7.1.47 · Transport in Plants
Phloem Sieve Tube Elements — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 9 parts Lesson 6 of 15 Unit quiz
Lesson summary
7.1.4 Phloem Sieve Tube Elements — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the structure of sieve tube elements and companion cells and explain how they are adapted for transport of assimilates.
- Cell types
- Living cells
- Functional unit
- Sap contents
- Why needed
- Seasonal sealing
Part 1 of 7The phloem tissue
The phloem tissue
Function
- Function it transports organic compounds (assimilates), particularly sucrose,
- from source to sink (for example roots), up and down the plant.
- (for example a leaf)
Sap
- Sap the organic compounds are dissolved in water to form phloem sap.
Cell types
- Cell types sieve tube elements, companion cells, phloem parenchyma for storage and fibres for strength.
- (the main conducting cells)
Living cells
- Living cells unlike xylem, the transport cells are alive, though sieve tube elements are very reduced.
Functional unit
- Functional unit each sieve tube element is partnered with at least one companion cell.
Part 2 of 7Sieve tube elements
| Structure | Function |
|---|---|
| Long, thin cells joined end to end into a sieve tube | Forms a continuous tube for assimilates |
| Sieve plates (perforated end walls) with sieve pores | Cytoplasm and sap flow from one element to the next |
| Cellulose cell wall | Strengthens the wall against the hydrostatic pressures that move assimilates |
| No nucleus, ribosomes or tonoplast; most organelles broken down | Less resistance to flow and more room for sap |
| Thin layer of cytoplasm lining the wall, with a few mitochondria and smooth ER | Reduces friction and leaves a wide central channel filled with sap |
- Sap contents
- sucrose, amino acids, hormones and ions.
- (about 10 to 25 per cent)
- sucrose, amino acids, hormones and ions.
Part 3 of 7Companion cells
| Structure | Function |
|---|---|
| Large nucleus, dense cytoplasm and many ribosomes | Provide metabolic support and make proteins for the sieve tube element |
| Many mitochondria | Provide ATP for active loading of sucrose |
| Transport proteins in the plasma membrane | Move assimilates into and out of the sieve tube element, using proton pumps and co-transport |
| Plasmodesmata linking to the sieve tube element | Organic compounds move from companion cell into the sieve tube |
| Wall ingrowths (transfer cells in many plants) | Increase surface area for loading sucrose |
Why needed
- Why needed the sieve tube element cannot make its own proteins or ATP because it has no nucleus.
Role
- Role companion cells load sucrose into the sieve tube and control the metabolism of the associated element.
Part 4 of 7Phloem compared with xylem
| Feature | Xylem vessel | Phloem sieve tube element |
|---|---|---|
| Living or dead | Dead | Living but reduced |
| Walls | Thick, lignified | Thin cellulose |
| End walls | Absent | Sieve plates with pores |
| Contents | Water and mineral ions | Sucrose, amino acids and other solutes |
| Direction | One way (up) | Up and down (source to sink) |
| Associated cell | None needed | Companion cell |
Part 5 of 7Sealing damaged sieve tubes
Problem
- Problem high pressure in a damaged sieve tube could cause loss of all the sap.
Callose
- Callose this polysaccharide is deposited rapidly over the sieve plate, and phloem proteins plug the pores, sealing the tube.
Seasonal sealing
- Seasonal sealing callose also closes the sieve plates for winter and is removed in spring.
Microscopy
- Microscopy look for transverse sieve plates, narrow companion cells alongside and no nuclei in the sieve tubes,
- and aniline blue stains callose to show the plates.
Part 6 of 7Evidence for phloem transport
Evidence for phloem transport
Ringing experiments
- Ringing experiments a strip of bark is removed, leaving the xylem intact;
- the stem swells above the ring and sugar concentration is higher above than below, showing downward movement of sucrose outside the xylem.
Radioactive tracers
- Radioactive tracers a leaf given carbon dioxide with 14C later shows the label in phloem sucrose and in sinks such as roots and fruits.
Aphid stylets
- Aphid stylets a stylet penetrates a single sieve tube;
- when the aphid is cut from it, sap continues to exude under pressure,
- and samples at different distances show different sucrose concentrations.
Pressure
- Pressure exudation under pressure supports pressure-driven flow.
Metabolic poisons and chilling
- Metabolic poisons and chilling they slow translocation,
- which shows active processes are needed at least for loading, though movement in the tube itself is passive mass flow.
Part 7 of 7More from the specification
More from the specification
Distribution of Vascular Bundles in Plant Organs
- In dicotyledonous plant stems, vascular tissue is arranged in a peripheral ring of discrete vascular bundles,
- whereas in mature roots, vascular bundles form a central stelar core with xylem arranged in an X-shape and phloem tissue distributed in clusters between the arms.
Structure and Adaptation of Phloem Sieve Tube Elements
- Mature phloem sieve tube elements are living elongated cells arranged end-to-end to form continuous transport tubes;
- they have perforated end walls called sieve plates, reduced peripheral cytoplasm,
- and lack nuclei and ribosomes to minimise resistance to mass flow of sap.
Companion Cells and Biological Drawing of Plant Tissues
- Companion cells connect to sieve tube elements via numerous plasmodesmata, carrying out metabolic functions and actively loading sucrose;
- when producing a biological tissue plan drawing from a light micrograph image, students must outline tissue boundaries without sketching individual cells.
Tissue
- A group of cells of similar structure that perform the same function.
Section
- Thin slice of a specimen.
Micrograph
- A photograph taken through a microscope.
Distribution
- How a species is spread through an ecosystem.
Glucose
- A simple sugar; the unit that starch, glycogen and cellulose are built from.
Vascular bundle
- Xylem and phloem in a vein, transporting water and sucrose.
Transverse section
- A thin slice cut at right angles to the long axis of an organ.
Vascular tissue
- Specialized tissue that transports water and food through a plant
Key terms
| Term | Meaning |
|---|---|
| Sieve tube element | A living phloem cell with no nucleus, joined end to end to carry assimilates. |
| Sieve plate | A perforated end wall between sieve tube elements. |
| Companion cell | A metabolically active cell linked to a sieve tube element that supports it and loads sucrose. |
| Plasmodesmata | Narrow cytoplasmic channels passing through cell walls, connecting adjacent cells. |
| Callose | A polysaccharide that seals sieve plates after damage. |
| Phloem sap | The solution of sucrose, amino acids and other substances in sieve tubes. |
| Transfer cell | A companion cell with wall ingrowths that increase surface area. |
Avoid these mistakes
1
Saying sieve tube elements have no cytoplasm — they have a thin layer of cytoplasm without a nucleus.
2
Saying companion cells transport sucrose through the plant — they load sucrose into the sieve tube.
3
Stating that phloem transports only downward.
4
Confusing sieve plates with pits in the xylem.
5
Saying phloem sap is mainly water with no sugar.
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