The transport of sugars through phloem from a source to a sink is called translocation. The leaf is usually the source, while a sink is fruits or roots. However, sink and source roles can be reversed when they need energy for growth. Therefore, food transport by phloem is bidirectional – both upward and downward and the direction is determined by the sucrose concentration.
Phloem tissue is made up of sieve tube cells, which are long columns with holes in their end walls. These holes are called sieve plates. However, most of the cellular functions of sieve tube cells inside the phloem are carried out by companion cells. The fluid that passes through these phloem cells is called phloem sap. It is mainly water and sucrose. A simple experiment called girdling can be used to illustrate the role of phloem cells in plants.
Phloem sap translocation from source to sink takes place by a mechanism called the pressure flow hypothesis. When plant leaves prepare glucose via photosynthesis, the glucose is then converted into sucrose. Sucrose then moves into the companion cells, after which it is ‘loaded’ into the sieve tube cells via active transport. This process of ‘loading’ sucrose into the phloem causes a hypertonic condition and sets up a water potential gradient. This, in turn, results in osmosis, which causes water to move into the phloem from adjacent xylem cells.
As osmotic pressure increases in the sieve tube, pressure flow begins and the phloem sap moves towards the sink, which has low osmotic pressure. This is followed by the sucrose from the phloem sap ‘unloading’ into the sink cells via active transport. The loss of sucrose produces a high water potential in phloem. This results in decreased osmotic pressure, which causes water to move out of the phloem cells and back into the xylem cells.