8.1.2Transport in Plants

Root Hair Cells

5 min read 9 parts Lesson 3 of 8 1 quiz

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Section 1 of 9Role of roots

Xylem vessels — longitudinal & transverse

Labelled root hair cell showing organelles

Root hair cell and palisade mesophyll cell

Roots anchor the plant in the soil and absorb water and mineral ions, such as nitrate and magnesium, from it. Absorption takes place mainly near the root tip, in the root hair cells. These are specialised epidermal cells on the surface of young roots, each with a long, thin extension that grows out into the soil between soil particles.

Section 2 of 9Structure and adaptations

Key Structures of the Respiratory System

Feature	Adaptation to function
Long, thin hair-like projection	Greatly increases the surface area for absorbing water and ions, and can reach between soil particles
Thin cell wall and surface layer	Short distance for water and ions to enter the cell
Large permanent vacuole containing sugars and salts	Keeps the cell sap concentrated so that water potential is lower than the soil water, which helps water enter by osmosis
Many mitochondria	Release energy by aerobic respiration for active transport of mineral ions
No chloroplasts	Underground, so no light for photosynthesis

Section 3 of 9Uptake of water by osmosis

Water Uptake by Osmosis — Soil to Xylem (v15)

Osmosis — Partially Permeable Membrane Simulation

Osmosis across a partially permeable membrane

Osmosis across a cell membrane (sucrose can't cross)

Soil water is a dilute solution of mineral ions, so it has a higher water potential (a higher concentration of water molecules) than the cytoplasm and vacuole of the root hair cell, which contain dissolved sugars, ions and proteins.

Osmosis

Water moves from the soil into the root hair cell by osmosis: the net movement of water molecules from a region of higher water concentration to a region of lower water concentration, through a partially permeable membrane.

The water then passes from cell to cell across the root cortex into the xylem, driven by the same difference in concentration from cell to cell,

and is carried up the plant.

Section 4 of 9Uptake of mineral ions by active transport

Root structure: whole root, cross-section and root hair cells in soil

Mineral Ions — Full 2D Simulation (Mg²⁺ & NO₃⁻)

Active Transport Across the Cell Membrane

Active transport: movement against concentration gradient (ATP)

The concentration of mineral ions such as nitrate is usually higher inside the root hair cell than in the soil water.

Diffusion would therefore move ions out of the cell, so the plant must take them in against the concentration gradient.

Active transport

This needs active transport: carrier proteins in the cell membrane use energy from respiration (ATP) to move ions from low to high concentration.

This is why root hair cells have many mitochondria, and why waterlogged soil,

which lacks oxygen and stops aerobic respiration, reduces mineral uptake and can damage the plant.

Section 5 of 9Comparing transport processes

	Diffusion	Osmosis	Active transport
What moves	Any particles	Water only	Ions and molecules
Direction	High to low concentration	High to low water concentration	Low to high concentration
Energy needed	No	No	Yes, from respiration
Membrane protein	Not always	Not needed	Carrier protein

Note that carrier proteins in active transport are specific to the particles carried. Root hair cells also take up some ions by facilitated diffusion where the concentration gradient is favourable.

Section 6 of 9Link to other topics

Surface Area : Volume — 3D Step-by-Step Simulation

The surface area to volume idea is the same as in lungs and villi: a larger surface area gives a faster exchange.

The water absorbed enters the xylem and is moved up the plant by transpiration,

and minerals taken up are used to make amino acids, nucleotides and chlorophyll.

Roots near the soil surface are often branched, increasing the area for absorption.

Understanding root hair adaptations lets you predict the effect of drought, flooding and fertiliser on plant growth.

Salty soil can reverse the water potential gradient so water leaves the plant, which is why crops wilt in over-fertilised or saline soil.

A practical way to see the effect is to compare plants grown in different conditions.

If seedlings are grown in a solution lacking nitrate, they are stunted and have yellow older leaves, showing that the ions must be taken in for protein synthesis.

Treating roots with a respiratory poison, or bubbling no air through a nutrient solution, reduces uptake of ions but not much of the water,

which shows that ions need respiration while water uptake by osmosis does not.
- (active transport)

In a question about a root hair cell you might also be asked to compare it with a cell in the upper epidermis of a leaf: the leaf cells are flat, transparent and lack hairs,

because their job is protection and letting light through.

Root hairs are short-lived,

so new ones form as the root grows,
which is why transplanting a seedling carefully, to avoid damaging them, helps it to establish quickly.

Section 7 of 9Exam-style questions

Explain how a root hair cell is adapted for the uptake of water. [3]3 marks
Explain why root hair cells contain many mitochondria. [3]3 marks
Explain why plants growing in waterlogged soil may have poor growth. [3]3 marks

Section 8 of 9Key terms

Term	Meaning
Root hair cell	A specialised root epidermis cell with an extension that absorbs water and ions.
Osmosis	The net movement of water through a partially permeable membrane from high to low water concentration.
Active transport	Movement of substances against a concentration gradient using energy from respiration.
Mineral ions	Ions such as nitrate and magnesium needed for plant growth.
Vacuole	A fluid-filled sac containing cell sap.
Surface area	The total area of the surface of a cell or structure.
Partially permeable membrane	A membrane that lets some substances pass but not others.

Section 9 of 9Exam technique

Saying that root hair cells absorb minerals by osmosis. Osmosis is only for water.

Saying that active transport is the movement from high to low concentration.

Forgetting that active transport needs energy from respiration, not from the soil.

Writing that roots 'suck' water. Water enters by osmosis down a concentration gradient.

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A ⇔ A = I / M ⇔ I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.