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How is the concentration of a substance within a cell related to the concentration of the same substance outside the cell? Many molecules can pass freely across cellular membranes. Small molecules and nonpolar molecules are the ones that can move most easily across the lipid bilayer and enter the cell. Diffusion is the process that causes these molecules to move in or out of the cell. Diffusion is driven by the random movements of individual molecules in solution. If the concentration of a molecule is greater outside a cell than inside the cell then, on average, more molecules will randomly enter the cell, than leave the cell. Molecules that are capable of diffusion across a cellular membrane will eventually reach equal concentrations inside and outside of the cell. At this point the rate at which individual molecules enter and leave will be equal.