3.6.6 The Mass Flow Hypothesis

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The mass flow hypothesis explains the movement of assimilates (e.g., sucrose) in the phloem from sources (e.g., leaves) to sinks (e.g., roots or growing tissues). This process relies on hydrostatic pressure gradients and the flow of water into and out of the sieve tube elements.

Steps in the Mass Flow Hypothesis:

Loading Sucrose at the Source:

Sucrose produced in the source (e.g., photosynthesising leaves) is actively transported into the phloem sieve tube elements by companion cells.
The increased sucrose concentration lowers the water potential in the sieve tubes.

Water Movement into the Sieve Tubes:

Water enters the sieve tubes from the xylem via osmosis due to the lower water potential.
This influx of water increases the hydrostatic pressure at the source end of the phloem.

Mass Flow Along the Sieve Tubes:

The high hydrostatic pressure at the source causes the sap (a mixture of water and dissolved sucrose) to flow down the phloem.
The sap moves towards areas of lower hydrostatic pressure at the sink.

Unloading Sucrose at the Sink:

Sucrose is actively transported or diffuses out of the sieve tubes into the sink cells.
This increases the water potential in the sieve tubes at the sink.

Water Movement Out of the Sieve Tubes:

Water moves out of the sieve tubes into the surrounding xylem via osmosis.
The removal of water reduces the hydrostatic pressure in the phloem at the sink.

Key Features:

Source:
- A region producing or releasing sucrose (e.g., leaves during photosynthesis).
- High hydrostatic pressure is established here due to water entry.
Sink:
- A region where sucrose is used or stored (e.g., roots or growing tissues).
- Low hydrostatic pressure is created here due to water exit.
Hydrostatic Pressure Gradient:
- Drives the mass flow of sap from source to sink.

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Summary:

The mass flow hypothesis relies on the hydrostatic pressure gradient to transport sucrose and other assimilates in the phloem.

Water potential changes drive water movement into and out of the sieve tubes.
Active transport and osmosis ensure a continuous pressure gradient.
Sucrose moves to where it is needed, such as for storage or growth.

Key Terms:

Mass Flow: The bulk movement of sap through the phloem due to pressure differences.
Source: A region that loads sucrose into the phloem (e.g., leaves).
Sink: A region where sucrose is removed from the phloem (e.g., roots or growing tissues).
Hydrostatic Pressure Gradient: A pressure difference that drives the flow of sap in the phloem.

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Exam Tips:

Explain how changes in water potential facilitate the movement of sucrose.
Use correct terminology such as source, sink, and hydrostatic pressure.
Be prepared to compare the mass flow hypothesis with alternative theories, such as cytoplasmic streaming.

The Mass Flow Hypothesis Simplified Revision Notes for A-Level AQA Biology

3.6.6 The Mass Flow Hypothesis

Steps in the Mass Flow Hypothesis:

Key Features:

Summary:

Key Terms:

Exam Tips:

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