The cytomembrane system cooperates in a number of processes including secretion (CB-18 through CB-27), autophagic digestion and degradation of extracellular material which becomes internalized. A central organelle in the latter two processes is the lysosome, a membrane vesicle filled with acid hydrolases. Lysosomal enzymes are synthesized in granular endoplasmic reticulum and transferred to the Golgi complex (or periGolgi membranes) for packaging into primary lysosomes (upper left of diagram).
Lysosomes supply the enzymes for degradation of worn out cell parts in a process called autophagy (process 5 in diagram). Lysosomes also provide the enzymes for degradation of particulate material taken in by phagocytosis (process 1) or fluid phase pinocytosis (process 2). Membrane bound structures in various phases of degradation are secondary lysosomes. Residual bodies resulting from degradation may be emptied from the cell or, in some cases, retained by the cell as lipofuscin (aging pigment). (Copenhaver et al., Bailey's Textbook of Histology, William and Wilkins Co., Baltimore, MD, 1971.)