Explain the differences between uniporters, symporters, and antiporters.

Because the hydrophobic interior of the lipid bilayer cell membrane is such a substantial barrier, diffusion is not possible for polar molecules and they must be transferred in (or out) of the cell using transport proteins (diffusion is not possible). Each transport protein is specialized for a certain molecule.

A gradient of any solute (Na+ for instance) across a membrane can be used to fuel the active transport of a second solute. The downhill movement of the first solute down its gradient provides the energy to drive the uphill transport of the second solute. The proteins that do this are called coupled transporters: if the transporter moves both solutes in the same direction across the membrane it is called a symport, if it moves them in opposite directions it is called an antiport. A transporter that moves only one type of solute (not coupled transport) is a uniport.

Uniports, symports, and antiports are used for passive and active transport.

Your answers should highlight differences in transport direction and energy input.