By the end of this lesson
- Describe substances transported by phloem.
- Explain source–sink translocation.
- Relate sieve tubes and companion cells to function.
BIG QUESTION
How can sugar made in one leaf move upward to a fruit and downward to a root?
Living sieve tubes transport assimilates between sources and sinks; direction depends on current demand.
Phloem transports assimilates
phloemPlant tissue that translocates sucrose and amino acids from sources, such as leaves, to sinks, such as roots, fruits and growing tissues. carries sucrose and amino acids between leaves and other plant parts by translocationTransport of sucrose and amino acids in the phloem from sources to sinks..
Sucrose moves carbon and chemical energy; amino acids supply nitrogen and carbon for protein synthesis.
Sources load and sinks unload
A sourceA part of a plant that releases sucrose and amino acids into the phloem, such as a photosynthesising leaf or a storage organ. releases assimilates, commonly a photosynthesising leaf or mobilised storage organ. A sinkA part of a plant that uses or stores sucrose and amino acids carried in the phloem, such as roots, fruits and growing buds. uses or stores them, such as a root, fruit, bud or growing tissue.
A tissue can change role with season or development.
Living tissues support pressure flow
Sieve-tube elements form columns joined by perforated sieve plates and contain little internal machinery. Companion cells provide metabolic support and active loading/unloading.
Water entry can generate pressure differences that drive sap from source to sink. Different sieve tubes may carry sap in opposite directions.
Source and sink examples
Direction is determined by relationship, not gravity.
| Tissue | Likely role | Reason |
|---|---|---|
| Mature illuminated leaf | Source. | Exports photosynthate. |
| Growing root tip | Sink. | Uses sugars and amino acids. |
| Developing fruit | Sink. | Growth and storage. |
| Storage root in spring | Source. | Mobilises stored carbohydrate. |
Language that earns precision
- translocation
- Transport of sucrose and amino acids in phloem.
- source
- Tissue that loads assimilates into phloem.
- sink
- Tissue that unloads and uses or stores assimilates.
- sieve tube
- Column of living conducting cells in phloem.
Common misconceptions
Claim: Phloem only transports sugar downward.
Correction: It transports from sources to sinks, which may lie in either direction.
Claim: Glucose is the main long-distance phloem sugar.
Correction: Sucrose is the principal named transport sugar.
Claim: Phloem cells are dead hollow tubes.
Correction: Sieve tubes are living and supported by companion cells.
Match the answer to the command word
Name sucrose, amino acids, leaves and other plant parts.
Use source and sink rather than up and down.
Recall, apply, then justify
Describe the role of phloem.
Hint
Substances and endpoints.
Suggested marking points
- Transports sucrose.
- Transports amino acids.
- Moves them between leaves and other parts by translocation.
Explain why a developing fruit acts as a sink.
Hint
Import, use and store.
Suggested marking points
- It imports sucrose and amino acids through phloem.
- They provide energy/materials for growth.
- Sugars may be stored or converted into other molecules.
Predict how removing a ring of bark containing phloem affects tissues above and below the cut.
Planning hint
Xylem may remain intact.
What a strong design includes
- Phloem translocation across the ring is interrupted.
- Sucrose can accumulate above the cut.
- Tissues below receive less assimilate.
- Roots may lose respiratory substrate and eventually die.
- Water may continue initially if xylem remains intact.
- Loss of roots later reduces water uptake and harms the whole plant.