According to the endosymbiotic theory, infoldings in the cell membrane of an ancestral prokaryotic cell gave rise to endomembrane components, including the nucleus, endoplasmic reticulum and golgi apparatus in what would become eukaryotic cells. the evolution of endomembrane components provided for compartmentalization in eukaryotes, leading to more efficiency in cell processes. the endomembrane system includes the endoplasmic reticulum. which statement does not describe the endoplasmic reticulum (er)?
a) the er is an extension of the nuclear envelope.

b) the er serves as a transport network for the proteins produced in its membrane.

c) the er provides separate microenvironments for each of the modifications that must take place to proteins.

d) the er serves a role in protein folding, lipid and steroid synthesis, carbohydrate metabolism and calcium storage