OrganisationDeep Dive
Phloem: The Food Transport System
Part of Plant Transport Systems — GCSE Biology
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 17 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 4 of 17
Practice
19 questions
Recall
24 flashcards
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:
- 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