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Both a unicellular and a multicellular organism are an architectural magnum opus, displaying incredibly high degrees of organization, and biology has not been able to answer two important questions that are crucial to understanding the nature of unicellular organisms. First, how do these microscopic beings determine the relative size, number, and spatial arrangement of organelles within the microscopic cell? We need to know where the “knowledge” necessary for producing and assembling the highly ordered structures comes from.Second, how do they determine their adaptive behavior in search of sources of food and in avoidance behavior? They have to search for and find these sources and get access to them. Only rarely provided with food by luck, they have to go after food sources (it is the prophet that goes to the mountain, not the reverse). Simple as this may seem, their foraging behavior implies that the unicellular is capable of discriminating between the inorganic and organic debris and localize the latter. On its way to the source, it may come across a barrier and has to figure out how to circumvent
it. Sometimes they have to use environmental cues such as light (phototaxis) or chemicals (chemotaxis) as leads to sources of food. Even their movement and necessary corrections in the direction of the source of food require precise calculations on the direction of the beating of cilia or flagella in ciliates or in determining the form, size, and direction of the beating of pseudopodia in amoeba. All these functions require adaptive changes in the structure of the cytoskeleton and microtubules of cilia and flagella and in the actin subunits of microfilaments. We need to know
where all the calculations necessary for determining these adaptive changes in structure and behavior are made.
Both the spatial arrangement of organelles within the cell and the adaptive behaviors mentioned above are not randomly occurring events that require specific information to take place. Hence, they point in the direction of a specialized control center that receives information on the internal and external environment and by processing that information, it produces instructions (chemical signals) that, via effectors, determine the spatial arrangement of organelles and adaptive foraging behaviors. But essentially, these are the functions of a control system. From a theoretical viewpoint as well, it clearly seems that complex systems such as unicellular organisms could not subsist, let alone reproduce and evolve, without a control system. The question, however, would arise whether central or separate local systems of control are responsible for coordinating vital functions in unicellulars. We need to know where the control system and its controller are located within the cell of a unicellular organism. And the only rational approach in looking for the “controller” of these functions is to trace back the possible causal chain by sequentially following the described functions to their proximal causes and farther upstream.
The reductionist Zeitgeist still makes many of us focus on separate organelles rather than on the whole unicellular organism; we see functions and behaviors of unicellulars as products of specialized organelles, including chromosomes (for cell reproduction), ribosomes (for protein synthesis), cytoskeleton (for cell shape and transport of molecules throughout the cell), Golgi apparatus (for processing and 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 selfregulated, 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 decades, 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 regulates 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 (cyclindependent 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 subordinate.
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 critical
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 controlling 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 central
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 unicellular 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.
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