Efficiency of energy conversion from metabolic substrates to ATP and mechanical and chemiosmotic energy.

The free energy available from substrate oxidation is a largely invariable figure in biological systems. The extent to which this energy is conserved in high-energy intermediates appears to be optimized during evolution to high efficiency. Since energy transformation by enzymatic and transport processes take place largely with fixed stoichiometrics, high efficiency can only be achieved by adapting free energy levels rather than by adapting turnover (number of molecules synthesized/hydrolyzed, etc.) The relatively small steps (equivalent with high efficiency) in free energy of various metabolic steps implicitly mean that changes in free energy leading to abolition of these steps (e.g., reduction of free energy of ATP) abolish the driving force and interrupt the (net) reaction. However, in the myocardium under limited energy supply (hypoxia) various protective mechanisms appear to be involved which keep free energy levels high for surviving of the cell but for the cost of function (contraction). Those mechanisms might play a role in the phenomena of hibernating and stunning.