Endocytosis is the process by which the plasma membrane folds inward to bring substances into the cell. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma membrane or cell membrane. The process opposite to endocytosis is exocytosis.
Through endocytosis animal cells engulf particulate material, such as cellular debris and microorganisms; macromolecules, such as proteins and complex sugars; and low-molecular-weight molecules, such as vitamins and simple sugars. Cells engage in at least three different types of endocytosis: 1) Phagocytosis (literally, cell-eating) is the process by which cells ingest solids, such as bacteria, viruses, or the remnants of cells which have undergone apoptosis; in phagocytosis the membrane invaginates enclosing the wanted particles in a pocket, then engulfs the object by pinching it off, and the object is sealed off into a large vacuole known as a phagosome. 2) Pinocytosis (literally, cell-drinking), which is how cells take in liquids. 3) Receptor-mediated endocytosis is a more specific active event where the cytoplasm membrane folds inward to form coated pits; in this case, proteins or other trigger particles lock into receptors/ ligands in the cell’s plasma membrane; it is then, and only then that the particles are engulfed.