The Mass Flow Hypothesis

1. Active Loading at the Source

• Proton Pumping: Companion cells use ATP to actively pump hydrogen ions ($H^+$) out of their cytoplasm and into the surrounding cell wall space.
• Cotransport: The resulting electrochemical gradient allows $H^+$ to diffuse back into the companion cell through a cotransporter protein, bringing a sucrose molecule with it against its concentration gradient.
• Entry to Phloem: Sucrose then moves from the companion cells into the sieve tube elements via small channels called plasmodesmata.

2. Generation of High Pressure

• The high concentration of sucrose in the sieve tube significantly lowers the water potential ($Ψ_s$).
• Water moves from the xylem (where $Ψ$ is higher) into the phloem by osmosis, which increases the volume of liquid and thus the hydrostatic pressure at the source.

3. Mass Flow and Unloading

• The sap moves down the pressure gradient toward the sink. At the sink, sucrose is actively or passively unloaded for use in respiration or storage as starch.
• The loss of solutes increases the water potential at the sink, causing water to move back into the xylem or surrounding cells by osmosis, which lowers the hydrostatic pressure and maintains the gradient.

• It is vital to distinguish between the mechanisms of transport in the xylem versus the phloem to avoid confusion during analysis.

• Mass Flow vs. Diffusion: Unlike diffusion, which relies on the random movement of individual molecules down a concentration gradient, mass flow involves the movement of the entire solvent and all dissolved solutes together due to a pressure difference.

Supporting Evidence

• Pressure Observations: When phloem sieve tubes are punctured (e.g., by aphid mouthparts), the sap oozes out under pressure, confirming the existence of positive hydrostatic pressure.
• Concentration Gradients: Studies show that sucrose concentration is consistently higher in leaves (sources) than in roots or fruits (sinks).
• Metabolic Inhibitors: If a plant is treated with chemicals that stop ATP production, translocation ceases, proving that active transport is essential for the process.

Contradictory Evidence

• Solute Velocity: Different solutes (e.g., amino acids vs. sugars) have been observed moving at different speeds within the same sieve tube, which contradicts the idea of a single 'mass' flow.
• Bidirectional Flow: Some experiments suggest that solutes can move in opposite directions simultaneously within the same sieve tube element, which a simple pressure gradient cannot explain.
• Sieve Plates: The presence of sieve plates creates significant resistance to flow; critics argue that if mass flow were the only mechanism, these structures would likely have been evolutionary disadvantages.

• Identify the Source and Sink: Always start by identifying which part of the plant is the source and which is the sink in the given scenario. Remember that a storage organ like a tuber can be a sink in summer but a source in early spring.

• Use Precise Terminology: Avoid saying 'sugar moves by diffusion.' Instead, use terms like 'active loading,' 'hydrostatic pressure gradient,' and 'bulk flow.'

• Link Xylem and Phloem: Examiners often look for the connection between the two tissues. Explain how water movement from the xylem is the physical cause of the pressure increase in the phloem.

• Evaluate, Don't Just Describe: If asked to 'evaluate' the hypothesis, you must provide both supporting evidence (like ringing experiments) and conflicting evidence (like different solute speeds).