An ATP molecule binds to the protein and is broken down into ADP and a phosphate group.
| Feature | Primary Active Transport | Secondary Active Transport | | :--- | :--- | :--- | | | ATP (or light) | Ion gradient (e.g., Na⁺ or H⁺) | | ATP Usage | Yes, directly | No | | Original Gradient Needed? | No (it creates the gradient) | Yes (it uses the gradient) | | Direction of Transport | Moves specific ions/molecules one way | Moves two molecules simultaneously | | Example | Sodium-Potassium Pump, Calcium Pump | SGLT (glucose uptake), Na⁺/Ca²⁺ exchanger |
Secondary active transport, on the other hand, does not directly use ATP to transport molecules. Instead, it relies on the concentration gradient of another molecule, typically an ion, to drive the transport of a second molecule against its concentration gradient. This process is often coupled with primary active transport, where the energy stored in the concentration gradient of an ion (e.g., sodium) is used to transport another molecule. Examples of secondary active transport include the cotransport of glucose and amino acids in the intestine.
Secondary active transport is a bit more clever. It doesn't use ATP directly. Instead, it hitches a ride on the energy stored in an electrochemical gradient created by primary active transport.