What is the primary driving force for translocation according to the Pressure Flow Hypothesis?

The primary driving force is the difference in hydrostatic pressure, or turgor pressure, between the source and the sink regions within the phloem. At the source, active loading of sugars causes water to enter osmotically, building high turgor pressure. At the sink, active unloading of sugars causes water to leave, leading to lower turgor pressure. This pressure gradient pushes the phloem sap from the high-pressure source to the low-pressure sink.

Are all steps of the Pressure Flow Hypothesis passive?

No, not all steps are passive. While the bulk flow (mass flow) of phloem sap itself is a passive process driven by the pressure gradient, the crucial steps of 'phloem loading' at the source and 'phloem unloading' at the sink are active transport processes. These steps require metabolic energy (ATP) to move sugars against their concentration gradients into and out of the sieve tube elements, respectively. This energy expenditure is fundamental to establishing and maintaining the pressure gradient.

What is the role of companion cells in the Pressure Flow Hypothesis?

Companion cells play a vital supportive role in the Pressure Flow Hypothesis. They are metabolically active cells closely associated with sieve tube elements. They assist in the active loading of sugars into the sieve tube elements at the source and their active unloading at the sink. Companion cells have numerous plasmodesmata connecting them to sieve tube elements, facilitating the transfer of substances and providing metabolic support, including ATP for active transport processes.

Can a 'source' become a 'sink' and vice-versa?

Yes, the roles of 'source' and 'sink' are not fixed and can change depending on the plant's developmental stage and physiological needs. For example, a developing potato tuber acts as a sink, receiving sugars from leaves. However, when the tuber sprouts in the spring, it mobilizes its stored starch into sugars, which are then transported to the growing shoots, making the tuber temporarily act as a source. Similarly, young leaves can be sinks, while mature leaves are sources.

How does water move between xylem and phloem in this hypothesis?

Water movement between xylem and phloem occurs primarily through osmosis. At the source, active loading of sugars into the phloem lowers its water potential, causing water to move from the adjacent xylem (higher water potential) into the phloem. At the sink, active unloading of sugars raises the phloem's water potential, causing water to move out of the phloem and back into the xylem or surrounding sink cells, maintaining the water cycle.

What types of organic solutes are transported by the phloem?

The primary organic solute transported by the phloem is sucrose, a non-reducing disaccharide. However, phloem sap also contains other substances, including amino acids, hormones, certain mineral ions, and even some proteins and RNA molecules. The composition can vary, but sucrose is by far the most abundant and critical for energy and carbon supply throughout the plant.

Pressure Flow Hypothesis

Biology

NEET UG

Version 1Updated 21 Mar 2026

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The Pressure Flow Hypothesis, also known as the Mass Flow Hypothesis, is the most widely accepted explanation for the translocation of sugars (primarily sucrose) through the phloem in vascular plants. Proposed by Ernst Münch in 1930, it posits that a gradient of turgor pressure drives the bulk movement of phloem sap from 'source' regions, where sugars are produced or mobilized, to 'sink' regions, …

Quick Summary

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.

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Key Concepts

Phloem Loading: Active Transport at the Source

Phloem loading is the critical initial step where sugars, mainly sucrose, are moved from their production…

Mass Flow: Pressure Gradient Driven Movement

Once sucrose is actively loaded into the sieve tubes at the source, the solute concentration inside these…

Phloem Unloading: Active Transport at the Sink

Phloem unloading is the process where sugars are removed from the sieve tube elements at the sink regions.…

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.

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.