The entry of particles or fluid into cells by methods other than diffusion or active transport across the plasma membrane. In phagocytosis, exhibited by some unicellular holozoic algae, protrusions of the outer regions of the cell, formed by flowing movements of the cytoplasm, surround food particles, enclosing them in a membrane-bound food vacuole. Lysosomes become associated with the food vacuoles, the intervening membranes break down, and hydrolytic enzymes are released to digest the particles. Pinocytosis is a process by which submi-croscopic particles (macromolecules or molecular aggregates) or droplets of extracellular fluid enter cells. The particles adhere to the plasma membrane (absorptive endocytosis) and an invagination forms. This becomes pinched off so taking the particles into the cytoplasm in a membrane-bound * vesicle - a pino-cytotic or endocytotic vesicle. Alternatively extracellular fluid enter a pitlike invagination that becomes pinched off and enters the cytoplasm (fluid endocytosis). Water is absorbed from the vesicles as they move through the cytoplasm. Eventually the membrane breaks down, releasing the contents into the cytoplasm. |
