Answer;
-Proteins
Step-by-step explanation;
-Proteins embedded in the cell membrane help control which substances exit and enter the cell. Channel proteins allow molecules to move from high concentration to low concentration.
-Cell is a fabulous machine that tightly controls what enters and exits. Part of how it does this is through a specialized cell membrane that only lets very small, hydrophobic molecules pass directly through it. Other molecules however, need a helper to get through the membrane, these molecules are called protein carriers.