Pressure Flow Hypothesis — Revision Notes

⚡ 30-Second Revision

Source: — Sugar production/release (e.g., leaves).
Sink: — Sugar utilization/storage (e.g., roots).
Phloem Loading: — Active transport of sucrose into sieve tubes at source (requires ATP).
Water Movement (Source): — Osmosis from xylem into phloem due to low water potential (high sugar conc.).
Turgor Pressure (Source): — High, due to water influx.
Mass Flow: — Bulk movement of sap from high pressure (source) to low pressure (sink).
Phloem Unloading: — Active transport of sucrose out of sieve tubes at sink (requires ATP).
Water Movement (Sink): — Osmosis from phloem into xylem/sink cells due to high water potential (low sugar conc.).
Turgor Pressure (Sink): — Low, due to water efflux.
Primary Sugar: — Sucrose.

2-Minute Revision

The Pressure Flow Hypothesis explains how sugars move in plants via the phloem. It starts at a 'source' (like a leaf), where sugars (mainly sucrose) are actively loaded into phloem sieve tubes. This active loading lowers the water potential inside the phloem, causing water to osmotically move in from the adjacent xylem.

This influx of water creates a high turgor pressure at the source. This high pressure then drives the bulk flow, or mass flow, of the sugary phloem sap towards a 'sink' (like a root or fruit), where sugars are needed.

At the sink, sugars are actively unloaded from the phloem. This removal of sugars raises the water potential in the phloem, causing water to osmotically move out and back into the xylem. This outflow of water reduces the turgor pressure at the sink, maintaining the pressure gradient that sustains the continuous flow of sap from source to sink.

Companion cells provide the necessary ATP for the active loading and unloading processes.

5-Minute Revision

The Pressure Flow Hypothesis, also known as the Mass Flow Hypothesis, is the most accepted model for the translocation of organic solutes, primarily sucrose, through the phloem. The entire process is driven by a pressure gradient established between a 'source' and a 'sink'.

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At the Source (e.g., mature leaf):

* Sugars (sucrose) produced during photosynthesis are actively transported from mesophyll cells into companion cells and then into the sieve tube elements of the phloem. This 'phloem loading' requires ATP, often involving sucrose-proton symporters.

* The high concentration of sucrose in the sieve tubes lowers their water potential. * Consequently, water moves by osmosis from the adjacent xylem into the sieve tube elements. * This influx of water creates a high hydrostatic pressure, or turgor pressure, at the source end of the phloem.

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Mass Flow:

* The high turgor pressure at the source pushes the phloem sap (water and dissolved sugars) through the sieve tubes towards regions of lower pressure. * This bulk movement is called mass flow, where all components of the sap move together.

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At the Sink (e.g., root, fruit, growing shoot):

* Sugars are actively transported out of the sieve tube elements into the surrounding sink cells, where they are either consumed for metabolism or converted into storage forms (e.g., starch). This 'phloem unloading' also requires ATP.

* As sugars leave the phloem, the solute concentration in the sieve tubes decreases, raising their water potential. * Water then moves by osmosis out of the sieve tubes and back into the xylem or into the sink cells.

* This outflow of water reduces the hydrostatic pressure, or turgor pressure, at the sink end of the phloem.

The continuous difference in turgor pressure between the source (high pressure) and the sink (low pressure) drives the unidirectional flow of sap from source to sink. The water that leaves the phloem at the sink is recycled back to the source via the xylem. Companion cells are crucial throughout, providing metabolic support and ATP for the active loading and unloading steps.

Prelims Revision Notes

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Definition: — Pressure Flow Hypothesis (Mass Flow Hypothesis) explains phloem transport of organic solutes (sugars).
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Proponent: — Ernst Münch (1930).
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Source: — Region of sugar production/release (e.g., mature leaves, storage organs mobilizing reserves).
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Sink: — Region of sugar utilization/storage (e.g., roots, fruits, growing meristems, young leaves).
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Primary Solute: — Sucrose (non-reducing disaccharide, stable for transport).
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Steps at Source:

* Sugar Synthesis: Glucose $o$ Sucrose in mesophyll cells. * Phloem Loading: Active transport of sucrose from mesophyll $o$ companion cells $o$ sieve tube elements. Requires ATP (e.g., sucrose-proton symporter). * Water Potential: High sucrose conc. $implies$ low water potential in sieve tubes. * Osmosis: Water moves from adjacent xylem (high water potential) $o$ sieve tubes. * Turgor Pressure: Water influx $implies$ high hydrostatic pressure in sieve tubes.

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Mass Flow: — High turgor pressure at source pushes phloem sap (water + solutes) through sieve tubes towards sink (region of lower pressure).
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Steps at Sink:

* Phloem Unloading: Active transport of sucrose from sieve tube elements $o$ sink cells. Requires ATP. * Water Potential: Low sucrose conc. $implies$ high water potential in sieve tubes. * Osmosis: Water moves from sieve tubes $o$ adjacent xylem/sink cells. * Turgor Pressure: Water efflux $implies$ low hydrostatic pressure in sieve tubes.

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Driving Force: — Positive hydrostatic pressure gradient (turgor pressure difference) between source and sink.
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Energy Requirement: — Overall process is energy-dependent due to active loading and unloading steps. Mass flow itself is passive.
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Role of Companion Cells: — Metabolically active, provide ATP and support to sieve tube elements for active transport.
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Direction of Flow: — Generally source to sink, but can be bidirectional depending on changing source/sink roles.
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Girdling Experiment: — Removal of phloem (bark) prevents sugar transport to roots, leading to root starvation and death, demonstrating phloem's role.

Vyyuha Quick Recall

S.L.O.W. P.U.L.S.E.

Source: Sugar made.
Loading: Active, into phloem.
Osmosis: Water in, from xylem.
Water Pressure: High at source.

Push: Mass flow.
Unloading: Active, out of phloem.
Low Pressure: At sink.
Sink: Sugar used.
Efflux: Water out, to xylem.