Pressure Flow Hypothesis

Definition

The pressure flow hypothesis explains how sugars are transported through the phloem in plants. This model describes the movement of sap, primarily composed of sugars, from source tissues, like leaves where photosynthesis occurs, to sink tissues, such as roots or fruits that require energy. The process relies on the generation of turgor pressure differences that drive the flow of sap through the phloem's vascular system.

5 Must Know Facts For Your Next Test

1. The pressure flow hypothesis relies on the creation of high turgor pressure in source cells, which forces sap into the phloem vessels.
2. At the source, sugars are actively loaded into phloem cells, creating a concentration gradient that draws water in via osmosis, increasing turgor pressure.
3. As the sap moves towards sink tissues, sugars are unloaded, decreasing the osmotic potential and allowing water to exit, which reduces pressure.
4. This process is dynamic; it can change direction based on seasonal needs, with leaves acting as sources during growth periods and roots serving as sinks during dormancy.
5. The efficiency of this transport mechanism is essential for plant growth and development, ensuring that energy produced in one part of the plant can be utilized in other areas.

Review Questions

• How does the pressure flow hypothesis explain the movement of sugars from source to sink in plants?
  • The pressure flow hypothesis describes how sugars produced at source tissues create a concentration gradient that draws water into phloem cells through osmosis, leading to increased turgor pressure. This pressure difference propels the sap down to sink tissues where sugars are actively unloaded. The process emphasizes both active transport at sources and passive flow due to pressure gradients throughout the phloem.
• Discuss how changes in turgor pressure impact sugar transport in plants according to the pressure flow hypothesis.
  • Turgor pressure plays a crucial role in the pressure flow hypothesis by creating a driving force for sugar transport. When sugars are loaded into phloem cells at a source, they cause an influx of water, raising turgor pressure. Conversely, when sugars are unloaded at sinks, turgor pressure decreases as water exits. These fluctuations ensure efficient movement of nutrients through the plant by responding to its metabolic demands.
• Evaluate the significance of the pressure flow hypothesis in understanding plant physiology and resource allocation during different growth stages.
  • The pressure flow hypothesis is vital for understanding how plants allocate resources efficiently throughout their lifecycle. By illustrating how sugar transport adapts based on growth phases—where sources may shift seasonally between leaves and roots—the model highlights how plants respond to environmental changes and internal needs. This adaptability is essential for maximizing energy use and supporting overall health, particularly during critical growth stages such as flowering or fruiting.

Related terms

Phloem: A type of vascular tissue responsible for the transport of nutrients and sugars produced by photosynthesis throughout the plant.

Turgor Pressure: The pressure of the cell contents against the cell wall, which is crucial for maintaining cell structure and driving sap flow in the phloem.

Source and Sink: Source refers to plant parts that produce or store carbohydrates (like leaves), while sink refers to parts that consume or store these carbohydrates (like roots or developing fruits).