. The initial gradual increase in the yield is due to a large concentration difference between extracting solvent and the cell wall, causing AP molecules to diffuse into the solvent. As extraction continues for a longer time, the concentration difference decreases and extraction becomes difficult due to interior location of cells. After 120 min, almost all AP molecules would have been extracted in the sol vent and TPP would attain equilibrium. Hence, even after increasing the time beyond 120min, the extraction yield remains almost constant.