This term qualifies systems or modes of catalysis often encountered in cellular biology, in which the functional components are not homogeneously distributed as in solution, but rather organized.
Model systems have been described using immobilized enzymes at the surface of matrices, or within porous lattices, surrounded by bulk solution in which reactants are dissolved. In such heterogeneous catalytic systems, the substrates and products have to diffusion to and from the enzymic phase in order to complete the catalytic cycle as sensed by an external observer (e.g., in bulk solution). The effective properties of such a heterogeneous system displays a channeling behavior for the reactants and may strikingly differ from these of the same system in solution. They include in addition to intrinsic properties of the catalyst(s), other features inherent to the non-homogeneous character of the system.
The use of localized probes to monitor the concentration of reactants in the immediate vicinity of the catalyst(s) helps to resolve the contribution of physical processes (diffusion, chemical or electrostatic partition, local changes in pH, etc.) to the observed behavior of heterogeneous systems.

last update, Dec 1999 - Claude Aflalo - suggestions are welcome...