Final answer:
Nuclear pores control the movement of materials into and out of the nucleus due to an interwoven meshwork of protein fibrils, and their regulated 'doors' made of protein, which facilitate the active transport of large molecules through the double membrane of the nuclear envelope.
Step-by-step explanation:
Nuclear pores restrict larger molecules from traversing the nuclear envelope due to their interwoven meshwork of protein fibrils. The nuclear envelope is a double membrane that encloses the nucleus and contains nuclear pores, which are protein complexes that control the movement of materials into and out of the nucleus. These pores are lined with proteins that act as 'doors' which can open to allow the selective transport of substances. The size of the pores and their protein composition prevents passive diffusion of large molecules, ensuring that transport is regulated and often requires energy in the form of ATP hydrolysis as part of active transport mechanisms.
The nuclear lamina, a meshwork of intermediate filaments, fortifies the shape and structure of the nuclear envelope. It works in conjunction with the nuclear pores, which are capable of expanding from their usual diameter of 10 nanometers up to 26 nanometers in response to specific signals. The active transport of nuclear proteins into the nucleus against a concentration gradient is an example of the selectivity and regulatory function that these nuclear pores facilitate.