Translocation of organic compounds in phloem

Translocation of organic compounds in phloem from source (shaded pink) to sink (shaded yellow). Water molecules are represented by blue balls; sucrose, by red balls. The movement of sucrose into and out of phloem occurs by dif-fusion through plasmodesmata, or by active transport across the cell membranes.

1 At a source region, sucrose moves into a companion cell, then into a sieve tube.

2 As a result of the increase in solute concentration, the fluid in the sieve tube becomes hyper-tonic. Water moves in from the surrounding cells, so phloem turgor increases.

3 The pressure difference pushes the fluid from the source to the sink. Pressure and solute concentrations decrease as the fluid moves from source to sink.

4 At a sink region, sucrose moves from the sieve tube into sink cells. Water fol-lows by osmosis.