How do carrier proteins differ from channel proteins in their role as gatekeepers of the cell? carrier proteins expend energy to pump substances across the membrane against a concentration gradient, whereas channel proteins provide passage via facilitated diffusion of substances without expending energy. carrier proteins bind to the substances they transport across the membrane via facilitated diffusion, whereas channel proteins provide a pore for substances to move across the membrane via facilitated diffusion. carrier proteins allow movement of substances in only one direction across the membrane, whereas channel proteins allow movement of substances in both directions. carrier proteins provide a means for large, polar substances to move across the membrane, whereas channel proteins provide a means for small, nonpolar substances to pass through the membrane?