3.6.4 Movement in the Phloem

Structure of Phloem Vessels:

1. Sieve Tube Elements:

• Long, narrow tubes joined end-to-end.
• Contain minimal cytoplasm and no nucleus to maximise space for sap flow.
• Sieve plates (perforated end walls) allow sap to move between cells.

2. Companion Cells:

• Located adjacent to sieve tube elements.
• Rich in mitochondria to produce ATP for active transport processes.
• Connected to sieve tubes by plasmodesmata, which allow transfer of substances.

3. Plasmodesmata:

• Microscopic channels linking sieve tube elements and companion cells.
• Enable communication and movement of materials such as sucrose.

Process of Translocation:

1. Active Loading at the Source:

• Companion cells actively pump H+ ions into surrounding tissues using ATP.
• This creates a diffusion gradient for H+ ions to return to the companion cells via facilitated diffusion.
• Co-transporter proteins bring sucrose molecules into the companion cells along with the returning $H+$ ions.
• Sucrose moves into sieve tube elements via plasmodesmata, increasing the sucrose concentration in the phloem.

2. Water Movement into Sieve Tubes:

• The influx of sucrose lowers the water potential in sieve tube elements.
• Water enters the sieve tubes from the xylem via osmosis, increasing the hydrostatic pressure in the sieve tube.

3. Mass Flow of Sap:

• Water and dissolved sucrose move down the sieve tube from areas of high hydrostatic pressure (at the source) to areas of low hydrostatic pressure (at the sink).
• This movement is driven by the pressure gradient.

4. Unloading at the Sink:

• Sucrose is actively transported or diffuses out of sieve tube elements into surrounding sink cells.
• The removal of sucrose increases the water potential in the phloem, causing water to leave via osmosis back into the xylem, reducing the pressure in the phloem at the sink.

Key Features of Translocation:

• Source: The site of sucrose production or release (e.g., leaves during photosynthesis).
• Sink: The site where sucrose is needed or stored (e.g., roots, meristems, or storage organs).
• Hydrostatic Pressure Gradient: Drives the mass flow of sucrose and water from source to sink.

Key Terms:

• Translocation: The movement of dissolved substances in the phloem.
• Source: A region where sucrose is loaded into the phloem.
• Sink: A region where sucrose is unloaded from the phloem.
• Mass Flow: The bulk movement of substances due to pressure differences.
• Plasmodesmata: Channels linking companion cells to sieve tube elements.