Final answer:
It is True that ER-bound and free ribosomes are structurally and functionally identical, with differences arising only in the proteins they synthesize based on localization, with ER-bound ribosomes producing proteins for export or membrane insertion, and free ribosomes producing proteins for the cytosol.
Step-by-step explanation:
The statement that ER-bound and free ribosomes, which are structurally and functionally identical, differ only in the proteins they happen to be making at a particular time is True. Both types of ribosomes are the machinery responsible for protein synthesis within the cell. Free ribosomes synthesize water-soluble proteins that function within the cytosol, while the ER-bound ribosomes are involved in the synthesis of proteins to be inserted into cell membranes, secreted from the cell, or utilized within the endomembrane system. Regardless of their location, the ribosomes are essentially the same in their molecular composition and function; they consist of a large and small subunit, composed of ribosomal RNA (rRNA) and proteins. The primary distinction between free and ER-bound ribosomes in eukaryotic cells is the destination of the proteins they produce.