The intracellular calcium ion concentration of ventricular muscle cells averages 10-4 mmol/L during diastole. The calcium ion concentration in transverse tubules (T-tubules) averages 2.5 mmol/L at rest. A protein transporter on the membrane of the T-tubule exchanges sodium for calcium. The transporter uses the transmembrane sodium gradient to fuel the exchange. Which of the following transport mechanisms best describes this type of transporter?
A. Facilitated diffusion
B. Primary active transport
C. Secondary active co-transport
D. Secondary active counter-transport
E. Simple diffusion

Secondary active counter transport: It is an active transport from the biological membrane that plays a role in the movement of a couple of protein ions down. The movement of another ions uphills sides against an electrochemical gradient. Thus the energy will be stored in the ions. This energy is used for another solution against a electrochemical energy or gradient energy.

It is of two types

Co-transport

Exchange transport.