3.6.5 The Sucrose Loading Mechanism

Steps in Sucrose Loading:

1. Active Transport of Hydrogen Ions:

• Companion cells use ATP to pump H+ ions out of the cell into the surrounding tissue.
• This creates a high concentration of H+ ions outside the cell and a steep diffusion gradient.

2. Facilitated Diffusion of H+ Ions:

• The H+ ions diffuse back into the companion cells down their concentration gradient through co-transporter proteins in the membrane.
• These co-transporter proteins simultaneously carry sucrose molecules into the companion cells.

3. Sucrose Movement into Sieve Tubes:

• The concentration of sucrose increases in the companion cells.
• Sucrose then diffuses into the sieve tube elements through plasmodesmata (microscopic channels connecting the two cell types).

Key Features of the Sucrose Loading Mechanism:

• Energy-Dependent:
  • ATP is required for the active transport of H+ ions.
  • This drives the secondary active transport of sucrose into the phloem.
• Plasmodesmata:
  • Channels that link the cytoplasm of companion cells and sieve tube elements, allowing the passive movement of sucrose.
• Source to Sink:
  • Sucrose loading allows the movement of sugars from source tissues (where sucrose is produced or stored) to sink tissues (where sucrose is used or stored).

Importance of Sucrose Loading:

1. Maintains Pressure Gradients:

• Sucrose entry into sieve tubes reduces water potential, causing water to enter from the xylem via osmosis, increasing hydrostatic pressure in the phloem.
• This drives the mass flow of sap to regions of lower pressure (the sink).

2. Efficient Transport:

• The active loading mechanism ensures a high concentration of sucrose in the phloem, maintaining a continuous flow of nutrients.

Key Terms:

• Active Loading: The ATP-driven transport of sucrose into the phloem.
• Co-Transport: The simultaneous movement of sucrose and H+ ions into companion cells.
• Plasmodesmata: Cytoplasmic connections between companion cells and sieve tube elements.
• Source: A region where sucrose is loaded into the phloem (e.g., leaves).
• Sink: A region where sucrose is unloaded from the phloem (e.g., roots, meristems).