Which type of transport involves moving two different molecules against their concentration gradients in opposite directions?

The correct answer involves a process known as antiport, which is a form of active transport. In antiport, two different molecules are transported across a membrane in opposite directions, with each molecule moving against its own concentration gradient. This typically requires the use of energy, often derived from ATP or the electrochemical gradient established by prior transport processes.

For example, in many cells, sodium ions (Na+) may be transported out of the cell while potassium ions (K+) are transported into the cell simultaneously. This exchange is crucial for maintaining electrochemical gradients and is an essential function of the sodium-potassium pump, a well-known antiporter.

In contrast, other transport mechanisms, such as symport, involve the movement of two different molecules in the same direction. Passive transport and facilitated diffusion refer to processes that do not involve moving substances against their concentration gradients and thus do not classify as antiport.