Some substances like CO2
(Which is cellular waste and requires to be excreted out by the cell) accumulates in high concentrations inside the cell. In the cell’s external environment, the concentration of CO2 is low as compared to that inside the cell. As soon as there is a difference of concentration of CO2 inside and outside a cell, CO2 moves out of the cell, from a region of high concentration, to a region of low concentration outside the cell by the process of diffusion. Similarly, O2 enters the cell by the process of diffusion when the level or concentration of O2 inside the cell decreases. Thus diffusion plays an important role in gaseous exchange between the cells as well as the cell and its external environment.