Autopoietic systems

This reaction scheme applies to the cases of a nuclear explosion, non-cyclic, and of increase in cell population and disease, which may appear to be expanding reaction patterns, but are also self-limiting (see Eigen in Further Reading). We also draw attention to autopoietic systems described by Bitbol and Luisi in Further Reading. 

In other words, it can be said that an autopoietic system organizes the production of its own components, so that these components are continuously regenerated and can therefore maintain the very network process that produces them. 

When you regard a living system you always find a network of processes or molecules that interact in such a way as to produce the very network that produced them and that determine its boundary. Such a network I call autopoietic. Whenever you encounter a network whose operations eventually produce itself as a result, you are facing an autopoietic system. It produces itself. The system is open to the input of matter but closed with regard to the dynamics of the relations that generate it. 

The most general property of an autopoietic system is the capability to generating its own components via a network process that is internal to the boundary. The boundary of the system must be of its own making, a product of the process of component production. Whether a given system is capable of making its own boundary or not is often the most discriminating criterion by which we recognize an autopoietic system. 

The question of the criteria of autopoiesis is formalized at length, but not always clearly, in the primary literature on autopoiesis. Varela, in his latest book (2000), has simplified these criteria into three basic ones, which can he expressed as follows Verify (1) whether the system has a semi-permeable boundary that (2) is produced from within the system and (3) that encompasses reactions that regenerate the components of the system. Thus, a virus is not an autopoietic system, as it does not produce the protein coat of its boundary or the nucleic acids (the host cell does this, and it is living). A computer virus is also not autopoietic, as it needs a computer system that is not produced hy the virus itself. A growing crystal is not autopoietic, as the components are not generated from an internalized network of reactions. 

Figure 8.3 The minimal autopoietic system. This system is characterized by two competitive reactions, one that builds the components of the boundary, and another one that destroys them. According to the relative value of these two velocity constants, the system can be in homeostasis, or grow, or die.

This notion, that reproduction is a consequence of the internal logic of life, can be visualized in Figure 8.3 (see also Luisi, 1996), which is an extension of the drawing of our Green Man (Figure 2.1) discussed in Chapter 2. It represents the various modes of existence of a minimal autopoietic system. This system is defined by a semi-permeable membrane formed by only one component S that allows the entrance of the nutrient A, which is transformed inside the system into S, the... 

This system is autopoietic, but can it be defined as living Intuitively, one would say no. On the other hand, at first sight, this oleate system is not very different from an amoeba that absorbs a nutrient and expels a by-product. What is the difference The question can be made more general are all autopoietic systems living, as emphasized by Gail Fleischacker in her review on autopoiesis (Fleischaker, 1988) -or is autopoiesis just one particular subsystem ... 

As a matter of fact, accephng the two assertions in the primary literature, one that autopoiesis is sufficient to characterize the organization of hfe, and the other, that cognition is equivalent to life, the conclusion should be reached that each autopoietic system is cognitive, and therefore living. This is actually what Heischacker refers to. 

I believe, however, that the assertion that every autopoietic system is living goes too far and in this section I would like to clarify the limits of this assertion. In doing so, I follow the lines of a paper recently presented in collaboration with Michel Bitbol (Bitbol and Luisi, 2004). The queshon of whether autopoiesis is the necessary and sufficient condition, or only the necessary one, has also been asked by Bourgine and Stewart, (2004) and earlier, in another context, by Weber (2002). 

Luisi, P. L. and Varela, F. J. (1990). Self-replicating micelles - a chemical version of minimal autopoietic systems. Orig. Life Evol. Biosph., 19, 633 3. 

The cell is an autopoietic system whose three-dimensional structures are built by assemblies that increase its complexity. 

In all cases, the rate versus [surfactant] profiles are characterized by a sharp change of the rate at [surfactant] > cac. This is the typical behavior of systems that operate on a cooperative basis [32]. Here the cooperativity occurs at the level of the association of the monomeric surfactant to form the aggregate with the onset of its new properties. The outburst of reactivity associated with the spontaneous formation of aggregates once the cac is reached has stimulated Luisi and coworkers [33,34] to introduce the provocative idea of autopoiesis . According to their definition, an autopoietic system is an organizational unit capable of selfmaintenance and, hence, self-reproduction. To illustrate this point they have [33],... 

Figure 4. The minimal autopoietic system. A closed boundary formed by only one molecular component S, with a reagent A which enters through the semipermeable boundary and is transformed into S with rate Vp. A competitive destruction reaction with rate vd transforms S into product(s) P which are eliminated. Depending upon the relative value of Vp and vd, three limit cases of the time development of the autopoietic system will occur, which simulate the three possible state of occurrence of a living cell.

Figure 17.7 Autopoietic systems and complex biochemical reactions in liposomes.

Thanks to autopoiesis, we can define minimal functions, implemented them in a synthetic conshuct, and study the behaviour of such system. Clearly, the system depicted in Figure 17.7b can be realized at different complexity levels, also depending on the chemical nature of the components. Living cells are autopoietic systems where L are of course the lipids (and proteins) constituting the boundary, whereas C is the whole metabolism (including genetic material), that produce itself and the boundary. 

Mavelli and Luisi (21) suggest that such autopoietic systems are a minimalist definition of a living system. They further suggest that self-reproduction is one of the possible kinetic aspects of autopoiesis and therefore autopoietic molecular aggregates can be considered as the most elementary chemical structures capable of simulating certain essential properties of the simplest living systems. In considerations of molecular evolution... 

---