Decide whether each of these statements is true or false, and then explain why.

A. Like the lumen of the ER, the interior of the nucleus is topologically equivalent to the outside of the cell.

B. Membrane bound and free ribosomes, which are structurally and functionally identical, differ only in the proteins they happen to be making at a particular time.

C. To avoid the inevitable congestion that would occur if two-way traffic through a single pore were allowed, nuclear pore complexes are specialized so that some mediate import while others mediate export.

D. The TOM complex is required for the import of all nucleus-encoded mitochondrial proteins.

E. There is no fundamental distinction between signaling molecules that bind to cell surface receptors and those that bind to intracellular receptors.

F. All small intracellular mediators (second messengers) are water soluble and diffuse freely through the cytosol.