IGCSE Biology 0610: How Root Hair Cells Are Adapted for Water Uptake

A student digs up a young bean seedling and notices that the region of the root just behind the growing tip is covered in thousands of tiny, thread-like outgrowths, each too small to see clearly without a hand lens. Her teacher explains that these are root hair cells, and that they are responsible for most of the water absorbed by the plant.

(a) Describe two structural features of a root hair cell that adapt it for absorbing water from the soil. [2]

(b) A root of a given length has thousands of small root hair cells covering it, rather than a single continuous absorbing surface of the same total length. Explain why having many small root hairs, rather than one smooth surface, increases the rate at which the root as a whole can take up water. [2]

(c) A pest destroys most of the root hairs on this seedling's roots, leaving the rest of the root surface undamaged. Suggest, with a reason, what effect this would have on the plant's rate of water uptake. [2]

Part (a): Structural features of a root hair cell

A root hair cell has a long, thin, hair-like extension that grows out from the main body of the cell into the tiny gaps between soil particles. This extension greatly increases the area of cell membrane in direct contact with the water film surrounding the soil particles. The cell also has a thin cell wall and cell membrane, which keeps the distance that water must cross to enter the cell as short as possible, allowing water to move in quickly.

Part (b): Why many small root hairs absorb water faster

If a root surface were smooth and unbroken, only the outer layer of cells lying flat against the soil would be in contact with soil water. By contrast, thousands of thin root hairs projecting outward from the same length of root expose a much greater total surface area of cell membrane to the surrounding soil water, since each individual hair adds its own absorbing surface. Water enters the root by crossing this cell membrane, so a larger total surface area in contact with soil water means more water can cross into the root at any one time, giving the whole root a faster overall rate of water uptake than a smooth surface of the same length could achieve.

Part (c): Effect of losing most of the root hairs

The root hairs are what give the root its very large surface area for absorbing water. If a pest destroys most of them, only the smaller area of the remaining root surface is left in contact with the soil water. With far less absorbing surface available, less water is able to cross into the root per unit time, so the rate at which the plant takes up water decreases, even though the rest of the root tissue is undamaged.

Final answers

(a) A long, thin, hair-like extension into the soil (increasing surface area); a thin cell wall and membrane (shortening the diffusion distance); very large numbers of root hair cells.
(b) Many small root hairs give a much greater total surface area in contact with soil water than one smooth surface of the same length, so more water can cross into the root per unit time.
(c) The rate of water uptake decreases, because destroying most of the root hairs removes most of the absorbing surface area, leaving less surface for water to cross into the root.