Diffusion is a process in which molecules move from an area of high concentration to an area of low concentration (down the concentration gradient). So, if the concentration of a material is higher outside a cell than inside the cell, then the material will enter the cell and vice versa. This happens in unicellular organisms as well, which rely on diffusion to move materials in and out. In multicellular organisms, however, diffusion has a role in maintaining homeostasis. For example, capillaries with their thin walls and narrow diameters enables blood cells to come close to the thin walls, across which gases can diffuse to surrounding tissue.
