3.6.7 Data on the Mass Flow Hypothesis

Steps in the Mass Flow Hypothesis:

1. Active Loading of Sucrose at the Source:

• Companion cells use ATP to actively transport H+ ions into surrounding tissue.
• This creates a diffusion gradient for H+ ions to re-enter the companion cells via co-transporter proteins.
• Sucrose molecules are brought into the companion cells along with H+ ions, increasing sucrose concentration.

2. Transfer of Sucrose to Sieve Tubes:

• Sucrose diffuses from companion cells into sieve tube elements through plasmodesmata (cytoplasmic channels linking the two cell types).

3. Water Movement into the Sieve Tubes:

• The addition of sucrose lowers the water potential inside the sieve tubes.
• Water enters the sieve tubes from the xylem by osmosis, increasing the hydrostatic pressure at the source.

4. Mass Flow Along the Sieve Tubes:

• The difference in hydrostatic pressure between the source (high pressure) and the sink (low pressure) causes water and dissolved sucrose to flow through the sieve tubes.

5. Unloading of 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, causing water to leave by osmosis back into the xylem, reducing hydrostatic pressure at the sink.

Supporting Data for the Mass Flow Hypothesis:

6. Pressure Gradients:

• Evidence shows higher hydrostatic pressure in the phloem at the source compared to the sink, consistent with the pressure-driven flow.

7. Radioactive Tracers:

• Radioactive isotopes like 14C-labelled CO2 can be used to trace the movement of sucrose through the phloem, demonstrating that assimilates move from source to sink.

8. Ring Experiments:

• Removing a ring of bark, including the phloem, causes sugars to accumulate above the ring, supporting the downward movement of sucrose.

9. Metabolic Inhibitors:

• Using inhibitors like cyanide, which stop ATP production, prevents active loading and halts translocation, demonstrating the role of active transport in sucrose loading.

Limitations of the Mass Flow Hypothesis:

10. Role of Sieve Plates:

• Sieve plates should impede flow, yet translocation is efficient, suggesting additional mechanisms may be involved.

11. Bidirectional Flow:

• The hypothesis struggles to explain how sap can flow in opposite directions simultaneously in different sieve tubes.

12. Functions of Companion Cells:

• Companion cells may have additional roles beyond sucrose loading, complicating the hypothesis.

Key Terms:

• Active Loading: The ATP-driven transport of sucrose into sieve tubes via companion cells.
• Plasmodesmata: Cytoplasmic channels linking companion cells and sieve tube elements.
• Hydrostatic Pressure Gradient: The difference in pressure driving sap from source to sink.
• Radioactive Tracers: Isotopes used to track the movement of assimilates in plants.