Practice question

Nuclear envelope architecture features inner membrane containing lamina-binding proteins emerin and lamin B receptor, outer membrane studded with ribosomes and functionally similar to rough ER, and intervening perinuclear space twenty to fifty nanometers wide that is direct extension of ER lumen. This topology allows shared environment for oxidative protein folding, calcium storage and lipid synthesis enzymes in outer membrane, and rapid diffusion of luminal chaperones BiP, calreticulin and protein disulfide isomerase between compartments. No such continuity exists with Golgi lumen because Golgi stacks are separate organelles connected via vesicles not tubules, lysosomal matrix maintains acidic pH generated by V-ATPase distinct from neutral ER, mitochondrial matrix is isolated by impermeable inner membrane and contains its own genome and translation system. Evidence includes EM serial sections showing membrane connections, and fluorescent protein diffusion experiments where photobleaching perinuclear GFP-Hsp47 recovers rapidly from ER pool, demonstrating single contiguous compartment essential for lipid supply to nuclear membranes and signal propagation.