IGCSE Biology / Transport in plants
Root hair cells and water uptake
Edexcel International GCSE Biology (4BI1), chapter 11: Transport in plants, specification point 2.55B. Free lesson.
Read the revision notes · IGCSE Biology past papers
Root hair cells absorb water from the soil and pass it across the root to the xylem. Water enters by osmosis, while mineral ions enter by active transport.
How a root hair cell is adapted
A root hair cell is one cell with a long, thin extension that reaches between soil particles. The extension gives the cell a large surface area for absorbing water. Thousands of root hairs together provide an enormous area. Just inside the cell wall is a partially permeable membrane, which allows water to pass through. The cell contents contain dissolved sugars and mineral ions, making them more concentrated than the soil water. The vacuole and cytoplasm contain dissolved substances. Mitochondria release energy for the active transport of mineral ions.
How water enters by osmosis
Soil water is usually dilute, meaning it contains a lot of water and few dissolved substances. The root hair cell contents are more concentrated, so their water concentration is lower than the water outside. Water crosses the partially permeable membrane from the soil into the cell by osmosis, moving down the water potential gradient. Osmosis is the movement of water through a partially permeable membrane, from a more dilute solution to a more concentrated solution. Water uptake works only while the soil water is more dilute than the cell contents.
How water reaches the xylem
After entering a root hair cell, water passes from cell to cell across the root. It then enters the xylem, which carries water up to the leaves. If soil is very dry or salty, its water can be more concentrated than the cell contents. The gradient then reverses, so the plant can lose water instead.
How water uptake differs from mineral uptake
Water and mineral ions enter root hair cells by different processes. Water moves down its gradient by osmosis and does not need energy. Mineral ions can be taken up against their gradient by active transport, which uses energy released by respiration. Water uptake stops if the soil water is more concentrated than the cell contents. Mineral ion uptake stops if the root has no oxygen for respiration. In waterlogged soil, respiration stops, so mineral uptake stops, but water uptake can continue.