Higher Pressure-Flow Hypothesis and Companion Cell Role

Higher Pressure-Flow Hypothesis and Companion Cell Role

The pressure-flow hypothesis explains how sugars move through phloem:

1. Active loading at source: Companion cells use ATP to actively transport sucrose from mesophyll cells into sieve tubes. This lowers the water potential of phloem sap at the source.
2. Osmotic water entry: Water enters the sieve tube from adjacent xylem by osmosis, increasing hydrostatic pressure at the source end.
3. Pressure-driven flow: The high pressure at the source pushes phloem sap along the sieve tubes toward regions of lower pressure (sinks — roots, growing tips, fruit).
4. Active unloading at sink: Sucrose is actively removed from sieve tubes at the sink. Water follows by osmosis, reducing pressure at the sink end and maintaining the pressure gradient.

Why companion cells matter: Sieve tube elements have no nucleus and cannot synthesise their own ATP. Companion cells are nucleated, metabolically active, and pass ATP to sieve tubes via plasmodesmata. Without companion cells, active loading and unloading would be impossible and translocation would stop.

Note: The cohesion-tension theory applies to xylem; the pressure-flow hypothesis applies to phloem. Do not mix these up.