Cytosis

Cytosis is an active transport mechanism for the movement of large quantities of molecules into and out of cells.

There are two types of cytosis:

Exocytosis, where a cell directs the contents of secretory vesicles out of the cell membrane. The vesicles fuse with the cell membrane and their content, usually protein, is released out of the cell.

Endocytosis, where a cell absorbs molecules, such as proteins, from outside the cell by engulfing it with the cell membrane. It is used by most cells, because many critical substances are large polar molecules that cannot pass through the cell membrane. Phagocytosis, (literally "cell-eating") is a form of endocytosis wherein large particles are enveloped by the cell membrane of a (usually larger) cell and internalized to form a phagosome, or "food vacuole." Pinocytosis, ("cell-drinking") is a form of endocytosis, a process in which small particles are taken in by a cell by splitting into smaller particles. The particles then form small vesicles which subsequently fuse with lysosomes to hydrolyze, or to break down, the particles.