Mechanism of Symport

To the right is shown an animation of the process of symport, the simultaneous transport of two different molecules or ions across across a lipid bilayer membrane in the same direction. In this animation, the chartreuse oval represents the plasma membrane of a cell. The smaller black ovals together represent the transmembrane transport protein, the symporter. The red circles represent molecules such as glucose whose concentration is higher inside the cell than outside the cell. The blue diamonds represent molecules or ions such as Na+ whose concentration is higher outside the cell than inside the cell. The blue diamonds are allowed to enter the cell (down their concentration gradient) if and only if there is a simultaneous uptake of the red circles which are moving against their concentration gradient. Note that at the beginning of the animation, the symporter is poised to accept the blue diamond, but not the red circle. Once the blue diamond binds, then the red circle is allowed to bind. When both molecules are bound, there is a conformational change in the symporter such that binding sites are open to the inside of the cell rather than to the outside. The molecules leave the tranporter sequentially. As the red circle unbinds, there is another conformational change in the symporter such that it opens to outside again, ready to bind another blue diamond. This animation does not model any specific known symporter. Rather, the animation is intended to provide an example of how a symporter might work.