Systems Biomechanics of the Cell - Systems Biomechanics of the Cell - page 34

Fig. 17 Orientation of the centrosome with respect to the attachment substrate as predicted by the
cell boundary energy model. 0 is orientation with the centrosome toward the substrate, 180° is
away from the substrate. Parameter values ( L and N ) mark the rows and columns of graphs.
Experimentally relevant subpopulations are indicated by numbers. Reproduced from Baratt et al.
( 2008 ) under the Creative Commons Attribution License
Emergent Irreversibility
Irreversibility is the hallmark of life that has long been recognized from the perspec-
tive of biochemical thermodynamics. That energy derived from the nonequilibrium
chemistry can be dissipated in the mechanical manifestations of life, such as move-
ment against viscosity and friction is obvious. This section considers some exam-
ples that demonstrate that the cell mechanics exhibits also nontrivial system-level
irreversibility that emerges specifically on the cellular level of structural organiza-
tion. In the light of these examples, the cell body-boundary mechanical interaction
appears in the general case irreversible. It may be posited that the irreversibility of
this type is responsible for the characteristic unidirectionality of the development of
biological form.
Let us recall from the previous section that in an effectively flat cell, displace-
ment of the centrosome from the center leads to deformation of microtubules that
adopt stable and metastable equilibrium forms. Examples of such forms are shown
in Fig. 18 . Calculations (Maly and Maly 2010 ) show that with an increasing devia-
tion of the centrosome from the center, a position will be reached beyond which the
metastable equilibrium form does not exist anymore. When this happens, the