The Na⁺/Ca²⁺ exchanger, which couples the movement of Na⁺ down its electrochemical gradient with the movement of Ca²⁺ against its electrochemical gradient, is a(n) because Ca²⁺ and Na⁺ move in different directions across the membrane.

Antiport.

Explanation:

Active transport can be achieved using different transporters:

Uniport (the transport of an only molecule).

Symport (the transport of two molecules in the same direction).

Antiport (two molecules are transported in opposite directions).

The Na⁺/Ca²⁺ exchanger is an antiport, that transports Na⁺ inside the cell and Ca²⁺ outside the cell (see attachment).

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