Atype of protein that allows a particular molecule or ion to freely cross the plasma membrane as it enters or exits the cell

Channel Protein. A channel protein is a protein that allows specific substances to enter or exit a cell membrane. They provides a selective pore gateway across the membrane for specific solute to enter or exit through. Since protein are essentially made up of amino acids; the makeup of the amino acid chains in the protein molecules determines the size and shape of the channel protein. Consequently, this determines what ions or molecules can be received by the channel protein to cross or exit the cell membrane.

The answer is a because - is an direction