Final answer:
The lumen or cisternal space is the hollow portion of the endoplasmic reticulum (ER) tubules. It is involved in protein synthesis, folding, and transport within the cell.
Step-by-step explanation:
The lumen or cisternal space refers to the hollow portion of the endoplasmic reticulum (ER) tubules.
The ER's membrane, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope.
The lumen or cisternal space plays a crucial role in protein synthesis, folding, and transport within the cell.