The concentration of sucrose in phloem sap can be as high as 0.5 M. The loading of sucrose into the phloem sieve tube element

Question

Quizplus · Accepted Answer

The high concentration of sucrose in phloem sap is achieved through an active transport mechanism that involves proton-sucrose symport. Protons are actively transported into the companion cell with the help of ATP hydrolysis, creating a gradient that facilitates the uptake of sucrose from the source cells into the companion cells. Once inside the companion cell, sucrose is actively transported into the sieve tube element through plasmodesmata via the symport mechanism. Therefore, the loading of sucrose into the phloem sieve tube element is linked to proton uptake (proton-sucrose symport).

The Concentration of Sucrose in Phloem Sap Can Be as High