A Number of Systems for Pumping Ions Across Membranes Are

question 54

Multiple Choice

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.

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