Final answer:
The endomembrane system's principal function is to modify, package, and transport proteins and lipids within eukaryotic cells, involving organelles like the ER and Golgi apparatus.
Step-by-step explanation:
The major function of the endomembrane system in eukaryotic cells is to modify, package, and transport lipids and proteins. The endomembrane system includes several key organelles and structures: the nuclear envelope, lysosomes, vesicles, the endoplasmic reticulum (ER), Golgi apparatus, and even the plasma membrane, which interacts with other endomembranous organelles. This system is essential for maintaining the cell's internal environment and for facilitating the movement of molecules within the cell, as well as to and from the cell membrane, thereby playing a critical role in cellular function and organization.