Autopoietic unit

From these simple, basic observations, Maturana and Varela (often referred to as the Santiago school) arrived at a characterization of living systems based on the autopoietic unit. An autopoietic unit is a system that is capable of sustaining itself due to an inner network of reactions that regenerate the system s components (Varela etal., 1974 Maturana and Varela, 1980 Luisi, 1997 Maturana and Varela, 1998 Varela, 2000 Luisa et al, 1996). 

Figure 8.2 The cyclic logic of cellular life. The cell, which is equivalent to an autopoietic unit, is an organized bounded system that determines a network of reactions that in turn produces molecular components that assemble into the organized system that determines the reaction network that... and so on.

The starting point is the interaction between the autopoietic unit and the environment. The living unit is characterized by biological autonomy and at the same time is strictly dependent on the external medium for its survival. There appears to be an apparent contradiction here the living must indeed operate within this contradiction. 

It was said earlier that the interaction with the environment, according to the theory of autopoiesis, must be implemented on the basis of the internal logic of the living. In other words, the consequence of the interaction between an autopoietic unit and a given molecule X is not primarily dictated by the properties of the molecule X, but by the way in which this molecule is seen by the living organism. 

Accordingly, changes, mutations, and evolution are seen as the result of the maintenance of the internal structure of the autopoietic organism. Since the dynamic of the environment may be erratic, the result in terms of evolution is a natural drift, determined primarily by the inner coherence and autonomy of the living organism. In this sense, Maturana and Varela s view (Maturana and Varela, 1980 1986) is close to Kimura s (1983) theory of natural drift and to Jacob s (1982) notion of bricolage. Evolution does not pursue any particular aim - it simply drifts. The path it chooses is not, however, completely random, but is one of many that are in harmony with the inner structure of the autopoietic unit. 

To this aim, let us consider Figure 8.7, keeping in mind also footnote 1. There is an internal cycle of three components. A, B, and C, and all this forms an autopoietic unit, in the sense illustrated in Figure 8.3, whereby, for example, the substance C is the membrane component. Let us now consider a substance X-Y that interacts with the autopoietic unit and is not recognized by the metabolic cycle. This... 

For example, the X to A transformation of Figure 17.7a is described without details, whereas in chemical terms, several molecular transformations require an additional reactant, and in biology, almost all transformation are catalyzed by enzymes. It follows that a slightly more complex description of autopoietic unit is required. In particular, if the X to A transformation (Figure 17.7a) refers to molecular systems, a catalyst C must be introduced in the autopoietic organization. Clearly, the catalyst C, being a single molecule, or a network of processes carried out by several molecules, must also be the product of its own activity, as required by autopoiesis. 

In Figure 17.7b we represent an autopoietic unit composed by boundary-forming molecules L and by a catalytic system C. The precursor(s) X are now transformed into L molecules thanks to the catalytic activity of C, which is also reproducing itself by uptak-ing the precursor(s) Y. Notice that all component of the systems (L and C) are produced from within, the systems is self-bounded, and its behaviour is determined by internal laws. Therefore the system in Figure 17.7b is autopoietic. Also in this case, this system interacts with the environment by taking up building blocks (X and Y) and releasing waste products (W and Z). 

Examples of self-bounded chemical structures which have the capacity to replicate are termed autopoi-etic self-reproducing systems. An autopoietic unit is regarded as a structure capable of self-maintenance by means of processes which all occur within its boundary, with the synthesis of copied structures also being possible. 

Luisi has a continuing interest in molecular evolution, autopoiesis, and the origin of life, and the matrix effect contributes to that debate. An autopoietic unit is a structure capable of self-maintenance, and this includes the option of self-replication. The work has been developed in a series of recent papers. 

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],... 

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