Final answer:
The most important difference between primary and secondary active transport is that primary active transport is directly dependent on ATP, while secondary active transport is indirectly dependent on ATP.
Step-by-step explanation:
Primary active transport is directly dependent on ATP, while secondary active transport is indirectly dependent on ATP.
Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP.
Secondary active transport does not directly require ATP: instead, it is the movement of material due to the electrochemical gradient established by primary active transport.