Water uptake by root hair cellsSpec 2.55B
In short
Root hair cells absorb water from the soil by osmosis and mineral ions by active transport. Soil water is more dilute than the cell sap, so water moves into the cell through the partially permeable membrane. The long, thin root hair gives a large surface area, and many mitochondria release energy for the active transport of mineral ions.
Root hair cells are found near the tips of roots. Each has a long, thin projection (the root hair) that reaches between the soil particles. They absorb water and mineral ions from the soil.
How water is absorbed
- Soil water is a dilute solution, so it has a higher concentration of water molecules than the cell sap inside the root hair cell.
- Water moves into the cell by osmosis: from the dilute solution outside to the more concentrated cell sap, through the partially permeable cell membrane.
- The water then passes from cell to cell across the root into the xylem, which carries it up the plant.
| Adaptation | How it helps |
|---|---|
| Long, thin projection (the root hair) | Large surface area, so more water can be absorbed |
| Thin cell wall and membrane | Short diffusion distance for water |
| Cell sap more concentrated than the soil water | Keeps a concentration gradient so water moves in by osmosis |
| Many mitochondria | Release energy for the active transport of mineral ions into the cell |
Mineral ions, such as nitrate, are usually at a lower concentration in the soil than inside the cell. They are taken in by active transport against the concentration gradient, which needs energy from respiration.
(opens full size in a new tab)For an 'explain' question, link each adaptation to its function: 'the root hair gives a large surface area, so more water can be absorbed'.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026