Loading sucrose at the source
At a source, sucrose is loaded into the phloem using an active mechanism involving companion cells.
- Companion cells use ATP to pump hydrogen ions (H+) from their cytoplasm into the surrounding cell wall.
- This establishes a concentration gradient for H+.
- Hydrogen ions move back into the companion cell down this gradient through a co-transporter protein.
- Sucrose is carried into the companion cell by the same co-transporter.
- Sucrose then moves from the companion cell into sieve tube elements through plasmodesmata.
Role of ATP
ATP is required for the active pumping of H+ out of the companion cell. The resulting H+ gradient then drives the co-transport of H+ and sucrose back into the cell.
ATP → H+ pumped out → H+ gradient → H+ returns with sucrose
Water enters at the source
The accumulation of sucrose and other solutes in the sieve tube lowers its water potential. Water consequently enters the phloem by osmosis, including from neighbouring xylem vessels.
This water entry increases the hydrostatic pressure in the phloem near the source.
Exam Tip: Give sucrose loading as a linked mechanism: ATP pumps H+ out → H+ returns through a co-transporter with sucrose → sucrose enters sieve tubes through plasmodesmata → phloem water potential falls.