A Number of Systems for Pumping Ions Across Membranes Are

question 77

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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