Sugar Transport: Pressure Flow Hypothesis in
Sugar Transport: Pressure Flow Hypothesis
The mechanism by which sugars are transported through the phloem, from sources to sinks, is called pressure flow. At the sources (usually the leaves), sugar molecules are moved into the sieve elements (phloem cells) through active transport. Water follows the sugar molecules into the sieve elements through osmosis (since water passively diffuses into regions of higher solute concentration). This water creates turgor pressure in the sieve elements, which forces the sugars and fluids down the phloem tubes toward the sinks. At the sinks, the sugars are actively removed from the phloem and water follows osmotically, so that conditions of high water potential and low turgor pressure are created, driving the pressure flow process.
This video explains the pressure flow hypothesis, an explanation for how sugars are transported through the vascular tissue of a plant.