The phloem structure
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of plant tissue types (from GCSE). You can test your knowledge on this below.
What is the function of the phloem?
The phloem transports organic substances, such as sucrose and amino acids both up and down the plant (in both directions)
How does translocation differ from transpiration?
- Translocation occurs in the phloem, moving sugars (sucrose) and other nutrients in both directions.
- Transpiration occurs in the xylem, moving water and mineral ions only upwards from the roots to the leaves.
What do we call the transport of organic substances in the phloem?
Translocation
Topic Explainer Video
Check out this @JoeDoesBiology video that explains the phloem structure or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Structure of the Phloem
The phloem is a living vascular tissue responsible for transporting organic substances (mainly sucrose) from sources (e.g., leaves) to sinks (e.g., roots, storage organs, or growing tissues).
Like the xylem, it is also formed from cells end to end, arranged in tubes. But it is purely a transport tissue- it is not used for support.
Sucrose (and other organic substances) move in the phloem by translocation. This is an example of mass transport.
What is a Source?
A source is any part of the plant that produces or releases sucrose into the phloem.
Examples of Sources:
- Leaves (Main Source):
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Photosynthesis produces glucose, which is converted into sucrose and loaded into the phloem.
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Sucrose is transported to growing regions or storage organs.
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- Storage Organs in Spring (e.g., tubers, bulbs):
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Stored starch is broken down into sucrose and released into the phloem for transport to growing tissues.
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Sources have a high concentration of sucrose.
What is a Sink?
A sink is any part of the plant that uses or stores sucrose, removing it from the phloem.
Examples of Sinks:
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Roots: Store sucrose as starch or use it for respiration.
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Developing Fruits & Seeds: Convert sucrose into storage compounds like fructose or starch.
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Growing Shoots & Leaves: Use sucrose for cell division and expansion.
Sinks have a low concentration of sucrose.
How the Direction of Transport Changes
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Summer/Autumn: Leaves (source) → Roots (sink) to store sugars.
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Spring: Roots (source) → New leaves (sink) as stored starch is converted into sucrose for growth.
Why is Sucrose Transported in the Phloem Instead of Glucose?
Sucrose is the main transported sugar in the phloem, rather than glucose, due to several key reasons:
1. Sucrose is Less Reactive than Glucose
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Sucrose is a non-reducing sugar, so it is chemically stable and does not react easily, preventing unwanted chemical changes during transport.
2. Sucrose is More Soluble
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Sucrose is highly soluble in water, allowing it to be efficiently transported in the phloem sap.
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This ensures smooth movement through the sieve tube elements.
3. Sucrose Provides an Efficient Energy Source
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Sucrose contains two sugar monomers (glucose + fructose) linked together, making it a compact energy store.
- Once it reaches a sink, sucrose can be easily broken down into glucose and fructose for respiration or stored as starch.
Phloem Structure Components
1. Sieve Tube Elements
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Main conducting cells of the phloem, arranged in long tubes.
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Lack a nucleus and contain few organelles, allowing efficient transport of sucrose.
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They contain very little cytoplasm (and this is pushed to the edges of the sieve tube elements) allowing efficient transport of sucrose.
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Connected end-to-end with sieve plates (perforated end cell walls that allow movement of sucrose between cells in the phloem).
2. Companion Cells
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Found next to sieve tube elements and contain a nucleus, mitochondria, and ribosomes.
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Actively transport sucrose into sieve tubes, using ATP.
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Connected to sieve tube elements via plasmodesmata (small cytoplasmic channels for communication and transport).
Key Terms
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Phloem: Living vascular tissue that transports sucrose and amino acids.
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Translocation: Movement of organic substances (e.g., sucrose) from sources to sinks.
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Sieve Tube Elements: Main phloem cells with few organelles, allowing transport.
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Companion Cells: Support sieve tube elements and use ATP for active transport.
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Sieve Plates: Perforated end walls between sieve tubes, allowing transport.
No answer provided.
Suggest and explain one way in which sieve cells are adapted for mass transport. (2 marks)
1. No / few organelles / very little cytoplasm / cytoplasm at edge / more room / hollow
2. So easier for sucrose to be transported / more flow of sucrose
Or
1. Sieve plates/ perforated end cell walls
2. To allow for sucrose to flow / move from cell to cell more easily
Suggest and explain one way in which companion cells are adapted for the transport of sugars between cells. (2 marks)
1. Mitochondria release energy / ATP
2. For active transport / uptake against concentration gradient
Or
3. Ribosomes / rough endoplasmic reticulum produce proteins
4. Proteins linked to transport e.g. carrier proteins / enzymes
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!