Final answer:
The mitochondrion is not part of the endomembrane system, which includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and the plasma membrane.
Step-by-step explanation:
The organelle that is not part of the endomembrane system is the mitochondrion. The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles, and although not within the cell, the plasma membrane is also considered a part of this system. However, the endomembrane system does not include the membranes of either mitochondria or chloroplasts.