Translocation of organic solutes in the phloem occurs via mass flow. Which of the following statements correctly describes the sequence of processes that establishes the hydrostatic pressure gradient at the source?
Proton pumps actively transport H+\text{H}^+H+ ions out of companion cells into the cell wall. H+\text{H}^+H+ ions then diffuse back into the companion cells down their concentration gradient via a co-transport protein, carrying sucrose with them. The accumulation of sucrose in the sieve tube element lowers its water potential, causing water to enter from the xylem by osmosis and generating high hydrostatic pressure.
Proton pumps actively transport H+\text{H}^+H+ ions into the companion cells from the cell wall. H+\text{H}^+H+ ions then diffuse out into the cell wall via a co-transport protein, carrying sucrose with them. The accumulation of sucrose in the sieve tube element increases its water potential, causing water to enter from the xylem by osmosis and generating high hydrostatic pressure.
Sucrose is actively transported directly from companion cells into the sieve tube elements. This increases the water potential in the sieve tube element, causing water to leave the sieve tube element and enter the xylem by osmosis, creating a localized high hydrostatic pressure.
Sucrose diffuses down its concentration gradient from the cell wall into the companion cells. H+\text{H}^+H+ ions are then co-transported out of the companion cells, which lowers the water potential of the companion cells, causing water to move out of the sieve tube elements and creating a localized low hydrostatic pressure.
