7.1.37 · Transport in Plants
Xylem Vessels Elements — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 9 parts Lesson 5 of 15 Unit quiz
Lesson summary
7.1.3 Xylem Vessel Elements — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the structure of xylem vessel elements and explain how each feature adapts them for transport of water and for support.
- Functions of xylem
- Xylem parenchyma
- Sclerenchyma
- Dead tissue
- Young vessels
- Older vessels
Part 1 of 7Types of xylem cell
Types of xylem cell
Functions of xylem
- Functions of xylem transport of dissolved minerals and water, structural support and food storage.
Vessel elements
- Vessel elements large cells with thickened walls and no end plates when mature, the main water-conducting cells in flowering plants.
Tracheids
- Tracheids long, narrow, tapered cells with pits, found in all vascular plants.
Xylem parenchyma
- Xylem parenchyma living cells that store food.
Sclerenchyma
- Sclerenchyma fibres and sclereids that give extra support.
Part 2 of 7How a vessel forms
1
A vessel element begins as a living cell from the cambium.
2
As it differentiates, lignin is laid down in the cell wall.
3
The cytoplasm and nucleus break down.
4
End walls between cells above one another dissolve, leaving a long, continuous, hollow xylem vessel.
- Dead tissue
- a mature vessel is dead, so water moves passively and not by living cells using energy.
Part 3 of 7Structure and function
| Feature | Function |
|---|---|
| Lignified walls | Lignin is strong, rigid and waterproof; it prevents collapse under the low pressure (tension) of the transpiration stream and supports the plant |
| No cytoplasm, nucleus or organelles | The lumen is empty, so there is no resistance to water flow |
| No end walls | A continuous, unbroken column of water allows rapid transport without impeding cohesive and adhesive forces |
| Narrow lumen | Strong adhesion of water to walls, with capillary action helping to maintain the water column and preventing it breaking |
| Pits (unlignified gaps in the wall) | Water moves sideways between vessels and into adjacent living cells, bypassing an air-blocked vessel |
| Cellulose in walls between lignin | Water adheres through hydrogen bonding with hydroxyl groups |
Part 4 of 7Patterns of lignification
Young vessels
- Young vessels lignin is in rings (annular) or spirals, which lets the vessel keep elongating as surrounding tissue grows.
- (spiral thickening)
Older vessels
- Older vessels lignin forms net-like (reticulate) patterns or fully lignified walls interrupted only by pits, giving maximum strength.
Pits
- Pits they are not holes but thin regions of the primary wall covered by a pit membrane, which lets water through but traps air bubbles and bacteria.
Part 5 of 7Observing xylem
1
Place a cut stem of celery or a white flower in water containing a dye such as eosin or methylene blue.
2
Leave it for some hours.
3
Cut a transverse section with a sharp blade and mount it in water.
4
Examine it under the microscope, where stained vessels show the routes taken by water.
Longitudinal section
- Longitudinal section it shows the lignin patterns.
Stains
- Stains safranin or phloroglucinol stain lignified walls red.
Drawing
- Drawing xylem vessels can be drawn from micrographs using biological drawing conventions.
Part 6 of 7Cohesion, adhesion and tension
Cohesion
- Cohesion hydrogen bonds between water molecules pull them up together as a continuous column.
Adhesion
- Adhesion attraction between water and the hydrophilic cellulose and lignin walls supports the column and creates capillary action,
- which alone raises water only tens of centimetres.
Driving force
- Driving force evaporation from leaves creates tension in the xylem.
Low pressure
- Low pressure pressure inside the xylem is below atmospheric, so rigid lignified walls stop the vessels collapsing.
Daily shrinking
- Daily shrinking stems may shrink slightly in diameter by day when transpiration and tension are greatest.
Cavitation
- Cavitation an air bubble breaks the column, and that vessel stops conducting, but pits let water detour into adjacent vessels.
Part 7 of 7More from the specification
More from the specification
Vascular Bundle Distribution in Plant Organs
- In vascular plants, xylem and phloem tissues are organized into distinct bundles;
- in roots, vascular tissue forms a central core,
- whereas in stems, bundles form a peripheral ring.
Xylem vs Phloem Structure and Function
- Xylem vessel elements consist of dead, lignified hollow tubes that transport water and mineral ions upward from roots,
- whereas phloem tissue contains living sieve tube elements and companion cells that translocate organic assimilates.
Translocation of Organic Compounds
- Sieve tube elements form continuous columns with perforated end plates to transport photosynthetic compounds bidirectionally between source and sink tissues.
- (sucrose and amino acids)
Phloem
- Tissue carrying sucrose and amino acids.
Assimilates
- Organic substances such as sucrose and amino acids transported in the phloem.
Compound
- A substance of two or more kinds of atoms chemically combined
Distribution
- How a species is spread through an ecosystem.
Glucose
- A simple sugar; the unit that starch, glycogen and cellulose are built from.
Companion cell
- A cell that supplies energy to a sieve tube.
Vascular bundle
- Xylem and phloem in a vein, transporting water and sucrose.
Organic compound
- Compound containing carbon and hydrogen.
Key terms
| Term | Meaning |
|---|---|
| Vessel element | A lignified, dead xylem cell that forms part of a continuous tube for water transport. |
| Lignin | A strong, waterproof polymer deposited in xylem walls. |
| Pit | A thin, unlignified area in a xylem wall allowing lateral movement of water. |
| Lumen | The hollow interior space of a vessel. |
| Tracheid | A narrow xylem cell with tapered, pitted end walls that remain. |
| Adhesion | The attraction between water molecules and the walls of the xylem. |
| Annular thickening | Lignin laid down as separate rings in the wall of a vessel. |
Avoid these mistakes
1
Saying lignin is made of cellulose — it is a different polymer deposited on the cellulose wall.
2
Saying water is actively pumped through xylem — transport is passive and driven by transpiration.
3
Saying pits are holes with no covering — they are thin areas of wall.
4
Describing xylem vessels as living cells.
5
Saying all end walls are dissolved in tracheids — in tracheids the end walls remain and are pitted.
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