Model membranes for the study of active transport phenomena

Models for active transport are prepared by the covalent bonding of molecules containing a catalytic site to the membrane asymmetrically with respect to its thickness. The membrane separates two compartments of unequal volume, both initially containing reactant at the same concentration. We observed the time dependence of the reactant and product concentrations, and of the osmotic pressure, and these results are compared with theoretical predictions. The two reactions studied are the hydrolysis of ethyl N-acetyl-L-tyrosinate catalyzed by alpha-chymotrypsin and the acid-catalyzed hydrolysis of triethoxymethane. The asymmetric membrane consumes reactant, and discharges product, at different rates through its two faces. The concentration of products, and of non-rate determining reactants, may exhibit maxima or minima with time, and computer simulation indicates that oscillatory behavior could be expected under appropriate conditions. Due to the different exit path lengths, the osmotic response (determined by the solute concentrations at the two membrane surfaces) may be opposite in sign to that expected from the concentrations in the external compartments.