Ans. Correct option. B. Cytoplasmic side of endoplasmic
reticulum.
Note that the membrane lipid (phospholipids) consist of polar
region (hydrophilic head consisting of glycerol, phosphate, etc.)
and the hydrophobic tail (consisting of long chain fatty acids).
Due to difference in polarity of the constituents, the
phospholipids are first synthesized at the cytoplasmic site of the
smooth ER. The cytoplasmic site of smooth ER serves as the source
of hydrophilic precursor (like glycerol-3-phosphate; CDP-choline,
etc.). The enzymes integrated in ER membrane use hydrophilic
precursors from cytoplasm and “burry” the hydrophobic tails of
resultant phospholipid molecule in the ER membrane
simultaneously.