Explain how osmosis moves water into a plant root hair cell when the cell is surrounded by soil water.

AQA A-Level Biology (7402) — 3.2.3 Transport Across Membranes · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Root hair cells have a large surface area and are in contact with water in the soil. The cell contents of a root hair cell contain dissolved minerals and sugars.

Model answer (5 marks)

Osmosis is the movement of water molecules through a partially permeable membrane.
The soil water has a higher water concentration than the cell contents, because the dissolved minerals and sugars inside the cell lower its water concentration.
Water therefore moves from the region of higher water concentration (soil water) to the region of lower water concentration (inside the cell) by osmosis.

Examiner tips

Use the term ‘osmosis’ and ‘partially permeable membrane’
Explain the concentration gradient caused by solutes inside the cell
Show the direction of water movement from soil to cell

Common mistakes

Confusing solute concentration with water concentration
Failing to mention the partially permeable membrane
Not linking the dissolved minerals/sugars to the lower water concentration inside the cell

Mark scheme (5 marks)

Osmosis is the movement of water molecules
through a partially permeable membrane
The soil water has a higher water concentration than the cell contents (or the cell contents have a lower water concentration than the soil water)
Water moves from a region of higher water concentration (soil water) to a region of lower water concentration (inside the cell)
The dissolved minerals / sugars inside the cell reduce the water concentration of the cell contents (explaining why the gradient exists)

Key terms in this question

osmosis