Phloem: The Food Transport System

Phloem: The Food Transport System

Structure of Phloem Tissue

Phloem tissue has a more complex structure than xylem:

• Sieve tube elements: Living cells that form the transport tubes
• Sieve plates: Perforated end walls with holes for substance flow
• No nucleus: Sieve tube elements lose their nucleus to make room for flow
• Companion cells: Neighboring cells that control the sieve tube elements
• Cytoplasm present: Unlike xylem, phloem cells remain alive

Translocation: Moving Sugars Through Plants

The movement of sugars in phloem is called translocation and follows the pressure-flow hypothesis:

1. Loading at source: Sugars are actively transported into phloem at leaves (sources)
2. Osmotic pressure: Water enters phloem by osmosis, creating high pressure
3. Mass flow: High pressure pushes sugars along the phloem tubes
4. Unloading at sink: Sugars are actively removed at roots/storage organs (sinks)
5. Pressure reduction: Water leaves phloem, reducing pressure and maintaining flow

The Role of Companion Cells

Companion cells are crucial for phloem function:

• Provide ATP for active transport of sugars
• Control loading and unloading processes
• Maintain sieve tube elements (which lack nuclei)
• Connected to sieve tubes by plasmodesmata