Pressure flow hypothesis

Definition

The pressure flow hypothesis explains how sugars and other organic compounds are transported in plants through the phloem. This process relies on the differences in pressure created by the loading of sugars into the phloem at the source and their unloading at the sink, leading to a flow that moves the nutrients efficiently throughout the plant.

5 Must Know Facts For Your Next Test

1. The pressure flow hypothesis relies on turgor pressure, which is the pressure of the fluid inside the cells, to drive the movement of sap through the phloem.
2. Loading of sugars into the phloem requires active transport, which helps create a high concentration of solutes and thus a gradient for water movement.
3. Water enters the phloem from surrounding xylem vessels due to osmotic pressure, aiding in creating a pressure difference necessary for flow.
4. At sinks, sugars are actively transported out of the phloem, lowering solute concentration and allowing water to exit, thus maintaining the pressure flow.
5. The process is bidirectional; while sugars move from sources to sinks, water can also move back into xylem vessels to maintain hydration and nutrient transport.

Review Questions

• How does the pressure flow hypothesis explain the movement of nutrients in plants?
  • The pressure flow hypothesis describes nutrient movement by highlighting the role of pressure differences in the phloem. Sugars are actively loaded at sources, creating a high concentration that draws in water through osmosis. This increases turgor pressure, causing sap to flow towards sinks where sugars are unloaded, thus demonstrating how pressure gradients facilitate nutrient transport throughout the plant.
• Discuss the roles of sources and sinks in relation to the pressure flow hypothesis.
  • In the context of the pressure flow hypothesis, sources are areas where photosynthesis occurs and sugars are produced, such as leaves. These sugars are then loaded into the phloem, increasing solute concentration and turgor pressure. Sinks, such as roots or fruits, utilize these sugars either for energy or storage. The dynamic interaction between sources and sinks ensures that nutrients are efficiently distributed according to plant needs.
• Evaluate how changes in environmental conditions could impact the efficiency of the pressure flow hypothesis in plants.
  • Environmental changes like drought can significantly affect the efficiency of the pressure flow hypothesis. During drought conditions, water availability decreases, leading to lower turgor pressure within plant cells. This reduction can hinder sugar loading into phloem and disrupt normal flow towards sinks. Additionally, insufficient water uptake affects overall nutrient transport and can lead to reduced growth or crop yield due to compromised resource distribution.

Related terms

Phloem: The vascular tissue in plants responsible for transporting sugars and other metabolic products downward from the leaves.

Source: Parts of the plant, like leaves, where sugars are produced through photosynthesis and loaded into the phloem.

Sink: Tissues or organs in the plant, such as roots or fruits, where sugars are consumed or stored.