endocytosis, the vesicles containing the particles become detached from the cell membrane and migrate toward the interior of the cell. Eventually, the vesicle membrane must disintegrate for the particle to truly enter the cell. In exocytosis, the process of endocytosis is reversed, and vesicles expel substances, often secretory substances (such as hormones) or wastes, after moving from the cell's interior to the cell membrane.
Yet despite the cell membrane's selectivity, it cannot regulate all exchanges of materials, including, most importantly, the exchange of water. Because the cell membrane cannot control the effects of osmotic pressure, in a hypertonic solution cells lose water and shrink while in a hypotonic solution cells gain water and swell. Only in a isotonic solution will a cell neither gain nor lose water because of osmosis.
