pinocytosis, a process by which liquid droplets are ingested by living cells. Pinocytosis is one type of endocytosis, the general process by which cells engulf external substances, gathering them into special membrane-bound vesicles contained within the cell. In pinocytosis, rather than an individual droplet of liquid traveling passively through the cell membrane, the droplet first becomes bound, or adsorbed, on the cell membrane, which then invaginates (forms a pocket) and pinches off to form a vesicle in the cytoplasm. It is believed that a vesicle may carry extracellular fluid to the opposite side of the cell, where it undergoes exocytosis. A droplet of fluid could thus be transported through the cell without disturbing its cytoplasm. Alternatively, the contents of the vesicle may be released to mix with the cytoplasm. See also phagocytosis.