Explain4 marksStandard
In an investigation into phloem transport, researchers isolated leaf discs from a plant and measured the uptake of radioactive sucrose into sieve tubes. They found that adding an inhibitor of ATP synthesis (such as cyanide) prevented sucrose uptake. In a separate experiment, they added a chemical that specifically blocks proton pumps in the plasma membrane of companion cells, and again sucrose uptake was inhibited. Using your knowledge of phloem loading, explain the roles of companion cells, proton pumps, and cotransporter proteins in the active loading of sucrose into sieve tubes.