Symporters are transport proteins that simultaneously transport two substances across the membrane in the same direction; one against the concentration gradient and one with the concentration gradient. Energy for transport of the substance moving against it's concentration gradient is provided by the potential energy stored in the electrochemical gradient of sodium ions (Na⁺) or protons (H⁺). Cells that line the small intestines, for example, use symporters to co-transfer sodium ions and glucose.