Final answer:
Proteins involved in nuclear transport cannot perform their function when they are bound to their ligands as they become saturated. Nuclear localization signals are crucial for the binding process and energy from ATP hydrolysis is needed for these proteins to cross the nuclear envelope. Carrier proteins are specific and finite, limiting the rate of transport.
Step-by-step explanation:
Proteins involved in nuclear transport cannot carry out their function when they are bound to certain molecules. Specifically, these proteins, when bound to their ligands, are saturated and can no longer facilitate the transport of additional materials. In the context of nuclear protein trafficking, this would relate to nuclear localization signals that enable the proteins to bind to nuclear transport receptor proteins in the cytosol. Once the proteins are bound, they change shape to move the molecule across the nuclear envelope. This process requires energy, typically from ATP hydrolysis, as they cross through nuclear pores.
For mRNA, once it is transported into the cytoplasm, the nuclear transport receptor is recycled back to the nucleus. The carrier proteins that facilitate this transport are specific for a single substance, which adds to the plasma membrane's overall selectivity. Furthermore, these carrier proteins are finite, making the rate of material transport maximal when all carrier proteins are bound to their ligands. In this state, even increasing the concentration gradient does not increase the transport rate.