Relate vascular cell structure to transport
The structure of xylem vessels, sieve tube elements and companion cells supports their distinct roles in plant transport.
AS Level · Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; identifying tissues from real slides, drawing biological sections, supervised practical competence and human subject-teacher review remain necessary.
Before you begin
- Distinguish plant cell walls from cell membranes and recognise that vascular tissues form pathways through roots, stems and leaves.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why do lignified xylem walls help water transport?
What you will learn
- Relate xylem vessel-element features to water transport and support
- Relate sieve tube and companion cell features to phloem transport
Xylem combines a pathway with mechanical support
A hollow vessel lumen provides a continuous route for water and dissolved mineral ions. Lignified walls strengthen the vessel and help resist collapse under tension as water is pulled through the plant. The vessel element is dead at maturity, so there is no cytoplasm obstructing its lumen.
Lignin supports the wall; it is not the liquid being transported. A thick wall and an open lumen contribute different functions, so do not use 'strong' as a substitute for explaining the route or the solution carried.
Phloem uses living conducting and support cells
Sieve tube elements form the conducting pathway for assimilates. Their reduced contents and perforated sieve plates allow solution to pass between elements. Companion cells contain a nucleus and many mitochondria and support loading and maintenance of the sieve tube elements.
The companion cell does not replace the sieve tube as the conducting tube. Structure supports the process, but transport direction depends on sources and sinks and can vary across the plant.
Pause and explain
Which feature most directly supports transfer of assimilates into a sieve tube element?
Worked example
Explain how two features of a mature xylem vessel support its function.
Show the worked solution
- The hollow lumen provides space for solution flow.
- Lignified walls strengthen the vessel and resist collapse.
Answer The hollow lumen provides a continuous pathway for water and dissolved mineral ions. Lignified walls strengthen the vessel and help it resist collapse under the tension created during transpiration.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Relate the lumen and lignified walls of a xylem vessel element to its roles.
Give me a hint
Give one transport link and one support link.
Compare my reasoning
- The lumen provides a continuous space for the transported solution.
- Lignin strengthens the wall against collapse under tension.
The hollow lumen provides a continuous route for water and dissolved mineral ions. Lignified walls strengthen the vessel and help prevent collapse while water is under tension.
Look for these in your work
- Lumen linked to transport
- Lignin linked to strength
Explain how sieve tube elements and companion cells work as an associated pair.
Give me a hint
Keep conducting and support roles separate.
Compare my reasoning
- Sieve tube elements conduct assimilates through phloem.
- Companion cells support loading and the living sieve-tube system.
Sieve tube elements provide the phloem conducting pathway for assimilates. Adjacent companion cells support transfer of assimilates into the sieve tubes and help maintain their activity.
Look for these in your work
- Sieve tube role stated
- Companion cell role stated
- Roles not conflated
A student says xylem and phloem must both carry water in the same way because they are vascular tissues. Correct the explanation.
Give me a hint
Distinguish the main contents and structures.
Compare my reasoning
- Xylem carries water and dissolved mineral ions through hollow vessels.
- Phloem carries assimilates in sieve tubes, with companion cells supporting loading.
Xylem vessels carry water and dissolved mineral ions through a hollow, lignified pathway. Phloem sieve tubes carry assimilates such as sucrose and amino acids, while companion cells support loading. The tissues have different structures and functions.
Look for these in your work
- Xylem contents and structure
- Phloem contents and associated cells
- Difference stated
Common mistakes
- Saying lignin transports water through the vessel wall
- Calling a companion cell the phloem vessel
- Assuming phloem transport is always downward
What can you explain now?
Why do lignified xylem walls help water transport?
Compare with the explanation
They strengthen the vessel and help it resist collapse
This supports a continuous lumen under tension.
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Make it stick
How do the structures of xylem vessels and phloem sieve tubes with companion cells support different transport roles?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.