Describe the mechanism of transport in the phloem, involving active loading at the source and removal at the sink.

The active transport of hydrogen ions out of the companion cells using ATP creates a hydrogen ion gradient.This causes the facilitated diffusion of H⁺ ions back into companion cells. This occurs through a cotransport protein as sucrose moves in with the hydrogen ions. Sucrose diffuses through the plasmodesmata from the companion cell into the sieve tube and this reduces the water potential in the sieve tube elements. Water follows by osmosis and increases the hydrostatic pressure in the sieve tube element. Water moves down the sieve tube elements from the higher hydrostatic pressure at the source to the lower hydrostatic pressure at the sink (carrying the sucrose with it). Sucrose moves either by diffusion or active transport, from the sieve tubes to the surrounding cells. This increases the water potential in the sieve tube element, so water molecules move into the surrounding cells by osmosis (reducing the hydrostatic pressure at the sink).