Control Systems in the Living World - Building the Most Complex Structure on Earth - page 23

secreting proteins from the cell), endoplasmic reticulum (for protein transport),
cilia, flagella, and pseudopodia (for cell movement), etc. We are so accustomed to
this view that we take it for granted that these organelles are self-controlled and self-
regulated, even though the supporting evidence is nowhere.
Unlike the metazoans, for which we have a clear picture of the control system
(especially for physiological and behavioral functions of animals), the picture of
the control system of single-celled organisms is blurred. However, in recent dec-
ades, contours of the cell's control system are gradually beginning to emerge before
our eyes.
As an example of a separate system, let us consider a control system that reg-
ulates the cell cycle ( Alberts et al., 2002 ). The control system that regulates both
DNA replication and cell mitosis consists of molecules of cyclin and Cdks (cyclin-
dependent kinases), which form complexes of cyclin-Cdk. The main activators
of Cdks are cyclins and cyclin-Cdk complexes that trigger sequential stages of the
cell cycle. But in less complex cells, where the level of cyclins and inactivation of
cyclin-Cdk complexes is determined by proteolytic enzymes of cyclin, the ultimate
regulator of the cell cycle is external to the system that regulates production of these
proteolytic enzymes, to which obviously the separate genomic control system is sub-
ordinate. The pending question then is: how do these enzymes know when to induce
or suppress their synthesis according to the sequential stages of the cell cycle?
Moreover, this control system of the cell cycle does not account for some of the criti-
cal events of the cycle, such as pole spindle formation and chromosome segregation.
Thus, although separate mechanisms of local control of the development and
functioning of organelles within the cell would exist, a “supersystem” for control-
ling and coordinating the separate systems would be necessary for the unicellular
organism to function properly. There is solid empirical evidence on a central control
of functions in metazoans (including humans), and they are basic topics of animal
physiology and animal behavior, respectively. There is also ample evidence of a cen-
tral control of the animal organogenesis ( Cabej, 2005, pp. 69 et seq, 2008, pp. 139
et seq, 2012, pp. 147 et seq ). Since this mechanism in multicellulars will be briefly
described later in this chapter, here I will only deal with the control systems in uni-
cellular organisms.
Theoretical considerations aside, even facts such as the perfect coordination
in space and time of the activity of cell organelles (e.g., ingestion and digestion of
foods and excretion of waste in the environment), coordination of movements of
appendages in locomotive behavior (phototaxis and chemotaxis of unicellulars),
which involves the repatterning of the whole cell cytoskeleton and body, formation
of pseudopods, coordination of thousands of cilia, and undulating motion of flagella,
all of which point in the direction of the existence of a central control system.
The time for proclaiming the discovery of a central control system within the cell
may not be on the horizon; hence, before we consider any speculative mechanisms
of the central regulation of cell structure, function, and behavior, I find it appropriate
to take a brief look at some facts and phenomena that represent counterinstances to
the supposed view of the self-regulation of cell organelles, which might also suggest
that a central control system is operative in unicellular organisms.