Uptake of Water and Mineral Salts by Plants

The root hair as the site of uptake

Root hairs are thin, tube-like extensions of epidermal cells near the root tip. They greatly increase the surface area of the root in contact with the soil, and their thin walls and close contact with soil water make them the main site of water and mineral uptake.

Water uptake by osmosis

The soil solution around the root hair usually has a higher water potential (more dilute) than the cell sap inside the root hair. Water therefore moves by osmosis from the soil, through the root hair's partially permeable cell membrane, into the cell sap.

This creates a water potential gradient that draws water across the cortex, cell to cell, towards the xylem in the centre of the root.

Mineral ion uptake by active transport

Mineral ions such as nitrate and potassium are often more concentrated inside root hair cells than in the surrounding soil, so they cannot enter by diffusion alone, which only moves substances from high to low concentration. Instead, root hair cells use active transport, which uses energy from respiration (ATP) to move ions against their concentration gradient, from a dilute soil solution into the more concentrated cell.

How it is examined

You may be asked to explain why water moves into a root hair by osmosis but mineral ions require active transport, to describe the structural features of a root hair that suit it for absorption, or to explain why root cells contain many mitochondria to support active transport.

Worked exam-style question

Question. The soil solution around a root hair cell has a water potential of -200 kPa, and the cell sap inside the root hair has a water potential of -700 kPa. The concentration of nitrate ions is 0.5 mmol dm⁻³ in the soil solution and 4.0 mmol dm⁻³ inside the root hair cell.

(a) State the direction water moves between the soil and the root hair cell, and name the process involved. (b) Explain your answer to (a) in terms of water potential.

(c) Explain why nitrate ions cannot enter the root hair cell by diffusion alone. (d) Name the process root hair cells use to absorb nitrate ions, and state the immediate energy source for this process.

Model answer. (a) Water moves from the soil solution into the root hair cell by osmosis. (b) Water moves from a region of higher (less negative) water potential to a region of lower (more negative) water potential across the partially permeable cell membrane; since -200 kPa is higher than -700 kPa, water moves inward.

(c) Diffusion only moves substances from a region of higher concentration to lower concentration, but nitrate is already more concentrated inside the root hair cell (4.0 mmol dm⁻³) than in the soil (0.5 mmol dm⁻³), so further movement inward would be against the concentration gradient. (d) Root hair cells absorb nitrate ions by active transport, using energy from ATP produced by respiration.

Practice question

Try this. A student places root hair cells in a solution containing a respiratory poison that stops ATP production in mitochondria. Predict what happens to the rate of mineral ion uptake by the cells, and explain your reasoning.

Exam tip

Key terms

These terms describe the structure and processes examined in this topic:

• Root hair cell, an epidermal cell near the root tip with a long, thin extension that increases the surface area for absorption.
• Osmosis, the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
• Active transport, the movement of substances against a concentration gradient using energy from respiration.
• Diffusion, the net movement of particles from a region of higher concentration to a region of lower concentration.

Source:SRC-DSKP-EN

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