Gel immobilised living cell systems represent a special form of hetero
geneous catalysis. Due to mass transfer limitations on substrate deliv
ery and product removal, time-dependent spatial variations in growth r
ate and biomass densities are created. A dynamic mathematical model ha
s been developed which was used to simulate the start-up dynamics of g
rowing yeast cells in calcium alginate beads. By coupling the transien
t model with the effectiveness factor calculation, the ''dynamic effec
tiveness factor'' could be calculated. The influence of the bead radiu
s and the initial biomass concentration on the dynamic effectiveness f
actor have been investigated.