What does countertransport (antiport) entail?

• A. Transporting two substances in the same direction
• B. Transporting one substance only
• C. Transporting two substances in opposite directions
• D. Independence from concentration gradients

Answer: (C)

Countertransport, also known as antiport, involves the movement of two substances across a membrane in opposite directions. This mechanism is a vital process in cellular function, particularly in maintaining the balance of ions and other molecules within cells. In antiport, while one substance is being transported into the cell, another substance is simultaneously being expelled out of the cell.

This process is often driven by the concentration gradients of the substances involved. For example, a common instance of countertransport is seen in the sodium-potassium pump, where sodium ions are transported out of the cell while potassium ions are brought into the cell. This action is essential for maintaining the electrochemical gradient necessary for cellular functions such as nerve impulse transmission and muscle contraction.

The other options present different transport mechanisms. Transporting two substances in the same direction refers to symport, while transporting one substance only does not accurately describe the nature of countertransport. Independence from concentration gradients is not applicable, as countertransport relies on these gradients to function effectively.