A molecule that is important for cell functioning cannot diffuse through the plasma membrane, but needs a protein that can open and close to allow the molecule to diffuse. What is this protein called?

Channel protein.

Explanation:

Channel protein have gate that opens and close which allow the passage of polar substance or ions to move to permeable membrane of the cell by diffusion. They are water filled and allow the movementof charged molecules inside and outside the cell membrane i.e polar molecules move through non polar bilayer of the cell membrane.

your answer is rna to the question.