Final answer:
The rough endoplasmic reticulum is the cytoplasmic structure mostly comprising of tubules and flattened sacs studded with ribosomes, responsible for the synthesis of proteins destined for the plasma membrane, other organelles, or secretion.
Step-by-step explanation:
The cytoplasmic structures that consist mostly of rough endoplasmic reticulum (RER) are expansive networks of membranous tubules and flattened sacs. The RER is characterized by the presence of ribosomes bound to its surface, giving it a 'rough' appearance. These ribosomes are the sites of protein synthesis, making proteins for use outside the cell, insertion into the plasma membrane, or for delivery to other organelles. Once synthesized, these proteins are packaged into transport vesicles that bud off from the RER and can then be sent to the Golgi apparatus for further processing, directly to the plasma membrane, or delivered to another organelle.