Trace water pathways from soil to root xylem
Water reaches the xylem through cell walls and cytoplasm, but the endodermal Casparian strip makes the apoplast route cross a membrane.
Section 7.2 · Transport mechanisms
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
Which route moves water through cell walls without crossing a plasma membrane at each cell?
What you will learn
- Describe apoplast and symplast pathways from soil towards root xylem
- Explain the roles of cellulose, lignin, the endodermis, Casparian strip and suberin
Two routes cross the root
In the apoplast pathway, water moves through cellulose cell walls and spaces between cells without crossing a plasma membrane at each cell. It can pass across cortex tissue towards the endodermis. The apoplast route is blocked at the endodermal Casparian strip, which contains suberin and lignin.
In the symplast pathway, water enters the cytoplasm after crossing a plasma membrane and moves through connected cytoplasm via plasmodesmata. At the endodermis, the Casparian strip forces apoplastic water to cross a membrane and enter the symplast before reaching the xylem.
The barrier changes entry into the stele
The waterproof Casparian strip prevents an unregulated wall-only route into the xylem. Crossing the endodermal membrane allows selective entry of water and dissolved substances into the inner root tissues; water can then reach the xylem.
Do not swap the materials: cellulose is a main component of plant cell walls; suberin and lignin contribute to the Casparian strip. The barrier redirects the apoplast route; it does not stop all water from reaching xylem.
Pause and explain
What happens to the apoplast route at the endodermal Casparian strip?
Worked example
Why does water moving apoplastically through the root cortex have to cross a membrane before entering the xylem?
Show the worked solution
- The Casparian strip blocks the wall-only apoplast path at the endodermis.
- Water must cross an endodermal membrane and enter the symplast.
Answer The suberised and lignified Casparian strip blocks the apoplast route at the endodermis. Water must cross a plasma membrane into the symplast before it can pass into the inner root tissues and xylem.
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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.
Compare apoplast and symplast routes for water moving across root cortex tissue.
Give me a hint
Name the route and the structures through which water moves.
Compare my reasoning
- Apoplast uses cell walls and spaces.
- Symplast uses cytoplasm linked through plasmodesmata after membrane entry.
The apoplast route moves through cellulose cell walls and spaces between cells without repeatedly crossing membranes. The symplast route moves through cytoplasm connected by plasmodesmata after water has crossed a plasma membrane.
Look for these in your work
- Apoplast route correct
- Symplast route correct
- Membrane crossing distinguished
Explain why an apoplast water pathway is interrupted at the endodermis.
Give me a hint
Include both the barrier and the route that must follow.
Compare my reasoning
- A suberin- and lignin-containing Casparian strip blocks wall-only passage.
- Water crosses the endodermal membrane into the symplast before reaching xylem.
The Casparian strip in the endodermal wall contains suberin and lignin and blocks the apoplast route. Water must cross a plasma membrane into the symplast before passing into the stele and xylem.
Look for these in your work
- Barrier identified
- Suberin/lignin included
- Membrane crossing explained
A tracer follows cell walls through the cortex, stops at a band of suberin, and then appears in the xylem. Interpret the observation.
Give me a hint
Use the Casparian-strip location and distinguish redirection from total blockage.
Compare my reasoning
- The wall-following part is apoplast movement.
- The barrier is the Casparian strip; passage requires membrane crossing into symplast.
The tracer initially follows the apoplast through the cortex. The suberised Casparian strip at the endodermis interrupts that wall-only route, so water must cross a membrane into the symplast before reaching the xylem. The strip redirects the pathway rather than preventing all water entry.
Look for these in your work
- Initial route identified
- Endodermis/Casparian strip identified
- Redirection distinguished from total blockage
Common mistakes
- Saying the Casparian strip blocks all water uptake
- Describing symplast water as moving only through cell walls
- Calling cellulose the waterproof material of the Casparian strip
What can you explain now?
Which route moves water through cell walls without crossing a plasma membrane at each cell?
Compare with the explanation
The apoplast pathway
Apoplastic movement uses cell walls and intercellular spaces.
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Make it stick
How do apoplast and symplast routes differ, and what does the Casparian strip require water to do?
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.