Explain mass flow in phloem sieve tubes down a hydrostatic pressure gradient from source to sink

Sourav PanOctober 29, 2024

Answered

Mass flow in phloem sieve tubes refers to the movement of phloem sap (which primarily contains sugars and other organic nutrients) from areas of high concentration (sources) to areas of low concentration (sinks) through a pressure-driven mechanism. This process is crucial for the distribution of nutrients throughout the plant. Here’s a detailed explanation of how this occurs:

1. Source and Sink

• Source: A source is any part of the plant where sugars (primarily produced during photosynthesis) are synthesized or stored, such as the leaves or storage organs (e.g., tubers).
• Sink: A sink is any part of the plant that requires sugars for growth, energy, or storage, such as roots, developing fruits, and young leaves.

2. Loading of Sugars into Sieve Tubes

• Active Transport: At the source, sugars (mainly sucrose) are actively transported into the phloem sieve tube elements from the mesophyll cells. This is accomplished by specific transport proteins in the cell membranes.
• Water Uptake: As sugars accumulate in the sieve tubes, they create a high solute concentration. This draws water into the phloem from surrounding tissues (e.g., xylem) through osmosis, increasing the turgor pressure within the sieve tubes.

3. Creation of Hydrostatic Pressure Gradient

• Pressure Build-Up: The influx of water due to osmosis raises the hydrostatic pressure in the phloem at the source. This pressure is higher than that in adjacent sink regions.
• Pressure Gradient: The difference in pressure between the source (high pressure) and sink (lower pressure) creates a hydrostatic pressure gradient.

4. Mass Flow Towards the Sink

• Flow of Phloem Sap: The increased pressure at the source forces the phloem sap to flow down the pressure gradient toward the sink. The flow is driven by mass flow, where the bulk movement of sap occurs due to the pressure difference, rather than by diffusion.
• Movement through Sieve Plates: As the sap moves through the sieve tubes, it passes through perforated sieve plates, which allow for efficient transport while maintaining a continuous flow of sap.

5. Unloading at the Sink

• Sugar Utilization: At the sink, sugars are unloaded from the phloem sieve tubes. This can occur via passive diffusion or active transport, depending on the sink tissue’s needs.
• Water Reabsorption: As sugars are removed, the osmotic pressure in the phloem decreases, leading to a loss of water back to the surrounding tissues or xylem, which helps maintain the pressure gradient for continuous flow.

Summary

• Mass Flow Mechanism: Mass flow in phloem sieve tubes occurs as a result of hydrostatic pressure differences created by the active loading of sugars at sources and the subsequent removal of sugars at sinks.
• Pressure Gradient: This pressure gradient drives the bulk movement of phloem sap, ensuring that nutrients produced during photosynthesis are efficiently distributed to areas of the plant where they are needed for growth and storage.