A number of systems for pumping ions across membranes are powered by ATP. Such ATP-powered pumps are often called ATPases, although they do not often hydrolyse 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 how is the ATPase acting?
A) ATPase activity must be powering an inflow of calcium from the outside of the cell into the SR.
B) ATPase activity must be transferring i 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) ATPase activity must be opening a channel for the calcium ions to diffuse back into the SR along the concentration gradient.

Question

A number of systems for pumping ions across membranes are powered by ATP. Such ATP-powered pumps are often called ATPases, although they do not often hydrolyse 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 how is the ATPase acting?
A) ATPase activity must be powering an inflow of calcium from the outside of the cell into the SR. 
B) ATPase activity must be transferring i 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) ATPase activity must be opening a channel for the calcium ions to diffuse back into the SR along the concentration gradient.

Quizplus · Accepted Answer

The ATPase is pumping calcium from the cytosol to the SR against the concentration gradient, as the concentration of calcium is much higher in the SR than in the cytosol. This requires energy input from ATP hydrolysis. Option A is incorrect as it proposes an inflow of calcium from the outside of the cell, which is not mentioned in the passage. Option B is also incorrect as it is not clear what 11eaa959_8661_0561_84bc_b5da5ecf354d_TB6149_11 represents. Option D is incorrect as it suggests that the ATPase is opening a channel for calcium diffusion, which does not require energy input from ATP hydrolysis.

A Number of Systems for Pumping Ions Across Membranes Are