Root hair cell structure and function

A root hair cell is a specialised epidermal cell found near the tip of a root. Its job is to absorb water and mineral ions from the soil and pass them into the plant.

Each root hair is a single outgrowth from one epidermal cell, not a separate structure, and thousands of them together give the root a huge absorbing surface. Exam questions usually show a labelled root hair cell and ask you to connect each feature either to the uptake of water by osmosis or to the uptake of mineral ions by active transport, so it is important to keep the two processes clearly apart.

Parts and functions

How structure suits function

A root hair cell has two distinct jobs, taking in water and taking in mineral ions, and its features are adapted to both. Water enters passively by osmosis, while mineral ions must be pumped in using energy.

In an exam, link each feature to the correct process: the shape and thin surfaces speed up absorption, the vacuole maintains the osmotic gradient, and the mitochondria and carrier proteins make active transport possible.

• Long, thin projection, a large surface area means more water and ions can be absorbed at once.
• Thin cell wall and membrane, the short diffusion distance lets water and ions cross quickly.
• Cell sap with a low water potential, water constantly moves in by osmosis down the water potential gradient from the soil.
• Many mitochondria, they release the ATP needed for active transport of mineral ions.
• Carrier proteins, they move mineral ions from the dilute soil into the cell against the concentration gradient.

Related processes

Water uptake is osmosis: the cell sap has a lower water potential than the surrounding soil water, so water moves into the root hair cell across its partially permeable membrane. From there it passes from cell to cell across the root cortex, still by osmosis, until it reaches the xylem vessels in the centre of the root.

Mineral ion uptake is active transport: ions such as nitrate and magnesium are usually more concentrated inside the cell than in the soil, so they must be pumped in against the gradient by carrier proteins using ATP. Once inside the plant, water rises through the xylem, pulled up by transpiration from the leaves, and the mineral ions are carried with it.

This links the root hair cell to transport in plants and to the wider water cycle in the plant.

Common labelling errors

Worked question

Question (in the style of Paper 2 Section A): The diagram shows a root hair cell. (a) Name the process by which water enters the cell. [1 mark] (b) Explain how the cell is adapted to absorb water quickly. [3 marks] (c) A root is left in waterlogged soil with very little oxygen.

Explain why the uptake of mineral ions falls. [3 marks]

Model answer: (a) Osmosis. (b) The long hair-like projection gives a large surface area for absorption, the cell wall and membrane are thin so the diffusion distance is short, and the cell sap has a low water potential so water moves in down the water potential gradient.

(c) With little oxygen there is less aerobic respiration in the mitochondria, so less ATP is released; because mineral ions are absorbed by active transport which needs ATP, their uptake falls.

Marking note: part (c) is a common trap, water uptake by osmosis is not affected by a lack of oxygen, but active transport of ions is.

Source:SRC-DSKP-EN

Frequently asked questions