Higher Pressure-Flow Hypothesis and Companion Cell Role
Part of Plant Transport Systems — GCSE Biology
This higher tier covers Higher Pressure-Flow Hypothesis and Companion Cell Role within Plant Transport Systems for GCSE Biology. Xylem and phloem structure, water and sugar transport, root hair adaptations, translocation, and practical investigations It is section 14 of 17 in this topic. This section is most useful once the core foundation idea is secure, because it adds the detail that pushes answers higher.
Topic position
Section 14 of 17
Practice
19 questions
Recall
24 flashcards
Higher Pressure-Flow Hypothesis and Companion Cell Role
The pressure-flow hypothesis explains how sugars move through phloem:
- 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.
- Osmotic water entry: Water enters the sieve tube from adjacent xylem by osmosis, increasing hydrostatic pressure at the source end.
- 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).
- 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.