Pressure Flow Hypothesis — Core Principles

Core Principles

The Pressure Flow Hypothesis explains how plants transport food (sugars) through the phloem. It begins at a 'source' (e.g., a leaf), where sugars like sucrose are actively loaded into phloem sieve tubes.

This increases the solute concentration, causing water to move in from the xylem via osmosis, building high turgor pressure. This high pressure pushes the sugary solution (phloem sap) through the sieve tubes towards a 'sink' (e.

g., root, fruit), where sugars are needed. At the sink, sugars are actively unloaded from the phloem. This reduces the solute concentration, causing water to move out of the phloem and back into the xylem, thus lowering the turgor pressure.

The continuous difference in turgor pressure between source and sink drives the mass flow of sap, ensuring efficient distribution of nutrients throughout the plant.

Important Differences

vs Xylem Transport

Aspect	This Topic	Xylem Transport
Substances Transported	Sugars (sucrose), amino acids, hormones, some mineral ions, water.	Water and dissolved mineral ions.
Direction of Flow	Bidirectional (from source to sink, which can vary).	Unidirectional (primarily upwards, from roots to leaves).
Driving Force	Positive hydrostatic pressure gradient (turgor pressure) established by active loading/unloading.	Negative pressure (tension) created by transpiration pull, aided by root pressure.
Energy Requirement	Requires metabolic energy (ATP) for active loading and unloading of solutes.	Largely passive, driven by physical forces (transpiration, cohesion-adhesion). No direct metabolic energy for water movement.
Conducting Cells	Sieve tube elements (living, anucleate, with companion cells).	Tracheids and vessel elements (dead at maturity, hollow tubes).
Mechanism	Pressure Flow Hypothesis (Mass Flow).	Cohesion-Tension Theory.

The transport mechanisms in xylem and phloem, though both part of the plant's vascular system, are fundamentally different. Xylem primarily moves water and minerals upwards from roots to leaves, driven by transpiration pull and cohesion-tension, a largely passive process. In contrast, phloem translocates organic solutes like sugars from sources to sinks in multiple directions, driven by a positive pressure gradient established through energy-dependent active loading and unloading of solutes. These distinct mechanisms ensure efficient distribution of both water and nutrients throughout the plant body.