Answer
The uptake of sucrose depends on having more protons (lower pH) outside the cell than inside. This gradient is used to drive sucrose uptake. An inhibitor of ATP regeneration would stop uptake of sucrose since ATP is normally hydrolyzed to create the proton gradient which drives this uptake.
Work Step by Step
Please see Fig. 7.18 which shows that the uptake of sucrose is driven by the passive uptake of protons by a symport protein at the same time as sucrose. That explains why acid (protons) put outside the cells would start sucrose uptake in the experiment. That figure also shows how the proton gradient is normally built up, by the breakdown of ATP to pump protons outside the cell.
