A Number of Systems for Pumping Ions Across Membranes Are

question 31

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 do not 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 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.

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