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A-Level Biology May/June 2025 Q1(c): In the apoplast pathway, water passes along the cell walls of adjacent plant cells and…
A-Level Biology · Paper 9700/22 · May/June 2025 · Question 1(c) · [3 marks]
In the apoplast pathway, water passes along the cell walls of adjacent plant cells and through the intercellular spaces. This is more efficient than the symplast pathway. Explain the structural features of plant cell walls that make the apoplast pathway an efficient pathway for the transport of water.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The plant cell wall is composed of cellulose microfibrils arranged in a loose, mesh-like network. This arrangement creates numerous microscopic spaces and pores between the fibres.
Furthermore, cellulose is a hydrophilic molecule. This allows water molecules to adhere to the surface of the cellulose fibres through hydrogen bonding.
Together, these features make the cell wall fully permeable to water, providing a continuous pathway with little resistance to water flow, which allows for rapid bulk movement.
How the marks are awarded
- B1 — Stating that the arrangement of cellulose fibres creates spaces or gaps for water to flow through. This is covered by: "arranged in a loose, mesh-like network. This arrangement creates numerous microscopic spaces and pores between the fibres."
- B1 — Identifying that cellulose is hydrophilic, which allows for the adhesion of water. This is covered by: "cellulose is a hydrophilic molecule. This allows water molecules to adhere to the surface of the cellulose fibres..."
- B1 — Explaining that the cell wall is fully permeable or offers a path of low resistance, allowing for continuous or rapid water movement. This is covered by: "...make the cell wall fully permeable to water, providing a continuous pathway with little resistance to water flow..."
Common mistakes
- Confusing the fully permeable cell wall with the partially permeable cell surface membrane, which is a feature of the symplast pathway.
- Stating that water moves 'through' cellulose fibres, rather than between them in the spaces of the mesh.
- Giving a vague answer like 'the cell wall has holes' without mentioning the specific structural component (cellulose microfibrils) and its arrangement.
- Failing to mention the hydrophilic nature of cellulose, which is a key chemical property enabling water adhesion and movement along the fibres.
Examiner tip: When asked to explain efficiency, always link specific structural features (e.g., arrangement of cellulose) directly to their functional advantages (e.g., creating a low-resistance pathway).
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