A model for Na+-glucose cotransport The inward Na+ gradient drives glucose transport into the cell against the glucose concentration gradient. External Na+ first binds to the negatively charged protein to allow glucose to bind with high affinity. Two Na+ ions and one glucose molecule are then transported across the membrane, where first the sugar and then Na+ are released into the cytoplasm due to the low affinity for glucose and the low intracellular concentration of Na+. The ligand-free protein then returns to the original conformation largely because of the negative membrane potential and the high external Na+ concentration