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Which of the following is not considered to be a part of the endomembrane system?

a)    golgi apparatus

b)   Smooth ER

c)    Rough ER

d)   Vesicles

e)    Mitochondria  

The correct answer is E. The name “Endomembrane System” can be slightly deceiving because it implies that anything with a membrane of its own can be part of the system (which would then include the mitochondria because it does indeed have a membrane of its own). However, this reasoning is incorrect. The Endomembrane system is basically a functional unit within a (usually) eukaryotic organism that helps transport proteins to the cell membrane. In this case, the system is defined as the set of organelles that then are involved in this process, which includes the entire ER (which includes the Rough + Smooth ER since they’re still one functional organelle that produces the protein), the golgi apparatus (which is involved in post-translational modification, and the vesicles themselves that transport the proteins from the ER to the golgi.