Which structure regulates the movement of substances between the nucleus and the cytoplasm?

The structure that regulates the movement of substances between the nucleus and the cytoplasm is the nuclear pore. Nuclear pores are large protein complexes that cross the nuclear envelope, which consists of two lipid bilayers. These pores allow for the selective exchange of molecules such as RNA and proteins. Smaller molecules can often pass freely, while larger ones require active transport mechanisms to facilitate their movement.

The nuclear envelope itself, while encompassing the nucleus and protecting it, does not directly mediate the transport of materials; it is the nuclear pores integrated into this envelope that perform that function. The nuclear membrane and the nucleolus do not serve as regulatory structures for translocation between the two cellular compartments as the nuclear pore does, which is specifically designed for this transport task.