Metabolic Dissipative Structures - Systems Biology of Metabolic and Signaling Networks - page 175

8.1 Introduction

Cells are complex metabolic systems characterized by continuous transformation
and renewal of macromolecular structures, highly coordinated catalytic behavior,
and emergent spatiotemporal metabolic rhythms.
Several studies about biochemical processes have shown that the concept of self-
organization is central to understanding the formation of the cell's biomolecular
architecture and its functional metabolic behavior (Glick 2007 ; Karsenti 2008 ;
Kauffman 1993 ). In general, self-organization can be defined as the spontaneous
emergence of macroscopic nonequilibrium dynamic structures, as a result of col-
lective behavior of elements interacting nonlinearly with each other, to generate a
system that increases its structural and functional complexity driven by energy
dissipation (Halley and Winkler 2008 ; Misteli 2009 ). Another important ingredient
of metabolic self-organization is given by nonlinear interaction mechanisms
between processes, involving, e.g., autocatalysis, product activation, substrate
inhibition.
It is well established that nonequilibrium states can be a source of order in the
sense that the irreversible processes may lead to a new type of dynamic state in
which the system becomes ordered in space and time. The emergent structures
cannot be directly predicted from the individual properties of their elements,
and this kind of self-organized process occurs only in association with energy
dissipation.
Metabolic self-organization is based on the concept of dissipative structures, and
its theoretical roots can be traced back to Ilya Prigogine (Nicolis and Prigogine
1977 ). According to this theory, an open system that operates far from thermody-
namic equilibrium is capable of continuously importing matter and energy from the
environment and, at the same time, exporting entropy. Consequently, the system's
entropy can be either maintained at the same level or decreased, in contrast with the
entropy of an isolated system which tends to increase towards a maximum at
thermodynamic equilibrium. Therefore, the total entropy in an open system can
decrease, and the negative entropy variation can be maintained over time by a
continuous exchange of matter and energy with the environment avoiding the
transition to thermodynamic equilibrium. A dissipative system works as an
energy-transforming mechanism that uses some of the energy inflow to produce a
new form of energy which has a higher thermodynamic value, i.e., lower entropy,
and the negative entropy variation corresponds to a positive variation in the
information of the system. This emergent information increases the complexity of
molecular organization, producing highly ordered macrostructures and functional
dynamic behavior (Ebeling et al. 1986 ; Klimontovich 1999 ; Prigogine et al. 1977 ).
In cells, dissipative self-organization is the main driving force of molecular
order, involved in all fundamental processes, e.g., cell division (Tyson et al. 1996 ),
mitosis (Bastiaens et al. 2006 ; Loose et al. 2008 ), genome organization (Misteli
2009 ), cell differentiation (Woodford and Zandstra 2012 ), bacterial chemotaxis