Hydrostatic Pressure Gradients
- Phloem sap containing sucrose and other organic solutes moves through the sieve tubes down a hydrostatic pressure gradient
- This is known as the pressure-flow hypothesis
- Hydrostatic pressure is the term used to describe the pressure exerted by a fluid on the walls of its container
- High hydrostatic pressure at the source is generated by the following process
- Sucrose is actively transported into the sieve tube elements at the source
- The raised solute concentration in the sieve tube causes water to follow the sucrose by osmosis
- Water cannot be compressed, so when the volume of water in the sieve tube increases, it presses against the rigid plant cell walls, resulting in a build-up of hydrostatic pressure in the sieve tube at the source
- Low hydrostatic pressure at the sink is maintained by the following process
- Sucrose is removed from the sieve tube elements when it reaches the sink
- Sink regions contain cells where sugars are either used in respiration or converted into starch for storage
- This lowers the solute concentration of the sieve tube contents
- Water leaves the sieve tube by osmosis, lowering the hydrostatic pressure inside the sieve tube at the sink
- Sucrose is removed from the sieve tube elements when it reaches the sink
- Phloem sap inside the sieve tube moves down a hydrostatic pressure gradient from high to low hydrostatic pressure
- The pressure difference between the source and the sink results in the mass flow of phloem sap from the high hydrostatic pressure area to the low hydrostatic pressure area
- The direction of the mass flow of phloem sap is determined by the hydrostatic pressure gradient, which in turn is determined by the relative locations of the source and sink regions in the plant; this means that phloem sap can flow either upwards or downwards within the sieve tube
The translocation of phloem sap occurs due to a hydrostatic pressure gradient between the source and the sinkExam Tip
Remember that the source will not always be the leaves and the sink will not always be the roots; phloem sap can move both up and down the plant depending on the location of the source and sink regions.
The hydrostatic pressure gradient is dependent on water moving in and out of the xylem vessels by osmosis.
The hydrostatic pressure gradient is dependent on water moving in and out of the xylem vessels by osmosis.