Bioenergetics of the Stress Response

Abstract

Energy is a property of matter which obeys the two principles of thermodynamics: energy conservation within a given system and the general trend toward a higher degree of disorder, i.e., the concept of entropy (diminution of the amount of energy available within a given system). Chemical reactions in biological and biomolecular systems are based on a succession of energy transmission provided by redox reactions involving the exchange of electrons between the oxidized and the reduced organic substrates. The major energy source of all cells in aerobic organisms is adenosine triphosphate (ATP). Oxidation reactions in nutrients allow ATP synthesis by oxidative phosphorylation. The most common chemical reaction to produce energy in cells is the hydrolysis of ATP to ADP and inorganic phosphate. Before the formalization of the principles of thermodynamics, Antoine-Laurent de Lavoisier (1743–1794) has already anticipated the key principle of bioenergetics among living organisms: “Life is a slow combustion sustained by respiration. Animals are composed of fuel elements. The food replaces loss of substances arising from the combustion of matters present in the body.” Indeed, living systems are open systems drawing their energy from substrates like nutrients. This is why living organisms are fundamentally different from inert material: biochemical reactions lead to an increase in energy availability, i.e., negative entropy. What has perhaps best characterizes a living system is the negative entropy to allow a dynamic and unstable balance between this open system and its environment. See from this thermodynamic perspective, homeostasis (degree of organization of the organism) is only the consequence of the accumulation of negative entropy. It should be therefore possible to consider the frontier between life and dying processes by estimating negative entropy. This opens up new prospects in fields like critical care medicine.