plant transport

First, a carrier protein (not a co-transport protein) moves H + ions from the cytoplasm of the companion cell to the cell wall, using active transport. 2. This creates a concentration gradient, and the H + ions are returned to the cytoplasm via a co-transport protein, which also transports sucrose. 3. Once the sucrose is inside the companion cell, it diffuses into the sieve tube element (this transport is not active). 4. This causes the water potential in the sieve tube element to decrease (adding solutes decreases the water potential). 5. Water moves from the xylem into the sieve tube element by osmosis. 6. This causes the hydrostatic pressure in the sieve tube element to increase. 7. At the sink cell, sucrose moving out of the sieve tube element decreases the hydrostatic pressure here. 8. As a result, the sucrose solution moves down the hydrostatic pressure gradient (since the sucrose solution is a fluid, it exerts hydrostatic pressure). 9. Finally, sucrose moves into the sink cell

transport of H + ions from cytoplasm to the cell wall via a carrier protein.