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A-Level Biology October/November 2024 Q6(a): The transport of water from the soil solution to the xylem of roots occurs by the apopl…
A-Level Biology · Paper 9700/22 · October/November 2024 · Question 6(a) · [4 marks]
The transport of water from the soil solution to the xylem of roots occurs by the apoplast and symplast pathways. Mineral ions can be transported dissolved in water. Describe the transport of water from the soil solution to the endodermis of roots by the apoplast pathway and explain why this pathway cannot continue at the endodermis.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
In the apoplast pathway, water moves from the soil solution into the root through the continuous network of cell walls and the intercellular spaces between cells, such as those of the cortex. This movement is passive.
This pathway cannot continue at the endodermis because the radial and transverse cell walls of the endodermal cells contain the Casparian strip. The Casparian strip is a band of tissue made of suberin, which is a waxy and waterproof substance. This impermeable layer blocks the apoplast pathway, forcing water to enter the cytoplasm of the endodermal cells (the symplast pathway) to continue its journey to the xylem.
How the marks are awarded
- M1 — The answer correctly identifies the apoplast pathway as movement through the cell walls.
- M1 — The answer includes the detail that water also moves through the intercellular spaces between cells.
- M1 — The explanation correctly identifies the Casparian strip as the structure that blocks the apoplast pathway at the endodermis.
- M1 — The answer provides detail on the Casparian strip, stating it is made of suberin and is waterproof or impermeable.
Common mistakes
- Confusing the apoplast and symplast pathways, for example by describing water crossing cell membranes or moving through the cytoplasm before the endodermis.
- Stating that water moves by osmosis through the cell walls; osmosis requires a partially permeable membrane, whereas the cell wall is fully permeable.
- Giving a vague explanation for the blockage, such as 'the endodermis is waterproof', without specifically naming the Casparian strip or suberin.
- Forgetting to describe the pathway to the endodermis and only explaining why it stops, thus missing the first two available marks.
Examiner tip: For questions with multiple command words like 'describe' and 'explain', structure your answer to clearly address each part of the question in turn.
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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