The transportation of sugars in plants occurs through the phloem via a process known as translocation, which involves the movement of sugars from a source to a sink. The source refers to the tissue where sugars enter the phloem, while the sink is where sugars exit. This movement is facilitated by bulk flow, primarily driven by differences in solute concentration.
When sugars are loaded into the phloem, this process is termed phloem loading. It occurs through secondary active transport, utilizing a proton pump to establish a proton gradient. The proton pump expels protons (H+) out of the cell, creating a high concentration of protons outside the cell membrane. This gradient allows a proton-sucrose symporter to transport sucrose into the phloem against its concentration gradient, as protons move back into the cell along their gradient.
The fluid within the phloem, known as phloem sap, is primarily composed of sucrose, along with other dissolved sugars, water, hormones, and minerals. The movement of this sap through the phloem is explained by the Pressure Flow Hypothesis, which posits that sugar concentration is higher at the source, such as in leaves where photosynthesis occurs. This high concentration of solutes in the phloem leads to a decrease in solute potential compared to the xylem, which is mostly water. Consequently, water moves from the xylem into the phloem, increasing turgor pressure in the phloem.
At the sink, where sugar concentration is lower, water exits the phloem and re-enters the xylem, resulting in decreased turgor pressure. This difference in turgor pressure—high at the source and low at the sink—creates a positive pressure gradient that drives the bulk flow of phloem sap from the source to the sink. Thus, the dynamics of solute concentration and water movement are crucial for the effective transport of nutrients throughout the plant.