Explain how water potential gradients drive the movement of water from the soil into the xylem of a plant root, referring to the apoplast and symplast pathways.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Water moves by osmosis from a region of higher (less negative) water potential to a region of lower (more negative) water potential.
Soil water has a higher (less negative) water potential than the cytoplasm of root hair cells and root cortex cells, so water enters the cells by osmosis.
In the apoplast pathway water travels through cell walls and intercellular spaces without crossing membranes until it reaches the Casparian strip in the endodermis, which blocks further apoplastic flow.
In the symplast pathway water moves through the cytoplasm of connected cells via plasmodesmata; active transport of ions into the xylem lowers the water potential in the xylem, maintaining the gradient that drives water into the xylem.
Soil water has a higher (less negative) water potential than the cytoplasm of root hair cells and root cortex cells, so water enters the cells by osmosis.
In the apoplast pathway water travels through cell walls and intercellular spaces without crossing membranes until it reaches the Casparian strip in the endodermis, which blocks further apoplastic flow.
In the symplast pathway water moves through the cytoplasm of connected cells via plasmodesmata; active transport of ions into the xylem lowers the water potential in the xylem, maintaining the gradient that drives water into the xylem.
Examiner tips
- Use the phrase ‘osmosis from higher to lower water potential’ first. Show the two pathways and mention the Casparian strip. Explain how ion transport lowers xylem water potential. Keep the answer concise and use the exact terminology from the mark scheme.
Common mistakes
- Confusing ‘higher’ with ‘more negative’ water potential. Forgetting to mention the Casparian strip as the apoplastic block. Failing to explain the role of ion transport in the symplast pathway.
Mark scheme (4 marks)
- Water moves by osmosis from a region of higher (less negative) water potential to a region of lower (more negative) water potential.
- Soil water has a higher (less negative) water potential than root hair cell cytoplasm / root cortex cells, so water enters by osmosis.
- In the apoplast pathway, water moves through cell walls and intercellular spaces without crossing membranes, until blocked at the Casparian strip in the endodermis.
- In the symplast pathway, water moves through the cytoplasm and plasmodesmata of connected cells; active transport of ions into the xylem lowers the water potential there, maintaining the gradient that drives water into the xylem.
Key terms in this question
water potential · symplast pathway
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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