Practice Test 10

A toxin blocks ATP production in cells. Shortly after, an ATP-dependent proton pump in the plasma membrane stops moving H$^+$ out of the cytosol, even though the H$^+$ concentration is higher outside than inside. When ATP is restored, the pump resumes transporting H$^+$ outward. Which mechanism best explains H$^+$ movement by this pump when ATP is present?

A membrane protein opens in response to a voltage change and allows Na$^+$ to cross. When open, Na$^+$ moves rapidly into the cell from high extracellular concentration to lower cytosolic concentration. The movement stops when the channel is blocked and is unaffected by ATP depletion. Which mechanism best explains Na$^+$ movement through this protein?