Integrated and Organized Cellular Energetic Systems

Bioenergetics as a part of biophysical chemistry and biophysics is a quantitative science that is based on several fundamental theories. The definition of energy itself is given by the first law of thermodynamics, and application of the second law of thermodynamics shows that the living cells can only function as open systems in which internal order (low entropy state) is maintained by increasing the entropy in surrounding medium (Schrodinger's principle of negentropy). For this mechanism, exchange of mass is needed that yields metabolism as the sum of catabolism and anabolism. The free-energy changes taking place during metabolic reactions obey rules of chemical thermodynamics, which deals with Gibbs free energy of chemical reactions and with electrochemical potentials. For application of these theories to the integrated systems in vivo, complex cellular organization should be accounted for: macromolecular crowding, metabolic channeling and functional coupling mechanisms caused by close and tight protein–protein interactions, compartmentation of enzymes, as well as both macrocompartmentation and microcompartmentation of substrates and metabolites in cells should be considered. For integration of these system components, effective methods of communication, including compartmentalized energy transfer systems, are required. These systems are represented in muscle cells by phosphotransfer networks, mostly by creatine kinase and adenylate kinase systems. System analysis of these networks shows their central role both in energy transfer and metabolic feedback regulation of mitochondrial function, in regulation of ion fluxes and cellular energetics during increased workload by synchronizing the cellular electrical and mechanical activities with energy supply and substrate oxidation.