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 diffusionB. primary acti*e transportC. secondary acti*e cotransportD. secondary acti*e countertransportE. simple diffusion

The correct answer is option D

SECONDARY ACTIVE COUNTER-TRANSPORT

Explanation: Secondary active transport is a type of transport that involves molecules across the cell membrane utilizing or making use of energy in other forms than Adenosine Triphosphate (ATP).

The energy that is being made use of comes from the electrochemical gradient created by pumping ions out of the cell. This can be either via antiport or symport.

Antiport or Counter-transport means that 2 different molecules or ions are being transported at the same time but opposite directions. One of the species is allowed to flow from high concentration to a lower concentration (often Sodium) while the other species is transported simultaneously to the other side.

Example is Na+-H+ counter-transport, the example and advantage of this transporter is clearly seen in the proximal tubules of the kidneys. The mechanism for concentrating H+ is not nearly as powerful as Primary active transport, however it can transport extremely large numbers and thus making it a key in H+ homeostasis in the body.

D. secondary active counter-transport

Explanation:

The protien transporter on the T-tubule is a secondary active counter transporter.

As the Ca2+ ions are pumped out of the cell and the Na+ ions are entered in the cell by a transporter which is using the energy derived from the sodium gradient to fuel the exchange. Hence thisis secondary active counter-transport.