Endocytosis

It is a process by which cells ingest external fluid, macromolecules, and large particles, including other cells. In endocytosis the external materials are enclosed by a portion of the cell surface membrane, which folds into itself and forms a pouch. The pouch then pinches off from the cell membrane and becomes a membrane – bound organelle called a vesicle. Some of the vesicles fuse with lysosomes, and their contents are digested by lysosomal enzymes.

Biologists distinguish three types of endocytosis, based on the kind of material that is taken into the cell.

1. Pinocytosis involves the transport of suspensions of finaly divided solids or fluids;
2. Phagocytosis is the movement of large particles or whole cells. Many unicellular organisms feed by phagocytosis. In addition, certain cells in animals use phagocytosis to ingest bacteria and viruses that invade the body. These cells, known as phagocytes, allow lysosomes to fuse with the vesicles that contain the ingested bacteria and viruses. Lysosomal enzymes then destroy the bacteria and viruses before they can harm the animal.
3. Receptor - mediated endocytosis : The membrane includes to form vesicles only in regions where particles have bound to specific receptors. The binding stimulates the infoldings.

Exocytosis 

Exocytosis is essentially the reverse to endocytosis. During exocytosis, waste materials such as solid and undigested remains from phagocytic vacuoles may be removed from cells or useful materials may be secreted. Secretion of enzymes form the pancreas is achieved in this way. Plant cells use exocytosis to export the materials needed to form cell walls.