Why is secondary active transport referred to as 'secondary'?

Secondary active transport is termed 'secondary' because it relies on the energy stored in concentration gradients created by primary active transport mechanisms. In primary active transport, ATP is used directly to pump ions or molecules across a cell membrane, creating an electrochemical gradient. This gradient establishes a difference in the concentration of specific ions across the membrane.

In secondary active transport, instead of using ATP directly, the transport process utilizes these existing concentration gradients. As ions move down their concentration gradient (from a region of higher concentration to a region of lower concentration), they can drive the transport of another substance against its concentration gradient. This coupling of the movement of one species with the movement of another is what distinguishes secondary active transport from primary transport, highlighting its reliance on the prior work of primary active transport processes.

The term 'secondary' emphasizes that it does not directly use ATP for the transport of the second molecule but instead harnesses the energy stored in the gradients established by the initial ATP-driven processes.