Explain how the structure of the root hair cells is adapted to absorb…

The idea

Root hair cells are epidermal cells on young roots that increase the surface area for absorption. Each has a long, thin extension projecting between soil particles, giving a large surface area over which water and mineral ions can enter. The cell wall is thin and the cytoplasm is close to the surface, so the diffusion pathway is short. Water moves into the cell by osmosis because the cell sap has a lower water potential than the surrounding soil solution; mineral ions are taken up by active transport, which needs energy from respiration, so root hair cells contain many mitochondria. The large surface area and short diffusion distance together raise the rate of absorption. A useful method is to link each structural feature to the process it speeds up: extension gives surface area, thin wall gives short distance, mitochondria give energy for active transport.

What earns the marks

• The long, thin extension of the root hair cell increases the surface area for absorption of water and mineral ions.
• The thin cell wall and short distance from the surface to the cytoplasm reduce the diffusion pathway, increasing the rate of movement.
• Water is absorbed by osmosis down a water potential gradient between the soil solution and the cell sap.
• Mineral ions are absorbed by active transport, which requires energy released by respiration.
• Numerous mitochondria in root hair cells supply the energy needed for active transport of mineral ions.

Common mistakes and how to put them right

• Saying water is absorbed by active transport.

  Put it right by stating water enters by osmosis, while mineral ions are taken up by active transport.

• Ignoring the surface-area benefit of the hair-like extension.

  Put it right by explaining that the extension greatly increases the area available for absorption.

• Claiming mineral ions enter only by diffusion.

  Put it right by noting that active transport is needed to move ions against a concentration gradient, using energy from respiration.

Exam tips

• For each structural feature you name, state the process it speeds up, such as surface area for absorption or short distance for diffusion.
• Distinguish clearly between osmosis for water and active transport for mineral ions in your answer.
• Link mitochondria to the energy needed for active transport rather than to photosynthesis.