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In-Depth Information
unicellulars to multicellulars, both metazoan and metaphyta) consists of a controller
that monitors the state of the system, compares it to the norm (set points), and sends
instructions to effectors to maintain/restore the normal state.
The maintenance within the homeostatic ranges of thousands of variables in cells
of unicellular and multicellular organisms is an incredibly complex undertaking. As
typical examples of the physical parameters maintained at normal ranges under brain
control are species-specific normal temperatures in warm-blooded animals and nor-
mal blood pressure. Thousands of chemical parameters, including the pH, are main-
tained at normal levels within the cells and in the body fluids. For example, glucose
is a basic source of energy in animals. The amount of sugar we take with each meal
would elevate the blood glucose to levels that are dangerous for the cell function and
for human health. In normal people, this does not happen: our body senses imme-
diately the higher-than-normal level of blood glucose and beta cells of the islets of
Langerhans are instructed to secrete insulin, which transforms glucose into a glu-
cose polymer (glucogen) that is stored in cells as a reserve source of energy. When
the blood glucose level drops (hypoglycemy), autonomic nerves activate the secre-
tion of the hormone glucagon, an antagonist of insulin, which, binding to its mem-
brane receptor in liver cells via a complex pathway, releases glucose molecules from
glucogen polymers, thus returning blood glucose to normal levels ( Taborsky, 2010 )
( Figure 1.12 ). Glucose homeostasis is regulated by the autonomic nervous system
that controls the secretion of insulin and glucagons by the pancreas, as well as the
metabolic state of liver muscles and fat tissue ( Thorens, 2011 ).
In all the cases studied as of yet, normal levels of homeostatic parameters in ver-
tebrates are maintained by hormones according to signals that ultimately originate in
the brain.
Maintenance of the homeostasis in a unicellular organism is less clear, and pres-
ently, we can speak only of speculative models of the control system and its control-
ler (see the section "The Control System in Unicellulars" later in this chapter).
Control Systems
How do these complex and nonequilibrium structures, which are improbable from
the viewpoint of physicists, arise in the real world? Improbable as they may be, liv-
ing systems would perish of the increasing entropy that results from their own func-
tion, were it not for the antientropic mechanisms that they evolved in the course of
evolution. They do not defy the second law because they cannot. But the antientropic
mechanisms they evolved help them resist the thermodynamic forces of structural
and energetic degradation temporarily until they grow up, mature, and produce off-
spring. These antientropic mechanisms only help living systems to buy the time nec-
essary to perpetuate their structure in the offspring. Thus, while failing in the short
term (the organism dies), the antientropic devices allow them to succeed in the long
term by repeating through generations an endless “relay race” against the second law.
The biological antientropic devices are unique built-in control systems that main-
tain a state of dynamic equilibrium within living organisms and enable them to resist
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