An experiment is designed to study the mechanism of sucrose uptake by plant cells. Cells are immersed in a sucrose solution, and the pH of the solution is monitored. Samples of the cells are taken at intervals, and their sucrose concentration is measured. The pH is observed to decrease until it reaches a steady, slightly acidic level, and then sucrose uptake begins. Rank the steps from first to last. To rank items as equivalent, overlap them. A proton Pump, using ATP, pumps protons outside.

Sucrose moves into the cell.

H+ moves down electrochemical gradient into the cell.

A steady proton gradient with a higher concentration of H+ out of the cell is maintained by the proton pump.

pH decreases.

The gradient becomes sufficient.

A proton Pump, using ATP, pumps protons outside

A steady proton gradient with a higher concentration of H+ out of the cell is maintained by the proton pump.

pH decreases;

The gradient becomes sufficient.

H+ moves down electrochemical gradient into the cell; Sucrose moves into the cell.

Explanation:

A proton Pump, using the energy of ATP hydrolysis, pumps protons outside the cell.

A steady proton gradient with a higher concentration of H+ out of the cell is maintained by the proton pump.

The continues outward pumping of protons by the proton pump causes the pH of the extracellular fluid to decrease

At some point the gradient becomes sufficient, and pH reaches a steady slightly acidic level.

H+ moves down its electrochemical gradient into the cell together with Sucrose through the hydrogen ion-sucrose co-transporter.

a

explanation: