722 Chapter 131 Intracellular Membrane Traffic

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•[Intracellular Membrane Traffic] During cell division and apoptosis, the Golgi apparatus fragments and disperses, leading to its reassembly in daughter cells. Correctly folded proteins from the endoplasmic reticulum (ER) are packaged into COPII-coated vesicles, forming vesicular tubular clusters that transport to the Golgi apparatus. Any ER proteins that escape are returned via COPI-coated vesicles. •[Golgi Apparatus Functions] The Golgi apparatus is essential for glycosylation and modifying proteins and lipids from the ER. It consists of polarized stacks of cisternae (cis, medial, and trans) connected to sorting stations. Proteins and lipids move through the Golgi in a cis-to-trans direction, utilizing vesicle transport and progressive maturation. The trans Golgi network sorts proteins for various destinations, including lysosomes. •[Lysosome Structure and Function] Lysosomes are membrane-enclosed organelles filled with hydrolytic enzymes that digest macromolecules, maintaining an acidic pH (4.5–5.0) for optimal enzyme activity. Lysosomal membrane proteins are highly glycosylated to prevent degradation, and the vacuolar H⁺ ATPase pumps protons into lysosomes to maintain a...