Cambridge A‑Level Biology – Transport Mechanisms: Mass Flow in Phloem
Transport Mechanisms – Mass Flow in Phloem Sieve Tubes
Learning Objective
Explain how solutes are transported in the phloem by mass flow down a hydrostatic pressure gradient from a source (e.g., mature leaf) to a sink (e.g., root, developing fruit).
Key Concepts
Source: Tissue that produces or releases soluble sugars (usually mature photosynthesising leaves).
Sink: Tissue that consumes or stores sugars (growing roots, buds, fruits, tubers).
Sieve tube elements: Living, elongated cells that form a continuous tube for transport.
Companion cells: Adjacent cells that maintain metabolic activity of sieve tubes.
Hydrostatic pressure gradient: Difference in turgor pressure between source and sink that drives bulk flow.
The Münch Pressure‑Flow Hypothesis
The widely accepted model for phloem transport is the Münch pressure‑flow hypothesis. It proposes that:
Photosynthates (mainly sucrose) are actively loaded into sieve tube elements at the source.
Loading raises the solute concentration, lowering water potential (\$\Psi_w\$) inside the tube.
Water enters osmotically from adjacent xylem, increasing turgor pressure (\$P\$) in the source region.
The pressure difference between source (\$P{\text{source}}\$) and sink (\$P{\text{sink}}\$) drives bulk flow of the sap.
At the sink, sucrose is actively or passively unloaded, reducing solute concentration, raising \$\Psiw\$, and causing water to exit the tube, lowering \$P{\text{sink}}\$.
Mathematical Description
The volumetric flow rate (\$Q\$) through a sieve tube can be expressed by an analogue of Hagen–Poiseuille’s law:
\$\$
Q = \frac{\pi r^4}{8 \eta L}\,\Delta P
\$\$
where:
\$r\$ = radius of the sieve tube element
\$\eta\$ = viscosity of the phloem sap
\$L\$ = length of the tube segment considered
\$\Delta P = P{\text{source}} - P{\text{sink}}\$ = hydrostatic pressure difference
The pressure difference is generated by osmotic loading and unloading:
\$\$
\Delta P = RT \,\Delta C
\$\$
with \$R\$ the gas constant, \$T\$ absolute temperature, and \$\Delta C\$ the difference in solute concentration between source and sink.
Step‑by‑Step Process
Step
Location
Key Events
1
Source (mature leaf)
Active sucrose loading via sucrose‑H⁺ symport; water influx from xylem; turgor pressure rises.
2
Along the sieve tube
Bulk flow of sap driven by pressure gradient; low resistance due to sieve plates.
3
Sink (root tip, developing fruit)
Active or passive sucrose unloading; water exits to surrounding tissues; turgor pressure falls.
4
Systemic adjustment
Continual recycling of water back to the xylem; maintenance of gradient by ongoing loading/unloading.
Factors Influencing Mass Flow
Concentration gradient (\$\Delta C\$): Greater loading at source or greater unloading at sink increases \$\Delta P\$.
Sieve tube radius (\$r\$): Flow rate is proportional to \$r^4\$; small changes have large effects.
Sap viscosity (\$\eta\$): Higher sugar concentrations increase viscosity, reducing flow.
Length of pathway (\$L\$): Longer pathways increase resistance.
Temperature (\$T\$): Affects both viscosity and the \$RT\$ term in the pressure equation.
Physiological Significance
Mass flow enables rapid, long‑distance distribution of carbohydrates, hormones, and signaling molecules, supporting growth, storage, and stress responses throughout the plant.
Suggested diagram: A longitudinal section of a plant showing source leaf, phloem sieve tubes, and sink organ with arrows indicating direction of bulk flow and pressure gradient.
Common Misconceptions
“Phloem transport is active throughout the pathway.” – Only loading and unloading require metabolic energy; the movement between source and sink is passive, driven by pressure.
“Phloem flow is unidirectional like xylem.” – Phloem can transport in multiple directions simultaneously, depending on the distribution of sources and sinks.
Summary
Mass flow in phloem sieve tubes is a pressure‑driven bulk movement of solutes and water from regions of high turgor pressure (source) to regions of low pressure (sink). The process relies on active loading and unloading of sugars, osmotic water movement, and the physical properties of the sieve tube network, as described by the Münch pressure‑flow hypothesis.