AAMC FL Practice Exam 2025 – The Comprehensive All-in-One Guide to Exam Success!

The primary characteristic of secondary active transport is that it relies on ion diffusion down a concentration gradient. In secondary active transport, the energy used to transport molecules against their concentration gradient does not come directly from ATP hydrolysis, as in primary active transport. Instead, it utilizes the energy created by the movement of ions down their electrochemical gradient, which often results from previous active transport processes.

For instance, if sodium ions are pumped out of a cell (using ATP in primary active transport), they create a concentration gradient that allows glucose or other molecules to be co-transported into the cell against their gradient, as they hitch a ride with the sodium ions moving back into the cell. This mechanism underscores the indirect use of ATP because the ATP is utilized to establish the ion gradient in the first place.

Thus, while transporting molecules against their concentration gradients, secondary active transport is essential in maintaining cellular functions and nutrient uptake, reflecting its dependence on ion diffusion to drive the movement of other substances.