(vesicles entering the exocytosis pathway) or by pumping in protons to
but autophagy refers specifically to a lysosomal degradation–recycling
acidify their contents (lysosomes destined for the intracellular sorting
pathway. Autophagosomes are seen in response to starvation and cell
pathway).
growth.
Within the Golgi stack proper, proteins undergo a series of sequen-
Late endosomes receive lysosomal enzymes from primary lysosomes
tial chemical modifications by Golgi resident enzymes synthesized
derived from the Golgi apparatus after late endosome–lysosome mem-
in the rough endoplasmic reticulum. These include: glycosylation
brane tethering and fusion followed by diffusion of lysosomal contents
(changes in glycosyl groups, e.g. removal of mannose, addition of
into the endosomal lumen. The pH inside the fused hybrid organelle,
N-acetylglucosamine and sialic acid); sulphation (addition of sulphate
now a secondary lysosome, is low (about 5.0) and this activates lyso-
groups to glycosaminoglycans); and phosphorylation (addition of