The kinetics of biological phase transitions manifested by sigmoid time curves: a review of approaches.

Experimental biological data showing a time course of sigmoid shape are customarily interpreted in terms of mass-action chemical kinetic theory of reactions in free solution. But the reactive sites in biological systems are often located in solid or structured cellular components through which cooperative interactions may occur, resulting in phase transitions. In inorganic systems, the time course of phase transitions often shows experimental curves of sigmoid shape, and this may be predicted theoretically. Therefore it is suggested that when sigmoid time curves are observed in biological systems containing solid or structured components, mathematical analysis based on cooperative interactions and phase transitions is more likely to be valid than analysis based on mass-action concepts.