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:
- Loading at source: Sugars are actively transported into phloem at leaves (sources)
- Osmotic pressure: Water enters phloem by osmosis, creating high pressure
- Mass flow: High pressure pushes sugars along the phloem tubes
- Unloading at sink: Sugars are actively removed at roots/storage organs (sinks)
- 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
This deep dive covers Phloem: The Food Transport System within Plant Transport Systems for GCSE Biology. Xylem and phloem structure, water and sugar transport, root hair adaptations, translocation, and practical investigations It is section 4 of 18 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Plant Transport Systems
Which substance does xylem tissue transport?
Explain how root hair cells are adapted for their function.
Quick recall flashcards
What is phloem tissue?
A plant tissue that transports sugars and amino acids from the leaves to other parts of the plant.
What is xylem tissue?
A plant tissue that transports water and mineral salts from the roots to the leaves.