The transporter in the figure is considered an example of secondary transport because it uses the energy stored in the electrochemical gradient of another molecule (such as sodium ions or protons) to drive the transport of a substrate against its concentration gradient. Unlike primary active transport, secondary transport does not directly use ATP for the transport process. Instead, it relies on the energy created by primary transporters that establish ion gradients across the membrane. Secondary transporters typically function as symporters or antiporters:
- Symporters move both the substrate and the driving ion in the same direction.
- Antiporters move the substrate and the driving ion in opposite directions.
This coupling mechanism allows uphill transport of the substrate by harnessing the downhill movement of the driving ion, which is moving down its electrochemical gradient. This results in the accumulation of the substrate on one side of the membrane without direct ATP consumption during the transport step itself.
