Autopoiesis - life maintaining and reproducing itself

One of the essential features of living systems is that they maintain and remake themselves. This of course is based on autocatalytic chemical networks. It takes more than an autocatalytic set of chemical reactions to make life, though. All known life is cellular, meaning that it is composed (topologically) of compartments, each wholly enclosed in a boundary. In the classic analysis of what fundamentally constitutes life, Maturana and Varela, 1980 discovered that self-maintenance and remaking of this boundary and its contents, by means of processes organised and enacted by the contents with its boundary was essential to all life. They termed this autopoiesis and labelled any system capable of it an autopoietic system. Along side, they recognised that it was also necessary for the autopoietic system to select what it needs and reject what is unfavourable, which in turn implies a need for recognising e.g. chemical features of the environment (both external and internal). They referred to this recognition as cognition. Such selection is a kind of information processing that results in a decrease of entropy for the system doing the selection, relative to its environment (that is because the probability of finding any particular molecule inside the system is different from the random expectation of finding it, based on the random mix outside). To be truly autopoietic, a system must be able to gather resources from the natural environment so as to utilise (make useful) these for the purpose of self-making (either repair or reproduction). Intriguingly, some quite simple chemical systems have been shown to exhibit this capability. For example, Zepik et al. (2001) experimented with fatty-acid ‘vesicles’ (self-contained droplets) and was able to demonstrate their  reproduction and self-maintenance by autonomous homeostatic processes.


Autopoiesis diagram following Luisi.Figure shows an autopoietic system consisting of an enclosing boundary made from components that are manufactured by a chemical process contained within. This manufacture requires that substrait material is selected from the external environment and imported into the internal environment. Newly constructed parts of the enclosing tegument are placed as part of it, renewing decayed parts, which are disposed of as waste (note brightness of colour coding indicates progressive decay of components). This diagram represent the sort of chemical system described in Zepik et al 2001 and is substantially based on diagrams drawn by Pier Luisi.


Bitbol and Luisi (2004) showed that autopoiesis and cognition are separately necessary conditions for life, not inseparably linked as apparently first thought by Maturana and Varela. They illustrated their point with reference to the the autopoietic fatty acid cells. From this work, it became clear that for a system to live, it must have at least the following three properties: autopoiesis, cognition (as described above) and an unbroken boundary to define its limits; this stops the ingredients of life from diffusing apart, rendering life's chemical reactions too thinly spread to work as a whole. This last element is a clear case of information embodied as living structure: the tegument or boundary ‘closes’ the living system in a thermodynamic sense, enabling a difference in the mixture of chemicals and their energies to persist on either side of it. The boundary has to be semi-permeable so that needed resources can be transported inwards and wastes ejected out through it. What is more, in practical living cells, the tegument is infiltrated with molecules that can selectively transport, and in some cases specifically recognise, needed or harmful chemicals. This ability to select is of course information generating. If the tegument were to be broken, then the contents would spill out and the external environment would easily invade, so disrupting the organisation of the cell that it would die. For this reason the tegument of living cells has never been broken since the beginning of life---it has only been divided by repeated fission. Indeed, every cell in your body is not only a distant relative of the first cells that ever existed, it is a fission product; the most recent living instantiation of those first cells (though none of the original material need remain). Multicellular organisms like plants and rabbits are really just colonies of specialising cells that cooperate by communicating information and sharing materials. In this way, the division among organisms (such as you and I) is just an elaboration of division among cells, for example in a bacterial colony. As a result, all life from its beginning to the presently living, is unified as a set of cells, related through replication from a common ancestor. What has been replicated is material in only a rather trivial sense: cells continually replace their material contents. What has been maintained and replicated throughout the whole history of life is the information content that organises this material into a structure that lives, by which we now mean it is information embodied in biochemistry and biological structure that maintains itself, using cognition and the information processing of self-control (homeostasis) and reproduction (information replication, which is a kind of communication).

Further Reading: we recommend Luisi, P. L. (2003). Autopoiesis: a review and a reappraisal. Naturwissenschaften 90:49–59.

References

Bitbol, M. and Luisi, P. (2004). Autopoiesis with or without cognition: defining life at its edge. Journal of the Royal Society Interface, 1(1):99–107.

Maturana, H. and Varela, F. J. (1980). Autopoiesis and Cognition: the Realization of the Living. D. Reidel Publishing Company, Dordrecht, NL. Translation of original: De Maquinas y seres vivos. Universitaria Santiago.

Zepik, H., Blochliger, E., and Luisi, P. (2001). A chemical model of homeostasis. Angewandte Chemie-International Edition, 40(1):199–202.



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