Final answer:
The nucleus is the organelle that is not a part of the endomembrane system, despite the nuclear envelope being part of it, as the nucleus functions separately from the system that processes proteins and lipids.
Step-by-step explanation:
The organelle that is NOT a part of the endomembrane system is the nucleus. The endomembrane system includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, as well as the plasma membrane. While the nuclear membrane is continuous with the rough ER and is often discussed in the context of the endomembrane system, the nucleus itself, which is encased by the nuclear membrane, is considered a separate entity and not a component of the endomembrane system that modifies, packages, tags, and transports proteins and lipids.