somal acid hydrolases to degrade the endosomal contents. The products
phosphate groups). Some modifications serve as signals to direct pro-
of hydrolysis either are passed through the membrane into the cytosol,
teins and lipids to their final destination within cells, including lyso-
or may be retained in the secondary lysosome. Secondary lysosomes
somes and plasma membrane. Lipids formed in the endoplasmic
may grow considerably in size by vesicle fusion to form multivesicular
reticulum are also routed for incorporation into vesicles.
bodies, and the enzyme concentration may increase greatly to form
large lysosomes (Fig. 1.5B).
Exocytic (secretory) pathway
Secreted proteins, lipids, glycoproteins, small molecules such as amines
Lysosomes
and other cellular products destined for export from the cell are trans-
ported to the plasma membrane in small vesicles released from the Lysosomes are membrane-bound organelles 80–800 nm in diameter,
trans-face of the Golgi apparatus. This pathway either is constitutive, in often with complex inclusions of material undergoing hydrolysis (sec-