Molecules move in and out freely from high and low concentration: phagocytosis active transport pinocytosis facilitated diffusion

Diffusion is the process in which particles in the cells move from an area of high concentration to an area of low concentration. This process usually occurs in lipid bilayers that are hydrophobic (water-fearing). Osmosis is the process in which particles in the cells move from a low concentration to a high concentration. This process usually occurs in glucose that are hydrophilic (water-loving). 

These processes occur until the cell reaches equilibrium or balance. It is part of the many communications that cells make to function properly in living organisms.