Sucrose Relocation into Phloem and Bulk Flow in a Sieve Tube

Sucrose is manufactured in mesophyll cells and can travel by symplast to sieve-tube elements. In some plants, sucrose would diverge from symplast near sieve tubes and travels through apoplast. Sucrose is then actively transported from apoplast into companion cells and sieve-tube elements. The proton pumps generate a hydrogen ion gradient that allows the buildup of sucrose with the help of cotransport proteins that couples sucrose transport to the diffusion of hydrogen ions back into the cell.

This is the bulk flow by positive pressure (pressure flow) in a sieve tube. Sugar is loaded into the sieve tubes at the source and that causes a reduction in the water potential inside the sieve tube elements. This causes the tube to undergo osmosis to take up water. The uptake of water then generates a positive pressure that forces the sap to flow along the tube. The pressure is relieved by the unloading of sugar and the consequent loss of water at the sink. In leaf-to-root translocation, the xylem would recycle water from sink to source. 

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