Consider the transport of protons and sucrose into a plant cell by the sucrose-proton cotransport protein. Plant cells continuously produce a proton gradient by using the energy of ATP hydrolysis to pump protons out of the cell. Why, in the absence of sucrose, do protons not move back into the cell through the sucrose-proton cotransport protein?

• A. Protons cannot move through membrane transport proteins.
• B. Protons are freely permeable through the phospholipid bilayer, so no transport protein is needed for protons.
• C. In the absence of sucrose, the ATP-powered proton pump does not function, so there is no proton gradient.
• D. Protons, unlike other substances, do not diffuse down their electrochemical gradient.
• E. The movement of protons through the cotransport protein cannot occur unless sucrose moves at the same time.