A number of systems for pumping 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. Small increases in calcium ions in the cytosol trigger a number of different intracellular reactions, so the cells must keep the calcium concentration quite low. Muscle cells also transport calcium from the cytosol into the membranous system called the sarcoplasmic reticulum (SR). If a muscle cell cytosol has a free calcium ion concentration of 10-⁷ in a resting cell, while the concentration in the SR can be 10-², then how is the ATPase acting?
A)The ATP must be powering an inflow of calcium from the outside of the cell into the SR.
B)ATP must be transferring Pi to the SR to enable this to occur.
C)ATPase activity must be pumping calcium from the cytosol to the SR against the concentration gradient.
D)The calcium ions must be diffusing back into the SR along the concentration gradient.
E)The route of calcium ions must be from SR to the cytosol, to the cell's environment.

Question

Quizplus · Accepted Answer

The question states that muscle cells transport calcium ions from the cytosol to the SR, where the concentration can be as high as 10-². This is against the concentration gradient and requires ATP-powered transport. Therefore, the ATPase activity must be pumping calcium from the cytosol to the SR, which is option C.

A Number of Systems for Pumping Across Membranes Are Powered