The Organization of living systems
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-producing systems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。
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 neighborhood interaction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
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Steven Rose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。
2.1 The essential idea of Autopoiesis
The fundamental question Maturana and Varela set out to answer is: what distinguishes entities or systems that we would call living from other systems, apparently equally complex, which we would not? How, for example, should a Martian distinguish between a horse and a car? This is an example that Monod (1974, p. 19) uses in addressing the similar but not identical question of distinguishing between natural and artificial systems.
This has always been a problem for biologists, who have developed a variety of answers. First came vitalism (Bergson, 1911; Driesch, 1908), which held that there is some substance or force or principle, as yet unobserved, which must account for the peculiar characteristics of life. Then system theory, 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 ( for example, Cannon, 1939; Priban, 1968). While this was a significant advance, such mechanisms could equally well be built into simple machines that would never qualify as living organisms.
A third approach, the most common recently, is to specify a list of necessary characteristics that any living organism must have - such as reproductive ability, information-processing capabilities, carbon-based chemistry, and nucleic acids (see, for example, Miller, 1978; Bunge, 1979). The first difficulty with this approach is that it is entirely descriptive and not in any real sense explanatory. It works by observing systems that are accepted as living and noting some of their common characteristics. However, this tactic assumes precisely that which is in need of explanation - the distinction between the living and the nonliving. The approach fails to define the characteristics particular to living systems alone or to give any explanation as to how such characteristics might generate the observed phenomena. Second, there is, inevitably, always a lack of agreement about the contents of such lists. Any two lists will contain different characteristics, and it is difficult to prove that every feature in a list is really necessary or that the list is actually complete.
Maturana’s and Varela’s work is based on a number of fundamental observations about the nature of living systems. They will be introduced briefly here but discussed in more detail in later chapters.
1. Somewhat in opposition to current trends that focus on the species or the genes (Dawkins,1978), Maturana and Varela pick out the single, biological individual (for instance, a single celled creature such as an amoeba) as the central example of a living system. One essential feature of such living entities is their individual autonomy. Although they are part of organisms, populations, and species and are affected by their environment, individuals are bounded, self-defined entities.
2. Living systems operate in an essentially mechanistic way. They consist of particular components that have various properties and interactions. The overall behavior of the whole is generated purely by these components and their properties through the interactions of neighboring elements. Thus any explanation of living systems must be a purely mechanistic one.
3. All explanations or descriptions are made by observers (i.e., people) who are external to the system. One must not confuse that which pertains to the observer with that which pertains to the observed. Observers can perceive both an entity and its environment and see how the two relate to each other. Components within an entity, however, cannot do this, but act purely in response to other components.
4. The last two lead to the idea that any explanation of living systems should be nonteleological, i.e., it should not have recourse to ideas of function and purpose. The observable phenomena of living systems result purely from the interactions of neighboring internal components. The observation that certain parts appear to have a function with regard to the whole can be made only by an observer who can interact with both the component and with the whole and describe the relation of the two.
To explain the nature of living systems, Maturana and Varela focus on a single basic example - the individual, living cell. Briefly, a cell consists of cell membrane or boundary enclosing various structures such as nucleus, mitochondria, and lysosomes as well as many (and often complex) molecules produced from within. These structures are in constant chemical interplay both with each other and, in the case of the membrane, with their external medium. It is a dynamic, integrated chemical network of incredible sophistication (see for example Alberts et al.,1989; Raven and Johnson,1991).
What is it that characterizes this as an autonomous, dynamic, living whole? What distinguishes it from machine such as a chemical factory which also consists of complex components and interacting processes of production forming an organized whole? It can not be to do with any functions or purposes that any single cell might fulfill in a larger multi-cellular organism since there are single-cellular organisms that survive by themselves. Nor can it explained in a reductionist way through particular structures or components of the cell such as the nucleus or DNA/RNA. The difference must stem from the way of the parts are organized as a whole. To understand Maturana and Varela’s answer, we need to look at two related questions - what is it that the cell does, that is what is it the cell produces? And what is it that produces the cell? By this I mean the cell itself rather than the results of their reproduction.
What does a cell do? This will be looked at in detail in Section 2.3 but, in essence, it produces many complex and simple substances which remain in the cell (become of the cell membrane) and participate in those very same production processes. Some molecules are excreted from the cell, through the membrane, as waste. What is it that produces the components of the cell? With the help of some basic chemicals imported from its medium, the cell produces its own constituents. So a cell produces its own 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 all that is meant by autopoiesis. The word means “self-producing” and that is what the cell does: it continually produces itself. Living systems are autopoietic - they are organized in such a way that their processes produce the very components necessary for the continuance of these processes. Systems which do not produce themselves are called allopoietic, meaning “other-producing” - for example, a river or a crystal. Maturana and Varela also refer to human-created systems as heteropoietic. An exemple is a chemical factory. Superficially, this is similar to cell, but it produces chemicals that are used elsewhere, and is itself produced or maintained by other systems. It is not self-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 some basic chemicals, everything within it would itself have to be produced by the machine itself. So, there would have to be machines to produce the various components. Of course, these machines themselves would have to be produced, maintained, and repaired by yet more machines, and so on, all within the same single entity. The machine would soon encompass the whole economy.
2. Suppose that you succeed. Then surely what you have created would be autonomous and independent. It would have the ability to construct and reconstruct itself, and would, in a very real sense, be no longer controlled by us, its creators. Would it not seem appropriate to call it living?
3. As life on earth originated from a sea of chemicals, a cell in which a set of chemicals interacted such that the cell created and re-created its own constituents would generate a stable, self-defined entity with a vastly enhanced chance of future development. This indeed is the basis for current research, to be described in section 2.4.1
4. What of death? If, for some reason, either internal or external, any part of the self-production process breaks down, then there is nothing else to produce the necessary components and the whole process falls apart. Autopoiesis is all or nothing - all the processes must be working, or the systems disintegrates.
This, then, is the central idea of autopoiesis: a living system is one organized in such a way that all its components and processes jointly produce those self-producing entity. This concept has nearly been grasped by other biologists, as the quotation from Rose at the start of this chapter shows. But Maturana and Varela were the first to coin a word for this life-generating mechanism, to set out criteria for it (Varela et al., 1974), and to explore its consequences in a rigorous way.
Considering the derivation of the word 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 not use it. Maturana coined the term in relation to the distinction between praxis (the path of arms, or action) and poiesis (the path of letters, or creation). However, it is interesting to see how closely Maturana’s usage of auto- and allopoiesis is actually foreshadowed by the German phenomenological philosopher Martin Heidegger. In the quotation at the start of Chapter 1, Heidegger uses the term poiesis as a bringing-forth and draws the contrast between the self-production (heautoi) of nature and the other-production (alloi) that humans do. Heidegger’s relevance to 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 will describe in more detail Maturana’s and Varela’s specification and vocabulary.
We begin from the observation that all descriptions and explanations are made by observers who distinguish an entity or phenomenon from the general background. Such descriptions always depend in part on the choices and processes of the observer and may or may not correspond to the actual domain of the observed entity. That which is distinguished by an observer, Maturana calls a unity, that is, a whole distinguished from a background. In making the distinction, the properties which specify the unity as a whole are established by the observer. For example, in calling something “a car,” certain basic attributes or defining features (it is mobile, carries people, is steerable) are specified. An observer may go further and analyze a unity into components and their relations. There are different, equally valid, ways in which this can be done. The result will be a description of a composite unity of components and the organization which combines its components 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 define and specify a system as a composite unity of a particular class, and determine its properties as such a unity … by specifying a domain in which it can interact as an unanalyzable whole endowed with constitutive properties.
[Structure] refers to the actual components and the actual relations that these must satisfy in their participation in the constitution of a given composite unity [and] determines the space in which it exists as a composite unity that can be perturbed through the interactions of its components, but the structure does not determine its properties as a unity.
Maturana (1978, p. 32)
The organization consists of the relations among components and the necessary properties of the components that characterize or define the unity in general as belonging to a particular type or class. This determines its properties as a whole. At its most simple, we can illustrate this distinction with the concept of a square. A square is defined in terms of the (spatial) relations between components - a figure with four equal sides, connected together at right angles. This is its organization. Any particular physically existing square is a particular structure that embodies these relations. Another example is a an airplane, which may be defined by describing necessary components such as wings, engines, controls, brakes, seating, and the relations between them allowing it to fly. If a unity has such an organization, then it may be identified as a plane since this particular organizatio would produce the properties we expect in a plane as a whole. Structure, on the other hand, describes the actual components and actual relations of a particular real example of any such entity, such as the Boeing 757 I board at the airport.
This is a rather unusual use of the term structure (Andrew, 1979). Generally, in the description of a system, structure is contrasted with process 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. The distinction between structure and organization is between the reality of an actual example and the abstract generality lying behind all such examples. This is strongly reminiscent of the philosophy of classic structuralism in which an empirical surface “structure” of events is related to an unobservable deep structure (“organization”) of basic relationships which generate the surface.
An existing, composite unity, therefore, has both a structure and an organization. There are many different structures that can realize the same organization, and the structure will have many properties and relations not specified by the organization and essentially irrelevant to it - for example, the shape, color, size, and material of a particular airplane. Moreover, the structure can change or be changed without necessarily altering the organization. For example, as the plane ages, has new parts installed, and gets repainted it still maintains its identity as a plane because its underlying organization has not changed. Some changes, however, will not be compatible with the maintenance of the organization - for example, a crash which converts the plane into a wreck.
The essential distinction between organization and structure is between a whole and its parts. Only the plane as a whole can fly - this is its constitutive property as a unity, its organization. Its parts, however, can interact in their own domains depending on all their properties, but they do so only as individual components. Sucking in a bird can stop an engine; a short circuit can damage the controls. These are perturbations of the structure, which may affect the whole and lead to a loss of organization or which may be compensable, in which can the plane is still able to fly.
With this background, we can consider Maturana’s and Varela’s definition of autopoiesis. A unity is characterized by describing the organization that defines the unity as a member of a particular class that is, which can be seen to generate the observed behavior of unities of that type. Maturana and Varela see living systems as being essentially characterized as dynamic and autonomous and hold that it is their self-production which leads to these qualities. Thus the organization of living systems is one of self-production - autopoiesis. Such an organization can, of course, be realized 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 by constituting and specifying its boundaries as surfaces of cleavage from the background through their preferential interactions within the network, is an autopoietic system. Maturana (1980b, p. 29)
The first part of this quotation details the general idea of a system of self-production, while the second specifies that the system must be actually realized in an entity that produces its own boundaries. This latter point, about producing boundaries, is particularly important when one attempts to apply autopoiesis to other domains, such as the social world, and is a recurring point of debate. Notice also that the definition does not specify that the realization must be a physical one, although in the case of a cell it clearly is. This leaves open the idea of some abstract autopoietic systems such as a set of concepts, a cellular automaton, or a process of communication. What might the boundaries of such a system be? And would we really want to call such a system “living”? Again, this is the subject of much debate - See section 3.3.2
This somewhat bare concept is further developed by considering the nature of such an organization. In particular, as an organization it will involve particular relations among 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 constitution concern the physical topology of the system (say, a cell) - its three-dimensional geometry. For example, that it has a cell membrane, that components are particular distances from each other, that they are the required sizes and shapes. Relations of specification determine that the components produced by the various production processes are in fact the specific ones necessary for the continuation of autopoiesis. Finally, relations of order concern the dynamics of the processes - for example, that the appropriate amounts of various molecules are produced at the correct rate and at the correct time. Specific examples of these relations will be given later, but it can be seen that these correspond roughly to specifying the “where”,”what”, and “when” of the complex production processes occurring in the cell.
It might appear that this description of relations “necessary” for autopoiesis has a functionalist, teleological tone. This is not really the case, as Maturana and Varela strongly object to such explanations. It is simply that, if such components and relationships do occur, they give rise to electrochemical processes that themselves produce further components and processes of the right types and at the right rates to generate an autopoietic system. But there is no necessity to this; it is simply a combination that does, or does not, occur, just as a plant may, or may not, grow depending on the combination of water, light, and nutrients.
In an early attempt to make this abstract characterization more operational, a computer model of an autopoietic cellular automaton was developed together with a six-point key for identifying an autopoitic system (Varela et al., 1974). The key is specified as follows:
i) Determine, through interactions, if the unity has identifiable boundaries. If the boundaries can be determined, proceed to 2. If not, the entity is indescribable and we can say nothing.
ii) Determine if ther are constitutive elements of the unity, that is, components of the unity. If these components can be described, proceed to 3. If not, the unity is an unanalyzable whole and therefore not an autopoietic system.
iii) Determine if the unity is a mechanistic system, that is, the component properties are capable of satisfying certain relations that determine in the unity the interactions and transformations of these components. If this is the case, proceed to 4. If not, the unity is not an autopoietic system.
iv) Determine if the components that constitute the boundaries of the unity constitute these boundaries through preferential neighborhood interactions and relations between themselves, as determined by their properties in the space of their interactions. If this is not the case, you do not have an autopoietic unity because you are determining its boundaries, not the unity itself. If 4 is the case, however, proceed to 5.
v) Determine if the components of the boundaries of the unity are produced by the interactions of the components of the unity, either by transformation of previously produced components, or by transformations and/or coupling of non-component elements that enter the unity trough its boundaries. If not, you do not have an autopoietic unity; if yes proceed to 6.
vi) If all the other components of the unity are also produced by the interactions of its components as in 5, and if those which are not produced by the interactions of other components participate as necessary permanent constitutive components in the production of other components, you have an autopoietic unity in the space in which its components exist. If this is not the case, and there are components in the unity not produced by components of the unity as in 5, or if there are components of the unity which do not participate in the production of other components, you do not have an autopoietic unity.
The first three criteria are general, specifying that there is an identifiable entity with a clear boundary, that it can be analyzed into components, and that it operates mechanistically, i.e., its operation is determined by the properties and relations of its components. The core autopoietic ideas are specified in the last three points. These describe a dynamic network of interacting processes of production (vi), contained within and producing a boundary (v) that is maintained by the preferential interactions of components. The key notions, especially when considering the extension of autopoiesis to nonphysical systems, are the idea of production of components, and the necessity for a boundary constituted by produced components.
These key criteria will be applied to the cell in the next section. This section will describe briefly embodiments of the autopoietic relations outlined above in the chemistry of the cell. Alberts et al. or Freifelder are good introductions to molecular biology, as is Raven and Johnson to the cell.
2.3 An illustration of Autopoiesis in the Cell
This section will describe briefly embodiments of the autopoietic relations outlined above in the chemistry of the cell. Alberts et al. are good introductions to molecular biology, as is Raven and Johnson to the cell.
2.3.1 Applying the Six Criteria
Zeleny and Hufford analyze a typical cell with the six key points. A schematic of two typical cells is shown in Fig 2. One is a eukaryotic cell, i.e., one that has a nucleus, and the other is a prokaryotic cell, 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, the nucleus, and the membranous network known as the endoplasmic reticulum. Thus, the cell is analyzable.
3. The components have electrochemical properties that follow general physical laws determining the transformations and interactions that occur within the cell. Thus, the cell is a mechanistic system.
4.The boundary of the cell is formed by a plasma membrane consisting of phospholipids molecules and certain proteins (fig 3). The lipid molecules are aligned in a double layer, forming a selectively permeable barrier; the proteins are wedged in this bilayer, mediating many 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 form the two layers with the heads outside. The integral proteins also have areas that seek or avoid water. The boundary is therefore self-maintained through preferential neighborhood relations.
5. The lipid and protein components of the boundary are themselves produced by the cell. For example, most of the lipid molecules required for new membrane formation are produced by the endoplasmic reticulum, which is itself a complex, membranous component of the cell. The boundary components are thus self-produced.
6. All of the other components of the cell (e.g., the mitochondria, the nucleus, the ribosomes, the endoplasimic reticulum) are also produced by and within the cell. Certain chemicals (such as metal ions) not produced by the cell are imported through the membrane and then become part of the operations of the cell. Cell components are thus self-produced.
2.3.2 Autopoietic Relations of Constitution, Specification, and Order
Apart from the six-point key, autopoiesis was also defined by three necessary types of relations. These can be illustrated as follows for a typical cell.
184.108.40.206 Relations of Constitution
Relations of constitution determine the three-dimensional shape and structure of the cell so as to enable the other relations of production to be maintained. This occurs through the production of molecules which, through their particular stereochemical properties, enable other processes to continue.
An obvious example is the construction of membranes or cell boundaries. In animal cells, the membrane surrounding the mitochondria, like that around the cell itself, serves to harbor cell contents and control the rate of reaction through diffusion. Various reactive molecules are distributed along the inner membrane in an appropriate order to allow energy-producing sequences to proceed efficiently. In plant cells, in addition to the plasma membrane, there is a cell wall, which consists of cellulose, a material made up of long, straight chains of glucose units packed together to form strong rigid threads. These give plants their rigidity.
A second example is the active sites on enzymatic proteins. These act as catalysts for most reactions, changing a particular substrate in an appropriate way to allow it to react more easily. Generally, the active site is found in certain specific parts of the enzyme molecule where the configuration of amino acids is structured to fit the particular substrate, sometimes with the help of “activators” or co-enzymes. The substrate molecule interlocks with the active site and in so doing changes appropriately so that it no longer fits, and thus frees itself.
220.127.116.11 Relations of Specification
These determine the identity, in chemical properties, of the components of the cell in such a way that through their interactions they participate in the production of the cell. There are two main types of structural correspondence, that among DNA, RNA, and the proteins they produce and that between enzymes and the substrates they catalyze.
Protein synthesis is particularly complex because each protein is formed by linking up to twenty different amino acids in a specific combination, often containing 300 or more units in all. This requires an RNA template molecule, tailor-made for each protein, containing specific spaces for each of the amino acids in order, together with an enzyme and t-RNA for each acid.
As already mentioned, enzymes are necessary to help most of the reactions in the cell, and again, each specific reaction requires an enzyme specific to the reaction and to the substrate involved. Hundreds of such enzymes are needed, and all must be produced by the cell.
18.104.22.168 Relations of Order
Relations of order concern the dynamics of the cell’s production processes. Various chemicals and complex feedback loops ensure that both the rate and the sequence of the various production processes continue autopoiesis. For instance, the production of energy through oxidation is controlled by the amount of phosphate and ADP (adenosine diphosphate) in the mitochondria. At the same time, reactions that use energy actually produce ADP and phosphate so that, automatically, a high usage of energy leads to a high production rate of these necessary substances.
2.3.3 Other Possible Autopoietic Systems
An interesting question leading from the idea of the cell as an autopoietic system is whether or not there are other instances of autopoietic systems. Are multicellular organisms also autopoietic systems? Maturana is equivocal, suggesting that organisms such as animals and plants may be second-order autopoietic systems, with the components being not the cells themselves but various molecules produced by the cells. On the other hand, he suggests that some cellular systems may not actually constitute autopoietic systems, but may be merely colonies. What about a system that appears to have a closed and circular organization but is not generally classified as living, such as the pilot light of a gas boiler? Finally, what about nonphysical systems such as the autopoietic automata mentioned in section 2.2.1 and described more fully in section 4.4, or systems such as a set of ideas or a society? These possibilities will be discussed in 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 its claims, autopoiesis would have had a major impact on the field of biology. In fact, for many years there was a noticeable reluctance to take the ideas seriously at all. In 1979, I wrote to an eminent British biologist - Professor Steven Rose at the Open University - querying the status of autopoiesis. He replied to the effect that he did not wish to comment on autopoiesis but that Maturana was a reputable biologist. One notable exception is Lynn Margulis, whose own theory, that eukaryotic cells evolved through the symbiosis of simpler units, is itself quite controversial.
However, recently interest has been growing in two areas: research into the origins of life and the creation of chemical systems that, although not living, display some of the characteristics of autopoietic self-production. Autopoiesis has also been compared with Prigogine’s dissipative structures. Varela has also pursued work on the nature of the immune system, viewing it as organizationally closed but not autopoietic. However, as this topic is very technical and not of primary 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 origin of life on Earth. In the first approach, based on study of the enzymes and genes, life is characterized as being molecular and a defining feature is the structure and function of the genes. In the second approach, life is characterized as cellular, and its defining feature is metabolic functioning within the cell. However, neither approach can really specify a standard or model for life against which important questions may be answered. In particular, at what point did prebiotic chemical systems become biotic living systems? And how could we recognize nonterrestrial living systems. Which might be radically different in structure from our own?
Fleischaker proposes that the concept of autopoiesis, together with notions of minimal cell, can provide a sound theoretical framework to tackle these questions within the second tradition mentioned above. Autopoiesis clearly does aim to provide a specific and operationally useful definition of life, although Fleischaker argues that the concept of autopoiesis does need some modification. This modification would restrict “living” systems to autopoietic system in the physical domain rather that allow the possibility of nonphysical living systems, a possibility which ( as mentioned above) is left open by the formal definition of autopoiesis. This will be discussed in Section 3.3.2
Given autopoiesis (or modified version) as a definition of life, the next step in theorizing about the origin of life is to consider how an elementary autopoietic system might have formed. Note that autopoiesis is all or nothing. A self-producing system either exists and produces itself or it does not - there can be no halfway stage. This leads to the idea of a theoretical “minimal” cell which could plausibly emerge, given the early conditions on earth. In fact, Fleischaker considers three different characterizations of minimal cells: a minimal cell representative of the evolved life forms that we know today; a minimal cell that would characterize both terrestrial and nonterrestrial life regardless of its constituents.
About the last, little can be put forward beyond the six-point autopoietic characteristics in the physical space; to be more specific would constrain the possibilities unnecessarily. On the other hand, we can be quite specific about a modern-day cell. Such a cell could be described as “a volume of cytoplasmic solvent capable of DNA-cycled, ATP-driven and enzyme-mediated metabolism enclosed within a phosphor-lipoprotein membrane capable of energy transduction”, This generalized specification can cover both prokaryotes (bacterial) and eukaryotes (algal, fungal, animal, and plant cells) even though there are important differences in their operation.
The most interesting minimal cell scenario concerns the origin of life. The first cell need be only a very basic cell without the later elaborations such as enzymes. Fleischaker suggests that such a cell must exhibit a number of operations (Fig.2.4):
1、The cell must demonstrate the formation and maintenance of a boundary structure that creates a hospitable inner environment and allows selective permeability for incoming and outgoing molecules and ions. The lipid bilayer found in contemporary cells is a good possibility since the hydropholic nature of lipid molecules leads them to form closed spheres in order to avoid contact with water. Lipid bilayers are also permeable in certain ways - for example, to flows of protons or sodium atoms - without the need for the complex enzymes prevalent in contemporary cells.
2. The cell must also demonstrate some form of active energy transduction to maintain it away from entropic chemical equilibrium. One possibility is an early form of photopigment system driven by light. Pigment molecules would become embedded in the membrane and act as proton pumps, leading to the concentration of variety of raw material in the cell.
3. The cell would also need to transport and transform material elements and use these in the production of the cell’s components and its boundary. A possible start in this direction would be the import of carbon dioxide and the physio-chemical transformation of its carbon and oxygen through light-driven carbon fixation.
What is important is not the particular mechanisms for any of these general operations but that whichever mechanisms are postulated, all operations need 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 interesting to look at attempts to identify or create chemical systems based on autopoietic criteria, and to consider whether or not these are living. We shall look at three examples: autocatalytic processes, osmotic growth, and self-replicating micelles.
22.214.171.124. Autocatalytic Reactions
A catalyst is a molecular substance whose presence is necessary for the occurrence of a particular chemical reaction, or which speeds the reaction up, but which is not changed by the reaction. The complex productions of contemporary cells (as opposed to cells that may have existed at the origin of life) require many catalysts, and this is one of the main functions of the enzymes. An autocatalytic process is one in which the specific catalysts required are themselves produced as by-products of the reactions. The process thus self-catalyzes. An example is RNA itself which, in certain circumstances, can form a complex surface that acts like an enzyme in reaction with other RNA molecules (Alberts et al.) Kauffman has a detailed discussion within the context of complexity theory.
Although this process can be described as a self-referring interaction, the system does not qualify as autopoietic because it does not produce its own boundary components and thus cannot establish itself as an autonomous operational entity (Maturana and Varela). Complex, interdependent chemical processes abound in nature, but they are not autopoietic unless they form self-bounded unities that embody the autopoietic organization.
126.96.36.199 Osmotic Growth
Zeleny and Hufford have suggested that a particular form of osmotic growth, studied by Leduc, can be seen as autopoietic. The growth is precipitation of inorganic salt that expands and forms a permeable osmotic boundary. This can be demonstrated by putting calcium chloride into a saturated solution of sodium phosphate. Interaction of the calcium and phosphate ions leads to the precipitation of calcium phosphate in a thin boundary layer. This layer then separates the phosphate from the calcium, water enters through the boundary by osmosis, and the increased internal pressure breaks the precipitated calcium phosphate. This break allows further contact between the internal calcium and the external phosphate, leading to further precipitation. 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 of their preferred neighborhood relations.
5. The boundary components are formed by the interaction of internal and external components following osmosis through the membrane.
6. The components (calcium chloride) are not produced by the cell but are permanent constituent components in the production of other components (the precipitate)
This hypothesis does cause problems, as Leduc’s system is clearly inorganic and not what would be called living. If it is accepted that the system does properly fulfill the criteria of autopoiesis, i.e., that it is an autopoietic system as currently defined, then either we must expand our concept of living or accept that autopoiesis is in need of redefinition to exclude such examples. In fact, it is debatable whether or not this osmotic growth does correctly fulfill the six criteria. It certainly meets the first three, but it is not clear that it is a dynamic network of processes of production.
As for the fourth criterion, the precipitate that forms the boundary is unlike a cell membrane. It is static and inactive, more like a stone wall than an active membrane. It is not formed through “preferential neighborhood interactions”; in fact, once formed, it does not interact at all. Considering the fifth criterion, the boundary components are not continuously produced by the internal processes of production. Rather, a split or rupture occurs and more boundary is precipitated at the split through the interaction of internal and external chemicals. It is only 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.
188.8.131.52 Self-replicating Micelles
An approach with more potential, currently being researched by Bachmann and colleagues, was first proposed by Luisi. It has been discussed by Maddox and Hadlington. A micelle is a small droplet of an organic chemical such as alcohol stabilized in an aqueous solution by a boundary or “surfactant” A reverse micelle is a droplet of water similarly stabilized in an organic solvent. Chemical reactions occur within the micelle, producing more of the boundary surfactant. Eventually, this leads to the splitting of the micelle and the generation of a new one, a process of self-replication. Experiments have been carried out with both ordinary and reverse micelles and with an enzymatically driven system.
In the reverse micelle experiments, the water droplets contain dissolved lithium hydroxide, one of the surfactants is sodium octanoate, and the other is 1-octanol, which is also a solvent. The other solvent is isooctane. The main reaction is one in which the components of the boundary are themselves produced at the boundary. Octyl octanoate is hydrolyzed using the lithium as a catalyst. This produces both the surfactants (sodium octanoate and 1-octanol). Since the lithium hydroxide is insoluble in the organic solvent, it remains within the water micelle, thus confining the reaction to the boundary layer. Once the system is initiated, large numbers of new micelles are produced, although the average size of the micelles decreases.
It is not clear that these systems could yet be called autopoietic. First, the raw materials(the water-lithium mixture or the enzyme catalyst) are not produced within the system. This limits the amount of replication which can occur; the system eventually stops. Even if these materials could be added on a regular basis, the system would still not be self-producing. Second, the single-layer surfactant does not allow transport of raw materials into the micelle. For this to happen, a double-layer boundary would be necessary, as exists in actual cell membranes. Moreover, the researchers themselves, and seem most interested in the fact that the micelles reproduce themselves, and seem to identify this as autopoietic. However, reproduction of the whole is quite secondary to the autopoietic process of self-production of components. Nevertheless, this does represent an interesting step toward generating real autopoietic systems.