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A-Level Biology May/June 2024 Q4(a): Describe and explain the roles of lignin and suberin in the transport of water through…
A-Level Biology · Paper 9700/23 · May/June 2024 · Question 4(a) · [4 marks]
Describe and explain the roles of lignin and suberin in the transport of water through the roots and stem of a plant.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
Suberin is found in the Casparian strip within the cell walls of the endodermis in the root. As suberin is impermeable to water, it blocks the apoplast pathway. This forces water to enter the symplast pathway by crossing the cell surface membrane of an endodermal cell, allowing the plant to control the entry of solutes into the xylem.
Lignin is found in the walls of xylem vessels. It is also impermeable to water, which prevents water from leaking out of the xylem as it is transported up the stem. Lignin also provides significant mechanical strength, which prevents the xylem vessels from collapsing inwards under the negative pressure created by the transpiration stream.
How the marks are awarded
- B1 — The answer correctly states that suberin is found in the Casparian strip / endodermal cells.
- B1 — The answer explains that suberin forces water to move from the apoplast pathway to the symplast pathway.
- B1 — The answer correctly states that lignin is found in the walls of xylem vessels.
- B1 — The answer explains that lignin strengthens the xylem walls to prevent their collapse under negative pressure/tension.
Common mistakes
- Confusing the locations and roles of lignin and suberin, for example, stating that lignin is in the Casparian strip.
- Providing a vague explanation for lignin's strength, such as 'it provides support', without linking it specifically to preventing the collapse of xylem under tension/negative pressure.
- Describing the apoplast and symplast pathways without explicitly stating that the Casparian strip (suberin) forces water to switch from one to the other.
- Writing extensively about only one of the substances, which limits the maximum marks available to three, as the question requires a description of both.
Examiner tip: For questions on plant anatomy, always connect the specific molecule or tissue (what) to its precise location (where) and its physiological role (why).
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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