A Number of Systems for Pumping Ions Across Membranes Are

A number of systems for pumping ions across membranes are powered by ATP. Such ATP-powered pumps are often called ATPases, although they don't often hydrolyze ATP unless they are simultaneously transporting ions. Because small increases in calcium ions in the cytosol can trigger a number of different intracellular reactions, cells keep the cytosolic calcium concentration quite low under normal conditions, using ATP-powered calcium pumps. For example, muscle cells transport calcium from the cytosol into the membranous system called the sarcoplasmic reticulum (SR) . If a resting muscle cell's cytosol has a free calcium ion concentration of 10-7 while the concentration in the SR is 10-2, then which of the following is the most likely mechanism by which the muscle cell ATPase maintains intracellular calcium concentrations?

A) The ATPase pumps calcium from the outside of the cell into the SR against the concentration gradient.
B) The ATPase pumps calcium from the cytosol into the SR against the concentration gradient.
C) The ATPase transfers ℗i to calcium ions so that they may diffuse into the SR.
D) The ATPase opens a calcium ion channel that allows calcium ions to diffuse back into the SR along the concentration gradient.