Why is life cellular?

The paradox of the cellular space

23 APRIL 2016,
The life of cells
The life of cells

It is well known that life is cellular and only cellular: all tissues and organs of all animals and plants are organized assemblies of cells. Thus, we can consider the cell as the elemental constituent of life on this planet. Cells are (more or less) spherical structures, closed by a membrane, as illustrated by the simplified figure 2. The cellular membrane, which closes the spherical structure, clearly discriminates the inside from the outside. This discrimination between inside and outside, between my world and the environment outside, is the first operational definition of life’s individuality.

But, this is not all. The closed membrane is also a means of protection, a warranty that the contents inside are not going to be diluted or poisoned by all agents swimming outside; and in this way, all that happens inside—also, at the level of evolution—is going to remain a property of the internal world. However, the membrane is, and must be, semi-permeable—and in a very selective way. Some particular compound-nutrients will be allowed to enter, while other compounds (no more useful) are going to be expelled.

Being open, being closed

This uptake of particular substances indicates a general principle: that the cell is thermodynamically open. The living remains, as such, living, only if it acts as an open system—permitting nutrients and energy to enter. In turn, this indicates that the living space and the environmental space are necessarily linked to each other, and form, in a kind of complementarity, the entire space of life.

The notion of thermodynamic openness is in apparent contrast to another general principle of cellular life, commonly referred to as operational closure—as particularly emphasized by the Santiago school of Maturana and Varela (subject of a previous article on WSI). This means the following: that all what is needed for the liver cell to be a liver cell, is contained in the cell itself. Or, in other words: the ant does not need any information from the outside world to be an ant. There are, in life, all kinds of stimuli from the outside, but, this is the important point—these act as triggers, but not as modifiers of the internal structure of the living. Thus, if I kick a dog, or I kick a snake, the dog and the snake are going to react on the basis of their dog-ness or snake-ness, from their internal circuit, from within. This consideration is the basis of the theory of cognition, of Maturana and Varela. From that, see the references indicated below.

The paradox of the cellular space

Inside this relative small space, there is an incredible chemical activity. In each of our liver cells, every second there are thousands of transformations: sugars being oxidized, proteins being hydrolysed, hormones being synthesized, proteins being made thanks of the information of nucleic acids, nucleic acids being synthesized thanks to the catalytic power of enzymes (special proteins)…

Figure 2, in its simplicity, then, does not yield the real complexity of a cell. And as soon as one considers the continuous series of transformations taken place inside each cell of each living organism, one realizes an apparent paradox. The paradox is given by the fact that despite all these transformations, a liver cell remains a liver cell, an amoeba remains an amoeba, and so on…(at least for a long period of observation, say during the so-called homeostasis).

How is this contradiction possible? How can we have self-maintenance and continuous chemical transformations at the same time, in the same place? Here lies the very essence and definition of cellular life, and life in general. In fact, the main function of the cell (and of the living in general) is self-maintenance: the cell maintains its integrity by re-generating from within all the compounds which are being destroyed. The cell, as an open system, does so thanks to the nutrients and energy that comes from the external milieu. It makes it so that there is a continuous conversion of these nutrients into the cell components—this is done to maintain a constant concentration of all reagents inside. The cell, and each living organism in general, can be seen as a factory that re-makes itself from within. Note that what has been said for a single cell is also valid for an entire organism—for the whole elephant or for any human being. I am re-making continuously my haemoglobin, or my hair, or my tissues, from within Autopoiesis (self-production) is the term that Maturana and Varela coined to illustrate this very important principle.

Autopoiesis is the invariant property of life
Conversely, the structure of the various cells in different organisms is the variable property. The membrane constituents can be different. There can be in that particular cell more or less lipids, more or less saccharides, and the concentration of some reagents may vary accordingly—but there is not a cell that does not comply to the general principle of autopoiesis—to the invariant mechanism. The invariant autopoietic mechanism, and the variable structure, form the two complementary aspects of life. Autopoiesis is thus a systems property, in the sense that what is important is the organization of the entire cell, namely the interactions among all cell constituents. This systemic view is in sharp contrast with the reductionist, DNA/RNA centered view, according to which life is essentially based and due to one type of molecule.

Space and localization

But the question that now we would like to ask is the following: where is life localized? Well, the answer is obvious: life is not localized—there is not a single spot, a single reaction, a single metabolic cycle, which may represent that specific bacterial life. Life is the entire net of reactions. Life is not, and cannot be localized, as it is a global, distributed property. And the same can be said for every living organism. Is there a point, a place, where the life of an elephant, or, for that matter, the life of a person, is localized? Of course not.

And this concept, of a systemic space complexity with a distributed quality without a centre of direction, is an important concept in the modern theory of complexity. Where is New York localized? Where is the centre of localization of a bee hive, of a termite’s nest, of a migrating bird formation? Or, for that matter, of a hospital, or of the Common Market? Again, we have a situation, as in the cellular space, which reflects life, but that space encompasses the entire network of relations— of all components with each other. In a way, this is a direct consequence of the system’s view of life, whose main tenet, is the following: that is the entire web of relations that makes up the main general properties.

Finale

This view from within has important consequences from a epistemic point of view. Clearly, the world seen by the fish is different from the world of the bat, and different from the world of the earth worm. There are all different kinds of “cognition.” Which means that the world is not seen as an objective reality, or equal for everyone—there are, then, as many worlds as there are many different organisms.

Now, if we translate this at the level of humankind, we have a similar situation. My view of a rose, as a western man well versed in romanticism and traditional European poetry, is different from the view of an Eskimo who may have never seen a rose. Clearly, this view from within brings about a problem with the notion of objectivity, and brings into question the subjectivity in science—including consciousness—and a theme which we can only point to here; but, has acquired considerable attention in the last two decades of literature.

Note to Fig. 2: A simplified representation of a cell (cross section), with the semipermeable membrane which permits the entrance of certain selected nutrients (N), the expulsion of certain catabolites (H); and then notice the web of reactions inside the cell. How large is a cell? The unit of measure is the micron (um), one thousand of a millimetre, and the smallest unicellular organisms –bacteria- may have a typical diameter of a few microns (Escherichia Coli in our intestine is about 2 um) with the red blood cell we are at about 9 um, with the amoeba we are at 90 um, a human egg cell is around 100 um, giant bacteria can reach 600 um, without forgetting that nerve cells-the neurons-can reach one meter of length.

References:

H. Maturana and F. Varela, The tree of Knoweledge, Shambala, 1998 (revised from the original 1987 version)
F. Varela, Principles of biological Autonomy, 1979, North Holland/Elsevier
F. Capra and P.L. Luisi, The systems view of life, Cambridge Univ. Press, 2014, italian edition Vita e natura. Una visione sistemica
P.L. Luisi, The Emergence of Life, Second edition, Cambridge Univ. Press, 2016

In collaboration with:
Aboca Museum