Question

You are studying a particular kind of Ca2+ transport protein in a purified system (the protein is the only transporter in the membrane). You find that if you add ATP to the interior of the membrane system, there is no change in the Ca?* concentration outside the cell. However, if you add an excess of Ca2+ to the interior of the cell, then you see an increase in the Ca2+ concentration outside the cell. What kind of transporter do you think you are studying? Ion channel Primary Active Transporter Antiporter • Coupled Na*/Ca2+ Symporter

Answer 1

The ion channel is a pore forming membrane protein that allows the passage of specific ions through the pore of the channel. The ion channels have high affinity for specific ions and on binding they undergo conformation change to move the ion across the membrane. The primary active transporter is membrane protein such as Na+/K+ pump that moves ions against electrochemical graident aross the membrane by hydrolysis of ATP. An antiporter is an exchanger or co-transport integral membrane protein that utilizes the electrochemical gradient established by primary active transport to move the ions or exchange of ions. For example, the Na+/ Ca2+ exchanger is involved in secondary transport and removes the cytoplasmic calcium with exchange of one calcium ion for three sodium ions. The symporter is an integral membrane protein that can transport two or more molecules simultaneously and is also known as a cotransporter. The H+/K+ pump is a symporter. In the above given scenario, the addition of ATP does notincrease calcium concentration outside the cell but addition of calcium to cell interior increases the calcium concentration outside the cell. This means, the transporter is an antiporter like Na+/ Ca2+ exchanger. Therefore, the option 3 is correct.