Which of the following best describes secondary active transport?

Secondary active transport is best described as the movement of molecules driven by ion gradients. This form of transport utilizes the energy created by the active transport of ions, typically sodium or hydrogen ions, across the cell membrane. When these ions are pumped out of the cell (often using ATP in primary active transport), they create a concentration gradient. This gradient can then be harnessed to move other substances into or out of the cell against their concentration gradients without direct use of ATP at that moment.

In secondary active transport, the movement of one substance occurs along with the movement of another, typically via a shared transport protein. For example, as sodium ions move back into the cell due to their concentration gradient, they can help pull glucose into the cell accompanying them, demonstrating the coupling of the two movements.

This mechanism is essential for maintaining various cellular functions, particularly in nutrient uptake and ion homeostasis.