Biology – 8.1 Transport in plants | e-Consult

Translocation is the process by which sugars (mainly sucrose) and amino acids are transported throughout a plant via the phloem. This movement occurs from regions where sugars are produced – known as sources – to regions where sugars are used or stored – known as sinks.

The primary mechanism driving translocation is the pressure flow hypothesis. This hypothesis states that the movement of phloem sap is driven by differences in pressure at the source and sink. At the source, sugars are actively loaded into the phloem, increasing the solute concentration and therefore the water potential. This causes water to move into the phloem by osmosis, increasing the pressure.

At the sink, sugars are actively unloaded from the phloem, decreasing the solute concentration and water potential. Water then moves out of the phloem by osmosis, reducing the pressure. This pressure difference between the source and sink creates a pressure gradient that drives the flow of phloem sap.

The phloem consists of sieve tubes and companion cells. Sieve tubes are living cells with sieve plates that allow for the flow of sap. Companion cells support the sieve tube cells and are involved in the loading and unloading of sugars. The direction of translocation can vary depending on whether a region is acting as a source or a sink.