Sugar Transport in Plants!

The pressure flow theory! The pressure flow theory is the mechanism by which sugars are transported through the phloem from the leaves (sources) to the roots (sinks). Leaves are called the sources because the sugars are made there and the roots are called the sinks because the sugars are stored there. At the sources, sugar molecules are transported into the sieve-elements (phloem cells) through active transport (the movement of a substance across a biological membrane against its concentration gradient with the help of energy input). Water flows the sugar molecules into the sieve elements through osmosis (since water passively diffuses into areas of higher solute concentration). This water creates tugor pressure (the force directed against a cell wall after the entering of water and swelling of a walled cell due to osmosis). This forces the sugars as well as fluids down the phloem tubes toward the sinks. When at the sinks, the sugars are actively removed from the phloem therefore creating the water to flow osmotically. This is so that the conditions of high water potential and low tugor pressure, which causes the pressure flow process, which enables and creates the transport of sugars throughout the plant so that the sugars can be used for the plant's metabolism. I hope this helped everyone better understand the pressure flow theory and the transport of sugar in plants !