Something similar to this happens in cells when, for example some substances like carbon dioxide (which is cellular waste and requires to be excreted out by the cell) accumulates in high concentration inside the cell in the cells external environment the concentration of the carbon dioxide is slow as compare to that inside the cell as soon as there is a difference of concentration of carbon dioxide inside and outside a cell carbon dioxide 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, oxygen enters the cell by the process of diffusion when the level of concentration of oxygen inside the cell decreases. Thus, diffusion place an important role in gases exchange between the cell as well as the cell and its external environment