Regulation of the intracellular concentration of substrates is essential for the maintenance of a stable cellular environment. Di!usion and reaction processes supply and consume substrates within cells and determine their steady-state concentrations. To realistically represent these processes by computer simulation they must be modeled in three dimensions. Yet threedimensional models are inherently computing intensive. This study describes a method, which substantially simpli"es the modeling of di!usion into a polyhedral body (a cube), that was used as a model representation of a cell. The method is applied to a case study of oxygen di!usion into nitrogen-"xing, rhizobia-infected cells in legume nodules. The method involved generating a one-dimensional representation of the three-dimensional problem to provide a &&surface area pro"le'' of three-dimensional di!usion. The one-dimensional models were signi"cantly easier to program, several orders of magnitude faster to solve and in this study were validated by assessing their results against those of comparable three-dimensional models of di!usion into the same body. The results show the one-dimensional method to be a close approximation of a three-dimensional source}sink problem with systematic di!erences below 10% for fractional oxygenation of leghemoglobin, cell respiration and nitrogenase activity. Larger di!erences between models (up to 45%) in the predicted average and innermost O concentrations had no e!ects on the physiological conclusions of the study, but were attributed to the poorer resolution of the three-than the one-dimensional model, and to an inherent simpli"cation in the derivation of the one-dimensional surface area pro"les. The one-dimensional modeling approach was found to be a simple, yet powerful tool for the study of di!usion and reaction in biological systems.