Uniporters are transport proteins that transport a substance across a membrane down a concentration gradient from an area of greater concentration to lesser concentration. The transport is powered by the potential energy of a concentration gradient and does not require metabolic energy. Interaction between the substrate and the uniporter causes a conformational change in the uniporter enabling the substrate to cross the membrane. Amino acids, nucleosides, sugars, and other small molecules are often transported across membranes by uniporters.