A plant transports dissolved sugars and amino acids in solution through a component of its vascular tissue called the phloem. The movement of fluid in the phloem, a process called translocation, can occur in any direction, up or down the plant. However, the fluid typically flows from source cells to sink cells. Source cells are cells that produce sugars and pump them into the phloem, whereas sink cells are cells that do not make enough sugars for their own growth and metabolism and must import them from the phloem.
The mechanism of phloem translocation is described in a model of phloem function called the pressure flow model. The accompanying animations describe this model, both in the plant and in a laboratory simulation.
In the accompanying animations, we described the pressure flow model of phloem translocation, which is currently the most widely accepted model for how translocation occurs.
In the first step of this model, sugar (mainly sucrose) is actively transported from source cells into the sieve tubes of the phloem. The addition of sucrose into the sieve tubes increases the concentration of this solute, causing water to flow into the sieve tubes by osmosis. With the entry of water, the sieve tube pressure near the source cells increases and forces the solution to move to regions of lower pressure.
At the regions of lower pressure, sink cells remove the sucrose by active transport. As the sink cells pull the solute out of the phloem, water leaves the phloem by osmosis, passing to neighboring tissues that have higher solute concentrations. The retreating water reduces the pressure in this region of the sieve tubes and encourages fluid to continue to flow from regions of higher pressure.
At different times of the year, a tissue may act as a source or a sink. For example, when leaves of a plant are young, they require more nutrients to grow than they can produce, and therefore they act as sinks. However, when the leaves have reached maturity, they make abundant sugars and act as sources. In the autumn, the plant translocates much of its sugar to storage organs such as stems and roots, which act as sinks and store it over winter in the form of starch. In the spring, these storage organs then act as sources, releasing this sugar for use by the rest of the budding and flowering plant.
Textbook Reference: Concept 25.4 Solutes Are Transported in the Phloem by Pressure Flow