小的细胞途径包含了生命的起源。第一个细胞依赖于一个非常基本的没有后期类似酶机制的细胞。Fleischaker建议必须能够实现一些操作：
1、细胞必须能够展示出一个具有边界的结构的形成和维护。这种结构应该能够创造出一个良好的内部环境，并且还能够有选择的输入和输出分子和金属离子。现代细胞研究中发现的磷质双层结构促成了一个封闭球的形成并且避免了与水的接触。磷质双层结构在某方面说也是可渗透的并且可以在没有复杂酶相互作用的条件下进行，例如，它可以渗透钠原子以及质子。
2、细胞还必须拥有某种保持活动能量转换结构来维持自身避免化学的均衡。一个最早的形式就是被光驱动的光合系统。色素分子将会作为质子泵潜入到细胞膜中去并引起细胞内新材质的集中。
3、细胞必须使用、转换它自己的材料来构建它需要的组件及边界。这个过程的一个可能开始是二氧化碳的输入以及碳和氧在光的作用下的转换。
最重要的不是形成这些基本操作的特定机制，而是所有的这些操作都需要成为一个连续的动态的自我生产的网络整体。
2.4.2化学自创生
除了理论上的最小细胞构建以外，如何根据自创生理论以及生命的标准来判断或者构建一个化学系统也是很有趣的。这里我们举三个例子：自催化过程，渗透性生长以及自复制胶体。
2.4.2.1.自催化反应
催化剂是一种能够促进化学反应发生或者加速化学反应的分子物质，但并不被化学反应本身所改变。细胞的复杂构建（正如人们设想的生命起源的时候就存在的细胞）需要很多的催化剂，这恰恰是酶的一个重要功能。一个自创生的过程就是一个化学反应的副产品所创造的特定催化剂的生产过程。因此整个过程就是自催化的。一个实例是RNA分子的自构建过程，在特定的环境下，RNA可以形成像酶一样能够与其他RNA分子反应的复杂表面。考夫曼曾经专门在复杂性理论中探讨过这些问题。
虽然这个过程可以用一种自指的相互作用来表达，但是整个系统并不是自创生的，因为它并没有生产它自己的边界，因此也就不是一个能够构建自身的主动的整体。复杂的，相互依赖的化学反应过程虽然是普遍存在于自然界的，但是它们并不是自创生的除非他们能够形成自我约束的边界，从而实现自创生组织。
2.4.2.2渗透性生长
Zeleny和Hufford建议曾经被Leduc研究的渗透性增长看作是自创生的。这种生长是由无机盐的扩展和沉淀形成的一个可渗透的边界。氯化钙与饱和的钠磷酸盐反应可以说明这个过程。钙和磷酸盐的交互形成了磷化钙沉淀的细边界层。这个层会将磷和钙分离，水会通过渗透穿过边界。提高的内部压力会打破沉淀的磷化钙。这种穿透会进一步促进内部的钙和外部的磷接触，因此形成进一步的沉淀。因此，这个沉淀层就会进一步增长。
Zeleny和Hufford声明这个系统满足自创生的六个标准：
1. 首先，渗透边界形成了渗透性生长是一个可区分的实体；
2. 其次，它可以被分解为更小的部件例如磷化钙边界以及氯化钙；
3. 整个过程服从物理规则；
4. 由于近邻偏好的关系，部件磷化钙聚集形成了边界；
5. 边界是由内部和外部部件在膜的边界处相互作用而形成的；
6. 组件（氯化钙）虽然不是被细胞产生但是其他部件（沉淀物）生产的永久组成物。
这种假设的确会引起一些问题，正如Leduc的系统明显是无机的并且不能被称为活的。如果我们接受该系统符合现在的自创生系统的标准，那么我们也就会不得不扩展我们对生命的理解。事实上，渗透性生长过程是否满足那六条标准是值得争议的。前三条是当然满足的，但是第四条并不满足，因为它并不是一个相互作用的网络在不断生产。
对于第四条，由沉淀形成的边界并不像细胞膜。它是一个静态的被动的边界，更像是石头墙而不是细胞膜。它并没有形成“近邻偏好的相互作用”；事实上，这种边界一旦形成，它就根本没有相互作用了。第五条：边界组件是由内部生产过程持续不断地生成这点也不满足，而是由边界的破裂以及破裂处的进一步沉淀形成的。最后，氯化物也不是系统自己生成的而是一开始就存在的一个单一部件。
2.4.2.3 自复制胶化物
另外一个现象是Bachmann及其同志们研究的。一个胶化物就是一个有机化学物的小液滴，如酒精滴在水中，并且被表面活性剂包围。反胶化物是水滴在有机溶剂中形成的。胶化物中会发生化学反应，不断产生更多的表面活性物。最后，这就会导致胶化物的分离并形成一个新的胶化物。整个过程是自复制的。普通胶化物和反胶化物以及在酶催化作用下的化学反应已经被实验证实。
在反胶化物实验中，水滴包含了溶解的氢氧化铝，表面活化剂是辛酸钠以及1-辛醇（也是一种溶液）。另外还有异辛烷溶液。主要的化学反应是边界组件不断的产生边界组件自己。当锂作催化剂的时候Octyloctanoate是可水解的。由于氢氧化铝在有机溶液中是不可溶解的，因此它会包在水的胶化物之中，大量的胶化物会生成，虽然它的尺寸会慢慢减小。
这些系统被称为是自创生的是值得怀疑的。首先，原始材料（水和铝的混合物或者酶催化剂）不是在系统内部产生的。这就限制了复制的发生次数，系统最终会停止下来。甚至如果这些材料可以被持续添加，系统仍然不是自生产的。第二，单层次表面活性剂不能被原始材料输送到胶体中。所以，一种如真实细胞膜的双层次边界就是必需的了。进一步，研究者们更加关注的是胶化物的自我复制，并认为这就是自创生的。然而，自复制是自创生的第二阶段。无论如何，这种现象说明这不是自创生的过程。
英文原文：
2.1 The essential idea of Autopoiesis
Thefundamental question Maturana and Varela set out to answer is: whatdistinguishes entities or systems that we would call living from othersystems, apparently equally complex, which we would not? How, forexample, should a Martian distinguish between a horse and a car? Thisis an example that Monod (1974, p. 19) uses in addressing the similarbut not identical question of distinguishing between natural andartificial systems.
This has always beena problem for biologists, who have developed a variety of answers.First came vitalism (Bergson, 1911; Driesch, 1908), which held thatthere is some substance or force or principle, as yet unobserved, whichmust account for the peculiar characteristics of life. Then systemtheory, with the development of concepts such as feedback, homeostasis,and open systems, paved the way for explanations of the complex,goal-seeking behavior of organisms in purely mechanistic term ( forexample, Cannon, 1939; Priban, 1968). While this was a significantadvance, such mechanisms could equally well be built into simplemachines that would never qualify as living organisms.
Athird approach, the most common recently, is to specify a list ofnecessary characteristics that any living organism must have – such asreproductive ability, information-processing capabilities, carbon-basedchemistry, and nucleic acids (see, for example, Miller, 1978; Bunge,1979). The first difficulty with this approach is that it is entirelydescriptive and not in any real sense explanatory. It works byobserving systems that are accepted as living and noting some of theircommon characteristics. However, this tactic assumes precisely thatwhich is in need of explanation – the distinction between the livingand the nonliving. The approach fails to define the characteristicsparticular to living systems alone or to give any explanation as to howsuch characteristics might generate the observed phenomena. Second,there is, inevitably, always a lack of agreement about the contents ofsuch lists. Any two lists will contain different characteristics, andit is difficult to prove that every feature in a list is reallynecessary or that the list is actually complete.
Maturana’sand Varela’s work is based on a number of fundamental observationsabout the nature of living systems. They will be introduced brieflyhere but discussed in more detail in later chapters.
1.Somewhat in opposition to current trends that focus on the species orthe genes (Dawkins,1978), Maturana and Varela pick out the single,biological individual (for instance, a single celled creature such asan amoeba) as the central example of a living system. One essentialfeature of such living entities is their individual autonomy. Althoughthey are part of organisms, populations, and species and are affectedby their environment, individuals are bounded, self-defined entities.
2.Living systems operate in an essentially mechanistic way. They consistof particular components that have various properties and interactions.The overall behavior of the whole is generated purely by thesecomponents and their properties through the interactions of neighboringelements. Thus any explanation of living systems must be a purelymechanistic one.
3. All explanations ordescriptions are made by observers (i.e., people) who are external tothe system. One must not confuse that which pertains to the observerwith that which pertains to the observed. Observers can perceive bothan entity and its environment and see how the two relate to each other.Components within an entity, however, cannot do this, but act purely inresponse to other components.
4. The lasttwo lead to the idea that any explanation of living systems should benonteleological, i.e., it should not have recourse to ideas of functionand purpose. The observable phenomena of living systems result purelyfrom the interactions of neighboring internal components. Theobservation that certain parts appear to have a function with regard tothe whole can be made only by an observer who can interact with boththe component and with the whole and describe the relation of the two.
Toexplain the nature of living systems, Maturana and Varela focus on asingle basic example – the individual, living cell. Briefly, a cellconsists of cell membrane or boundary enclosing various structures suchas nucleus, mitochondria, and lysosomes as well as many (and oftencomplex) molecules produced from within. These structures are inconstant chemical interplay both with each other and, in the case ofthe membrane, with their external medium. It is a dynamic, integratedchemical network of incredible sophistication (see for example Albertset al.,1989; Raven and Johnson,1991).
Whatis it that characterizes this as an autonomous, dynamic, living whole?What distinguishes it from machine such as a chemical factory whichalso consists of complex components and interacting processes ofproduction forming an organized whole? It can not be to do with anyfunctions or purposes that any single cell might fulfill in a largermulti-cellular organism since there are single-cellular organisms thatsurvive by themselves. Nor can it explained in a reductionist waythrough particular structures or components of the cell such as thenucleus or DNA/RNA. The difference must stem from the way of the partsare organized as a whole. To understand Maturana and Varela’s answer,we need to look at two related questions – what is it that the celldoes, that is what is it the cell produces? And what is it thatproduces the cell? By this I mean the cell itself rather than theresults of their reproduction.
What doesa cell do? This will be looked at in detail in Section 2.3 but, inessence, it produces many complex and simple substances which remain inthe cell (become of the cell membrane) and participate in those verysame production processes. Some molecules are excreted from the cell,through the membrane, as waste. What is it that produces the componentsof the cell? With the help of some basic chemicals imported from itsmedium, the cell produces its own constituents. So a cell produces itsown components, which are therefore what produces it in a circular,ongoing process (Fig. 2.1)
It produces,and is produced by, nothing other than itself. This simple idea is allthat is meant by autopoiesis. The word means “self-producing” and thatis what the cell does: it continually produces itself. Living systemsare autopoietic – they are organized in such a way that their processesproduce the very components necessary for the continuance of theseprocesses. Systems which do not produce themselves are calledallopoietic, meaning “other-producing” – for example, a river or acrystal. Maturana and Varela also refer to human-created systems asheteropoietic. An exemple is a chemical factory. Superficially, this issimilar to cell, but it produces chemicals that are used elsewhere, andis itself produced or maintained by other systems. It is notself-producing.
At first sight this may seem an almost trivial idea, yet further contemplation reviews how significance it is. For example:
1. Imagine try to build autopoietic machine. Save for energy and somebasic chemicals, everything within it would itself have to be producedby the machine itself. So, there would have to be machines to producethe various components. Of course, these machines themselves would haveto be produced, maintained, and repaired by yet more machines, and soon, all within the same single entity. The machine would soon encompassthe whole economy.
2. Suppose that you succeed. Then surely what you have created would beautonomous and independent. It would have the ability to construct andreconstruct itself, and would, in a very real sense, be no longercontrolled by us, its creators. Would it not seem appropriate to callit living?
3. As life on earth originated from a sea of chemicals, a cell in whicha set of chemicals interacted such that the cell created and re-createdits own constituents would generate a stable, self-defined entity witha vastly enhanced chance of future development. This indeed is thebasis for current research, to be described in section 2.4.1
4. What of death? If, for some reason, either internal or external, anypart of the self-production process breaks down, then there is nothingelse to produce the necessary components and the whole process fallsapart. Autopoiesis is all or nothing – all the processes must beworking, or the systems disintegrates.
This,then, is the central idea of autopoiesis: a living system is oneorganized in such a way that all its components and processes jointlyproduce those self-producing entity. This concept has nearly beengrasped by other biologists, as the quotation from Rose at the start ofthis chapter shows. But Maturana and Varela were the first to coin aword for this life-generating mechanism, to set out criteria for it(Varela et al., 1974), and to explore its consequences in a rigorousway.
Considering the derivation of theword itself, Maturana explains that he had the main idea of a circular,self-referring organization without the term autopoiesis. In fact,biology of cognition, the first major exposition of the idea, does notuse it. Maturana coined the term in relation to the distinction betweenpraxis (the path of arms, or action) and poiesis (the path of letters,or creation). However, it is interesting to see how closely Maturana’susage of auto- and allopoiesis is actually foreshadowed by the Germanphenomenological philosopher Martin Heidegger. In the quotation at thestart of Chapter 1, Heidegger uses the term poiesis as a bringing-forthand draws the contrast between the self-production (heautoi) of natureand the other-production (alloi) that humans do. Heidegger’s relevanceto Maturana’s work will be considered further in Section 7.5.2
2.2 Formal Specification of Autopoiesis
Now that I have sketched the idea in general terms, this section willdescribe in more detail Maturana’s and Varela’s specification andvocabulary.
We begin from the observation that all descriptions and explanationsare made by observers who distinguish an entity or phenomenon from thegeneral background. Such descriptions always depend in part on thechoices and processes of the observer and may or may not correspond tothe actual domain of the observed entity. That which is distinguishedby an observer, Maturana calls a unity, that is, a whole distinguishedfrom a background. In making the distinction, the properties whichspecify the unity as a whole are established by the observer. Forexample, in calling something “a car,” certain basic attributes ordefining features (it is mobile, carries people, is steerable) arespecified. An observer may go further and analyze a unity intocomponents and their relations. There are different, equally valid,ways in which this can be done. The result will be a description of acomposite unity of components and the organization which combines itscomponents together into a whole.
Maturana and Varela draw an important distinction between the organization of a unity and its structure:
[Organization]refers to the relations between components that defineand specify a system as a composite unity of a particular class, anddetermine its properties as such a unity … by specifying a domain inwhich it can interact as an unanalyzable whole endowed withconstitutive properties.
[Structure] refers to the actual components and the actual relationsthat these must satisfy in their participation in the constitution of agiven composite unity [and] determines the space in which it exists asa composite unity that can be perturbed through the interactions of itscomponents, but the structure does not determine its properties as aunity.
Maturana (1978, p. 32)
Theorganization consists of the relations among components and thenecessary properties of the components that characterize or define theunity in general as belonging to a particular type or class. Thisdetermines its properties as a whole. At its most simple, we canillustrate this distinction with the concept of a square. A square isdefined in terms of the (spatial) relations between components – afigure with four equal sides, connected together at right angles. Thisis its organization. Any particular physically existing square is aparticular structure that embodies these relations. Another example isa an airplane, which may be defined by describing necessary componentssuch as wings, engines, controls, brakes, seating, and the relationsbetween them allowing it to fly. If a unity has such an organization,then it may be identified as a plane since this particular organizatiowould produce the properties we expect in a plane as a whole.Structure, on the other hand, describes the actual components andactual relations of a particular real example of any such entity, suchas the Boeing 757 I board at the airport.
Thisis a rather unusual use of the term structure (Andrew, 1979).Generally, in the description of a system, structure is contrasted withprocess to refer to those parts of the system which change only slowly;structure and organization would be almost interchangeable. Here,however, structure refers to both the static and dynamic elements. Thedistinction between structure and organization is between the realityof an actual example and the abstract generality lying behind all suchexamples. This is strongly reminiscent of the philosophy of classicstructuralism in which an empirical surface “structure” of events isrelated to an unobservable deep structure (“organization”) of basicrelationships which generate the surface.
Anexisting, composite unity, therefore, has both a structure and anorganization. There are many different structures that can realize thesame organization, and the structure will have many properties andrelations not specified by the organization and essentially irrelevantto it – for example, the shape, color, size, and material of aparticular airplane. Moreover, the structure can change or be changedwithout necessarily altering the organization. For example, as theplane ages, has new parts installed, and gets repainted it stillmaintains its identity as a plane because its underlying organizationhas not changed. Some changes, however, will not be compatible with themaintenance of the organization – for example, a crash which convertsthe plane into a wreck.
The essentialdistinction between organization and structure is between a whole andits parts. Only the plane as a whole can fly – this is its constitutiveproperty as a unity, its organization. Its parts, however, can interactin their own domains depending on all their properties, but they do soonly as individual components. Sucking in a bird can stop an engine; ashort circuit can damage the controls. These are perturbations of thestructure, which may affect the whole and lead to a loss oforganization or which may be compensable, in which can the plane isstill able to fly.
With this background,we can consider Maturana’s and Varela’s definition of autopoiesis. Aunity is characterized by describing the organization that defines theunity as a member of a particular class that is, which can be seen togenerate the observed behavior of unities of that type. Maturana andVarela see living systems as being essentially characterized as dynamicand autonomous and hold that it is their self-production which leads tothese qualities. Thus the organization of living systems is one ofself-production – autopoiesis. Such an organization can, of course, berealized in infinitely many structures.
A more explicit definition of an autopoietic system is
A dynamic system that is defined as a composite unity as a network of productions of components that,
a) through their interactions recursively regenerate the network of productions that produced them, and
b) realize this network as a unity in the space in which they exist byconstituting and specifying its boundaries as surfaces of cleavage fromthe background through their preferential interactions within thenetwork, is an autopoietic system. Maturana (1980b, p. 29)
Thefirst part of this quotation details the general idea of a system ofself-production, while the second specifies that the system must beactually realized in an entity that produces its own boundaries. Thislatter point, about producing boundaries, is particularly importantwhen one attempts to apply autopoiesis to other domains, such as thesocial world, and is a recurring point of debate. Notice also that thedefinition does not specify that the realization must be a physicalone, although in the case of a cell it clearly is. This leaves open theidea of some abstract autopoietic systems such as a set of concepts, acellular automaton, or a process of communication. What might theboundaries of such a system be? And would we really want to call such asystem “living”? Again, this is the subject of much debate – Seesection 3.3.2
This somewhat bare conceptis further developed by considering the nature of such an organization.In particular, as an organization it will involve particular relationsamong components. These relations, in the case of a physical system,must be of three types according to Maturana and Varela (1973):constitution, specification, and order. Relations of constitutionconcern the physical topology of the system (say, a cell) – itsthree-dimensional geometry. For example, that it has a cell membrane,that components are particular distances from each other, that they arethe required sizes and shapes. Relations of specification determinethat the components produced by the various production processes are infact the specific ones necessary for the continuation of autopoiesis.Finally, relations of order concern the dynamics of the processes – forexample, that the appropriate amounts of various molecules are producedat the correct rate and at the correct time. Specific examples of theserelations will be given later, but it can be seen that these correspondroughly to specifying the “where”,”what”, and “when” of the complexproduction processes occurring in the cell.
Itmight appear that this description of relations “necessary” forautopoiesis has a functionalist, teleological tone. This is not reallythe case, as Maturana and Varela strongly object to such explanations.It is simply that, if such components and relationships do occur, theygive rise to electrochemical processes that themselves produce furthercomponents and processes of the right types and at the right rates togenerate an autopoietic system. But there is no necessity to this; itis simply a combination that does, or does not, occur, just as a plantmay, or may not, grow depending on the combination of water, light, andnutrients.
In an early attempt to makethis abstract characterization more operational, a computer model of anautopoietic cellular automaton was developed together with a six-pointkey for identifying an autopoitic system (Varela et al., 1974). The keyis specified as follows:
i) Determine,through interactions, if the unity has identifiable boundaries. If theboundaries can be determined, proceed to 2. If not, the entity isindescribable and we can say nothing.
ii) Determine if ther areconstitutive elements of the unity, that is, components of the unity.If these components can be described, proceed to 3. If not, the unityis an unanalyzable whole and therefore not an autopoietic system.
iii) Determine if the unity is a mechanistic system, that is, thecomponent properties are capable of satisfying certain relations thatdetermine in the unity the interactions and transformations of thesecomponents. If this is the case, proceed to 4. If not, the unity is notan autopoietic system.
iv) Determine if the components that constitute the boundaries of theunity constitute these boundaries through preferential neighborhoodinteractions and relations between themselves, as determined by theirproperties in the space of their interactions. If this is not the case,you do not have an autopoietic unity because you are determining itsboundaries, not the unity itself. If 4 is the case, however, proceed to5.
v) Determine if the components of the boundaries of the unity areproduced by the interactions of the components of the unity, either bytransformation of previously produced components, or by transformationsand/or coupling of non-component elements that enter the unity troughits boundaries. If not, you do not have an autopoietic unity; if yesproceed to 6.
vi) If all the other components of the unity are also produced by theinteractions of its components as in 5, and if those which are notproduced by the interactions of other components participate asnecessary permanent constitutive components in the production of othercomponents, you have an autopoietic unity in the space in which itscomponents exist. If this is not the case, and there are components inthe unity not produced by components of the unity as in 5, or if thereare components of the unity which do not participate in the productionof other components, you do not have an autopoietic unity.
Thefirst three criteria are general, specifying that there is anidentifiable entity with a clear boundary, that it can be analyzed intocomponents, and that it operates mechanistically, i.e., its operationis determined by the properties and relations of its components. Thecore autopoietic ideas are specified in the last three points. Thesedescribe a dynamic network of interacting processes of production (vi),contained within and producing a boundary (v) that is maintained by thepreferential interactions of components. The key notions, especiallywhen considering the extension of autopoiesis to nonphysical systems,are the idea of production of components, and the necessity for aboundary constituted by produced components.
Thesekey criteria will be applied to the cell in the next section. Thissection will describe briefly embodiments of the autopoietic relationsoutlined above in the chemistry of the cell. Alberts et al. orFreifelder are good introductions to molecular biology, as is Raven andJohnson to the cell.
2.3 An illustration of Autopoiesis in the Cell
This section will describe briefly embodiments of the autopoieticrelations outlined above in the chemistry of the cell. Alberts et al.are good introductions to molecular biology, as is Raven and Johnson tothe cell.
2.3.1 Applying the Six Criteria
Zeleny and Hufford analyze a typical cell with the six key points. Aschematic of two typical cells is shown in Fig 2. One is a eukaryoticcell, i.e., one that has a nucleus, and the other is a prokaryoticcell, which does not.
1. The cell has an identifiable boundary formed by the plasma membrane. Thus, the cell is identifiable.
2.The cell has identifiable components such as the mitochondria, thenucleus, and the membranous network known as the endoplasmic reticulum.Thus, the cell is analyzable.
3. The components have electrochemical properties that follow generalphysical laws determining the transformations and interactions thatoccur within the cell. Thus, the cell is a mechanistic system.
4.The boundary of the cell is formed by a plasma membrane consisting ofphospholipids molecules and certain proteins (fig 3). The lipidmolecules are aligned in a double layer, forming a selectivelypermeable barrier; the proteins are wedged in this bilayer, mediatingmany of the membrane functions. A lipid molecule consists of two parts– a polar head, which is attracted to water, and a hydrocarbon (fatty)tail, which is repelled. In solution, the tails join together to formthe two layers with the heads outside. The integral proteins also haveareas that seek or avoid water. The boundary is thereforeself-maintained through preferential neighborhood relations.
5. The lipid and protein components of the boundary are themselvesproduced by the cell. For example, most of the lipid molecules requiredfor new membrane formation are produced by the endoplasmic reticulum,which is itself a complex, membranous component of the cell. Theboundary components are thus self-produced.
6. All of the other components of the cell (e.g., the mitochondria, thenucleus, the ribosomes, the endoplasimic reticulum) are also producedby and within the cell. Certain chemicals (such as metal ions) notproduced by the cell are imported through the membrane and then becomepart of the operations of the cell. Cell components are thusself-produced.
2.3.2 Autopoietic Relations of Constitution, Specification, and Order
Apart from the six-point key, autopoiesis was also defined by threenecessary types of relations. These can be illustrated as follows for atypical cell.
2.3.2.1 Relations of Constitution
Relations of constitution determine the three-dimensional shape andstructure of the cell so as to enable the other relations of productionto be maintained. This occurs through the production of moleculeswhich, through their particular stereochemical properties, enable otherprocesses to continue.
An obvious example is the construction of membranes or cell boundaries.In animal cells, the membrane surrounding the mitochondria, like thataround the cell itself, serves to harbor cell contents and control therate of reaction through diffusion. Various reactive molecules aredistributed along the inner membrane in an appropriate order to allowenergy-producing sequences to proceed efficiently. In plant cells, inaddition to the plasma membrane, there is a cell wall, which consistsof cellulose, a material made up of long, straight chains of glucoseunits packed together to form strong rigid threads. These give plantstheir rigidity.
A second example is the active sites on enzymatic proteins. These actas catalysts for most reactions, changing a particular substrate in anappropriate way to allow it to react more easily. Generally, the activesite is found in certain specific parts of the enzyme molecule wherethe configuration of amino acids is structured to fit the particularsubstrate, sometimes with the help of “activators” or co-enzymes. Thesubstrate molecule interlocks with the active site and in so doingchanges appropriately so that it no longer fits, and thus frees itself.
2.3.2.2 Relations of Specification
These determine the identity, in chemical properties, of the componentsof the cell in such a way that through their interactions theyparticipate in the production of the cell. There are two main types ofstructural correspondence, that among DNA, RNA, and the proteins theyproduce and that between enzymes and the substrates they catalyze.
Protein synthesis is particularly complex because each protein isformed by linking up to twenty different amino acids in a specificcombination, often containing 300 or more units in all. This requiresan RNA template molecule, tailor-made for each protein, containingspecific spaces for each of the amino acids in order, together with anenzyme and t-RNA for each acid.
Asalready mentioned, enzymes are necessary to help most of the reactionsin the cell, and again, each specific reaction requires an enzymespecific to the reaction and to the substrate involved. Hundreds ofsuch enzymes are needed, and all must be produced by the cell.
2.3.2.3 Relations of Order
Relations of order concern the dynamics of the cell’s productionprocesses. Various chemicals and complex feedback loops ensure thatboth the rate and the sequence of the various production processescontinue autopoiesis. For instance, the production of energy throughoxidation is controlled by the amount of phosphate and ADP (adenosinediphosphate) in the mitochondria. At the same time, reactions that useenergy actually produce ADP and phosphate so that, automatically, ahigh usage of energy leads to a high production rate of these necessarysubstances.
2.3.3 Other Possible Autopoietic Systems
An interesting question leading from the idea of the cell as anautopoietic system is whether or not there are other instances ofautopoietic systems. Are multicellular organisms also autopoieticsystems? Maturana is equivocal, suggesting that organisms such asanimals and plants may be second-order autopoietic systems, with thecomponents being not the cells themselves but various moleculesproduced by the cells. On the other hand, he suggests that somecellular systems may not actually constitute autopoietic systems, butmay be merely colonies. What about a system that appears to have aclosed and circular organization but is not generally classified asliving, such as the pilot light of a gas boiler? Finally, what aboutnonphysical systems such as the autopoietic automata mentioned insection 2.2.1 and described more fully in section 4.4, or systems suchas a set of ideas or a society? These possibilities will be discussedin more detail in Section 3.3.
2.4．Applications of Autopoiesis in Biology and Chemistry
One would have expected that, given the importance and nature of itsclaims, autopoiesis would have had a major impact on the field ofbiology. In fact, for many years there was a noticeable reluctance totake the ideas seriously at all. In 1979, I wrote to an eminent Britishbiologist – Professor Steven Rose at the Open University – querying thestatus of autopoiesis. He replied to the effect that he did not wish tocomment on autopoiesis but that Maturana was a reputable biologist. Onenotable exception is Lynn Margulis, whose own theory, that eukaryoticcells evolved through the symbiosis of simpler units, is itself quitecontroversial.
However,recently interest has been growing in two areas: research into theorigins of life and the creation of chemical systems that, although notliving, display some of the characteristics of autopoieticself-production. Autopoiesis has also been compared with Prigogine’sdissipative structures. Varela has also pursued work on the nature ofthe immune system, viewing it as organizationally closed but notautopoietic. However, as this topic is very technical and not ofprimary relevance, it cannot be pursued here.
2.4.1 Minimal Cells and the Origin of Life
There are two main lines of approach to theories concerning the originof life on Earth. In the first approach, based on study of the enzymesand genes, life is characterized as being molecular and a definingfeature is the structure and function of the genes. In the secondapproach, life is characterized as cellular, and its defining featureis metabolic functioning within the cell. However, neither approach canreally specify a standard or model for life against which importantquestions may be answered. In particular, at what point did prebioticchemical systems become biotic living systems? And how could werecognize nonterrestrial living systems. Which might be radicallydifferent in structure from our own?
Fleischaker proposes that the concept of autopoiesis, together withnotions of minimal cell, can provide a sound theoretical framework totackle these questions within the second tradition mentioned above.Autopoiesis clearly does aim to provide a specific and operationallyuseful definition of life, although Fleischaker argues that the conceptof autopoiesis does need some modification. This modification wouldrestrict “living” systems to autopoietic system in the physical domainrather that allow the possibility of nonphysical living systems, apossibility which ( as mentioned above) is left open by the formaldefinition of autopoiesis. This will be discussed in Section 3.3.2
Givenautopoiesis (or modified version) as a definition of life, the nextstep in theorizing about the origin of life is to consider how anelementary autopoietic system might have formed. Note that autopoiesisis all or nothing. A self-producing system either exists and producesitself or it does not – there can be no halfway stage. This leads tothe idea of a theoretical “minimal” cell which could plausibly emerge,given the early conditions on earth. In fact, Fleischaker considersthree different characterizations of minimal cells: a minimal cellrepresentative of the evolved life forms that we know today; a minimalcell that would characterize both terrestrial and nonterrestrial liferegardless of its constituents.
About thelast, little can be put forward beyond the six-point autopoieticcharacteristics in the physical space; to be more specific wouldconstrain the possibilities unnecessarily. On the other hand, we can bequite specific about a modern-day cell. Such a cell could be describedas “a volume of cytoplasmic solvent capable of DNA-cycled, ATP-drivenand enzyme-mediated metabolism enclosed within a phosphor-lipoproteinmembrane capable of energy transduction”, This generalizedspecification can cover both prokaryotes (bacterial) and eukaryotes(algal, fungal, animal, and plant cells) even though there areimportant differences in their operation.
Themost interesting minimal cell scenario concerns the origin of life. Thefirst cell need be only a very basic cell without the laterelaborations such as enzymes. Fleischaker suggests that such a cellmust exhibit a number of operations (Fig.2.4):
1、The cell mustdemonstrate the formation and maintenance of a boundary structure thatcreates a hospitable inner environment and allows selectivepermeability for incoming and outgoing molecules and ions. The lipidbilayer found in contemporary cells is a good possibility since thehydropholic nature of lipid molecules leads them to form closed spheresin order to avoid contact with water. Lipid bilayers are also permeablein certain ways – for example, to flows of protons or sodium atoms –without the need for the complex enzymes prevalent in contemporarycells.
2. The cell must also demonstrate some form of active energytransduction to maintain it away from entropic chemical equilibrium.One possibility is an early form of photopigment system driven bylight. Pigment molecules would become embedded in the membrane and actas proton pumps, leading to the concentration of variety of rawmaterial in the cell.
3. The cell would also need to transport and transform materialelements and use these in the production of the cell’s components andits boundary. A possible start in this direction would be the import ofcarbon dioxide and the physio-chemical transformation of its carbon andoxygen through light-driven carbon fixation.
Whatis important is not the particular mechanisms for any of these generaloperations but that whichever mechanisms are postulated, all operationsneed to be part of a continuous network to form a dynamic,self-producing whole.
2.4.2 Chemical Autopoiesis
Beyond theoretical constructs of minimal cells, it is also interestingto look at attempts to identify or create chemical systems based onautopoietic criteria, and to consider whether or not these are living.We shall look at three examples: autocatalytic processes, osmoticgrowth, and self-replicating micelles.
2.4.2.1. Autocatalytic Reactions
A catalyst is a molecular substance whose presence is necessary for theoccurrence of a particular chemical reaction, or which speeds thereaction up, but which is not changed by the reaction. The complexproductions of contemporary cells (as opposed to cells that may haveexisted at the origin of life) require many catalysts, and this is oneof the main functions of the enzymes. An autocatalytic process is onein which the specific catalysts required are themselves produced asby-products of the reactions. The process thus self-catalyzes. Anexample is RNA itself which, in certain circumstances, can form acomplex surface that acts like an enzyme in reaction with other RNAmolecules (Alberts et al.) Kauffman has a detailed discussion withinthe context of complexity theory.
Althoughthis process can be described as a self-referring interaction, thesystem does not qualify as autopoietic because it does not produce itsown boundary components and thus cannot establish itself as anautonomous operational entity (Maturana and Varela). Complex,interdependent chemical processes abound in nature, but they are notautopoietic unless they form self-bounded unities that embody theautopoietic organization.
2.4.2.2 Osmotic Growth
Zeleny and Hufford have suggested that a particular form of osmoticgrowth, studied by Leduc, can be seen as autopoietic. The growth isprecipitation of inorganic salt that expands and forms a permeableosmotic boundary. This can be demonstrated by putting calcium chlorideinto a saturated solution of sodium phosphate. Interaction of thecalcium and phosphate ions leads to the precipitation of calciumphosphate in a thin boundary layer. This layer then separates thephosphate from the calcium, water enters through the boundary byosmosis, and the increased internal pressure breaks the precipitatedcalcium phosphate. This break allows further contact between theinternal calcium and the external phosphate, leading to furtherprecipitation. Thus the precipitated layer grows.
Zeleny and Hufford argue that this system fulfills the six autopoietic criteria:
1. It is distinguishable entity because of its precipitate boundary.
2. It is analyzable into components such as the calcium phosphate boundary and the calcium chloride.
3. It follows mechanistic laws.
4. The boundary components (calcium phosphate) aggregate because oftheir preferred neighborhood relations.
5. The boundary components are formed by the interaction of internaland external components following osmosis through the membrane.
6. The components (calcium chloride) are not produced by the cell butare permanent constituent components in the production of othercomponents (the precipitate)
Thishypothesis does cause problems, as Leduc’s system is clearly inorganicand not what would be called living. If it is accepted that the systemdoes properly fulfill the criteria of autopoiesis, i.e., that it is anautopoietic system as currently defined, then either we must expand ourconcept of living or accept that autopoiesis is in need of redefinitionto exclude such examples. In fact, it is debatable whether or not thisosmotic growth does correctly fulfill the six criteria. It certainlymeets the first three, but it is not clear that it is a dynamic networkof processes of production.
As for thefourth criterion, the precipitate that forms the boundary is unlike acell membrane. It is static and inactive, more like a stone wall thanan active membrane. It is not formed through “preferential neighborhoodinteractions”; in fact, once formed, it does not interact at all.Considering the fifth criterion, the boundary components are notcontinuously produced by the internal processes of production. Rather,a split or rupture occurs and more boundary is precipitated at thesplit through the interaction of internal and external chemicals. It isonly because of, and at, the rupture that new boundary is produced.Finally, chloride, which is introduced artificially at the beginning,is not produced by the system, and eventually runs out.
2.4.2.3 Self-replicating Micelles
An approach with more potential, currently being researched by Bachmannand colleagues, was first proposed by Luisi. It has been discussed byMaddox and Hadlington. A micelle is a small droplet of an organicchemical such as alcohol stabilized in an aqueous solution by aboundary or “surfactant” A reverse micelle is a droplet of watersimilarly stabilized in an organic solvent. Chemical reactions occurwithin the micelle, producing more of the boundary surfactant.Eventually, this leads to the splitting of the micelle and thegeneration of a new one, a process of self-replication. Experimentshave been carried out with both ordinary and reverse micelles and withan enzymatically driven system.
Inthe reverse micelle experiments, the water droplets contain dissolvedlithium hydroxide, one of the surfactants is sodium octanoate, and theother is 1-octanol, which is also a solvent. The other solvent isisooctane. The main reaction is one in which the components of theboundary are themselves produced at the boundary. Octyl octanoate ishydrolyzed using the lithium as a catalyst. This produces both thesurfactants (sodium octanoate and 1-octanol). Since the lithiumhydroxide is insoluble in the organic solvent, it remains within thewater micelle, thus confining the reaction to the boundary layer. Oncethe system is initiated, large numbers of new micelles are produced,although the average size of the micelles decreases.
Itis not clear that these systems could yet be called autopoietic. First,the raw materials(the water-lithium mixture or the enzyme catalyst) arenot produced within the system. This limits the amount of replicationwhich can occur; the system eventually stops. Even if these materialscould be added on a regular basis, the system would still not beself-producing. Second, the single-layer surfactant does not allowtransport of raw materials into the micelle. For this to happen, adouble-layer boundary would be necessary, as exists in actual cellmembranes. Moreover, the researchers themselves, and seem mostinterested in the fact that the micelles reproduce themselves, and seemto identify this as autopoietic. However, reproduction of the whole isquite secondary to the autopoietic process of self-production ofcomponents. Nevertheless, this does represent an interesting steptoward generating real autopoietic systems.
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