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⁺). Sugars, phosphate ions, and sulfate ions are examples of molecules or ions that can be transported across bacterial membranes by symporters.