Final answer:
Secondary active transport carriers, such as symporters and antiporters, have two binding sites. They use the energy of the electrochemical gradient established by primary active transport to move substances across the cell membrane without directly using ATP. Symporters move substances in the same direction, whereas antiporters move them in opposite directions.
Step-by-step explanation:
Secondary active transport carriers, such as symporters and antiporters, have two binding sites. One site is for the ion, usually sodium (Na+), which moves down its concentration gradient created by primary active transport like the sodium-potassium pump. The energy from this movement powers the transport of another molecule, often glucose or an amino acid, against its concentration gradient. This second molecule binds to the other site on the carrier protein, and both are then translocated across the cell membrane.
A symporter carries two different ions or molecules in the same direction, using the flow of ions like sodium into the cell to facilitate the uptake of another substance like glucose. An antiporter also carries two substances but in opposite directions, such as the Na+-K+ ATPase pump that exchanges sodium for potassium ions across the membrane.
The secondary active transport process relies on the electrochemical gradient established by primary active transport, and while it utilizes energy from this gradient, it does not directly use ATP to transport molecules.