2.7.13Inquiry Process
Translocation in Plants — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 85 of 160
Part 1 of 4Structure of Phloem
Translocation is the movement of organic compounds in the phloem.
Sieve tube elements
- Living cells joined end to end with perforated sieve plates between them, forming a tube through which sap flows.
Reduced contents
- Sieve elements have little cytoplasm and lack a nucleus, which leaves a clear route for sap.
Companion cells
- Each sieve element is connected to a companion cell by plasmodesmata;
- the companion cell has many mitochondria to supply ATP.
Sap
- Phloem sap carries sucrose, amino acids and other organic solutes in a water solution.
Part 2 of 4Pressure-Flow Mechanism
1
At a source such as a mature leaf, companion cells use ATP to pump hydrogen ions out, creating a proton gradient.
2
Sucrose is moved into the companion cell by co-transport with hydrogen ions, and then passes into the sieve tube: active loading.
3
The high solute concentration lowers water potential in the sieve tube, so water enters by osmosis and hydrostatic pressure rises.
4
At a sink, such as a root or growing fruit, sucrose is unloaded and used or stored, so water potential rises and water leaves.
5
The pressure difference between source and sink drives mass flow of sap along the phloem.
Part 3 of 4Sources, Sinks and Evidence
Sources, Sinks and Evidence
Source
- A region that produces or releases sugars, such as photosynthesising leaves or storage organs in spring.
Sink
- A region that uses or stores sugars, such as roots, fruits, seeds and growing shoot tips.
Changing roles
- A storage organ can be a sink in summer and a source when the plant grows in spring.
Aphid experiments
- Aphid stylets inserted into phloem exude sap under pressure, showing that sap is under positive pressure.
| Feature | Xylem | Phloem |
|---|---|---|
| Substance | Water, minerals | Sucrose, amino acids |
| Cells | Dead | Living |
| Direction | Upwards | Source to sink |
Part 4 of 4Exam Checklist & Tips
2.7.13 – Key Points for the Exam
- Translocation The transport of organic solutes in the phloem.
- Source A part of the plant that loads sugars into the phloem.
- Sink A part of the plant that removes sugars from the phloem.
- Companion cell A cell that supports a sieve element and carries out active loading.
- Mass flow Bulk movement of liquid along a pressure gradient.
Common Mistakes
- Saying phloem transport only goes upwards; it can go up or down depending on source and sink.
- Describing sieve tubes as dead; they are living but lack many organelles.
- Forgetting that active loading uses ATP.
- Claiming that sucrose moves by diffusion alone; it moves by mass flow.
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