Biology – Transport mechanisms | e-Consult

Translocation is the process by which sugars, primarily sucrose, are transported from source tissues (e.g., mature leaves where photosynthesis occurs) to sink tissues (e.g., roots, developing fruits, growing stems). This movement occurs through the phloem, specifically via sieve tubes.

The driving force behind translocation is the creation of a pressure gradient. At the source, sugars are actively loaded into the sieve tubes, increasing the solute concentration and therefore the water potential within the sieve tube. This causes water to move into the sieve tube from the adjacent xylem via osmosis, increasing the pressure (turgor pressure) in the sieve tube.

This increased pressure at the source creates a positive pressure (hydrostatic pressure) within the sieve tubes. At the sink, sugars are actively unloaded from the sieve tubes, decreasing the solute concentration and water potential. Water then moves out of the sieve tubes by osmosis, reducing the pressure at the sink. This pressure gradient, from high pressure at the source to lower pressure at the sink, drives the bulk flow of phloem sap.

The sieve tubes are living cells with modified walls that lack nuclei and vacuoles. They are connected to companion cells, which provide metabolic support. The sieve plates, located at the ends of the sieve tubes, facilitate the flow of phloem sap between cells. The movement of phloem sap is not solely due to pressure gradients; it also involves mass flow, where the bulk movement of sap is driven by the pressure gradient.

In summary: Active loading at the source creates a high solute concentration and high water potential, drawing water into the sieve tubes and establishing positive pressure. Active unloading at the sink creates a low solute concentration and low water potential, causing water to move out of the sieve tubes and establishing a lower pressure. The pressure gradient between source and sink drives the bulk flow of phloem sap.