Minimal autopoietic system

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... 

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. 

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.

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. 

All the models mentioned thus far are based on autopoietic self-reproduction experiments. The experimental implementation of a homeostatic mode of the autopoietic minimal system, which is also illustrated in Figure 8.3, proved to be much more difficult, and was realized only in 2001 (Zepik et al., 2001). It is based on the oleic acid surfactant system and is schematized in Figure 8.5 (respecting the theoretical scheme of Figure 8.3) there are two competitive reactions, the reaction Up forms oleate surfactant from the hydrolysis of the anhydride and the other reaction destroys oleate via oxidation of the double bond. 

A physical system can be said to be living if it is able to transform external energy/matter into an internal process of self-maintenance and self-generation. This common sense, macroscopic definition, finds its equivalent at the cellular level in the notion of autopoiesis. This can be generalised to describe the general pattern for minimal life, including artificial life. In real life, the autopoietic network of reactions is under the control of nucleic acids and the corresponding proteins. 

Compartmentalized protein synthesis, as noticed above, combines cell-free expression with liposome technology. Several important systems have been designed and experimentally done, as shown by the several reports collected in Table 17.1. There are, however, goals not yet achieved, snch as the simnltaneous protein synthesis (inside the vesicle) and vesicle self-reprodnction, as done in the case of Qj8 replicase experiment. Moreover, these two processes shonld occur not only simultaneonsly, bnt mnst be fnnc-tionally coupled, i.e. the first process should affect the second, as tentatively done in the case of lipid-synthesizing liposomes. However, this would not snffice. In order to make an autopoietic minimal cell, the self-reproduction (or self-replication) of all internalized metabolic components is reqnired, so that the issue of death by dilution is avoided. 

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