The ability of the Golgi apparatus to sort and dispatch proteins to particular destinations is dramatically illustrated by lysosomes. Lysosomes are specialized vesicles derived from the Golgi apparatus that degrade damaged or unneeded macromolecules (Fig. 5.22). They contain a variety of enzymes that break down macromolecules such as proteins, nucleic acids, lipids, and complex carbohydrates. Macromolecules destined for degradation are packaged by the Golgi apparatus into vesicles. The vesicles then fuse with lysosomes, delivering their contents to the lysosome interior.

The formation of lysosomes also illustrates the ability of the Golgi apparatus to sort key proteins. The enzymes inside the lysosomes are synthesized in the RER, sorted in the Golgi apparatus, and then packaged into lysosomes. In addition, the Golgi apparatus sorts and delivers specialized proteins that become embedded in lysosomal membranes. These include proton pumps that keep the internal environment at an acidic pH of about 5, the optimum pH for the activity of the enzymes inside. Other proteins in the lysosomal membranes transport the breakdown products of macromolecules, such as amino acids and simple sugars, across the membrane to the cytosol for use by the cell.

108

The function of lysosomes underscores the importance of having separate compartments within the cell bounded by selectively permeable membranes. Lysosomal enzymes cannot function in the normal cellular environment, which has a pH of about 7, and many of a cell’s enzymes and proteins would unfold and degrade if the entire cell were at the pH of the inside of a lysosome. By restricting the activity of these enzymes to the lysosome, the cell protects proteins and organelles in the cytosol from degradation.