Diffusion is the main method by which small molecules move across the cell membrane. Diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration. The concentration of a substance is the amount of the substance in a given volume. Many substances can move in and out of cells by diffusion alone. Once a substance has passed into a cell through the cell membrane,it can continue to diffuse throughout the cytoplasm down its concentration gradient. Thus,diffusion aids in the even distribution of materials within a cell.
