2.2.6Scaling Up

Transport of Water and Mineral Ions in Plants — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

5 min read 9 parts Lesson 13 of 16

📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision

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Part 1 of 7Why plants need transport

Plants must obtain water and mineral ions from the soil and carbon dioxide from the air, and they make sugars in the leaves. These substances need to be moved over long distances, so plants have two types of transport tissue grouped in vascular bundles: xylem, which carries water and minerals up from roots, and phloem, which carries sugars to where they are needed.

Part 2 of 7Root hair cells and entry of water

Water and mineral ions enter through root hair cells.

Each root hair is a long thin extension of an epidermal cell, so there is a very large surface area.

Water

Water moves into the root hair by osmosis.

It moves from the soil water (high water concentration) into the cell cytoplasm and vacuole.
The cell has a lower water concentration because of dissolved solutes.

Mineral ions

Mineral ions such as nitrate are taken in by active transport.

They are at a lower concentration in the soil than in the cell.
Active transport uses carrier proteins and energy from respiration.

Root hair cells contain many mitochondria.

Water then passes from cell to cell across the root cortex, again down a water concentration gradient, until it reaches the xylem.

Part 3 of 7Xylem structure

Xylem vessels are formed from dead cells joined end to end.

Xylem structure

The end walls break down, forming a continuous hollow tube.

Lignin

The cell walls are thickened and strengthened with lignin, a rigid, waterproof substance, deposited in rings or spirals.

This has two effects: it allows the tubes to withstand the low pressure of water being pulled up without collapsing and it provides support to the plant.

Xylem has no cytoplasm or nucleus, so water flows easily with little resistance.

Pits in the walls allow water to move sideways between vessels.

Part 4 of 7The movement of water upward

Water evaporates from the surfaces of mesophyll cells in the leaf and diffuses out of the stomata (transpiration).

Transpiration stream

This loss lowers the pressure at the top of the xylem and pulls water upward from the roots, in a continuous stream called the transpiration stream.

Water molecules stick together (cohesion), so a continuous column is pulled up the xylem rather than breaking.

The stream also carries mineral ions to the leaves,

where nitrate is used to make amino acids and magnesium to make chlorophyll,
and it supplies water for photosynthesis, cools the leaf by evaporation and keeps the cells turgid,
so that the plant is supported.

Part 5 of 7Evidence and links

Practical evidencea cut celery stalk placed in water containing a coloured dye shows the dye moving up in the xylem, and a stained cross-section reveals the coloured xylem in a ring or in bundles.

Evidence and links

Dissolved minerals are shortage-sensitive: nitrate deficiency causes stunted growth and yellow leaves,

and magnesium deficiency causes yellowing between the veins.

Transpiration is described in more detail in the next lesson; the effects of the environment on water uptake follow in the lesson after that.

Part 6 of 7Following water from soil to leaf

It helps to follow a single water molecule on its journey.

It begins in the soil water in contact with a root hair.

It enters by osmosis because the cell sap has a lower water concentration.

It then moves along cells of the root cortex, each with a slightly lower water concentration than the last, until it reaches the xylem in the centre of the root.

Once in the xylem it is carried up the stem, pulled by transpiration, in a column held together by cohesion.

In the leaf it leaves the xylem in the veins, enters the mesophyll cells,

and either evaporates from their surface and diffuses out through the stomata,
or is used in photosynthesis,
or helps to keep cells turgid.

This pathway explains why plants wilt when soil is dry or salty: the water concentration in the soil is no higher than in the cells, so water cannot enter by osmosis, and the cells lose their turgor. It also explains why plants given too much fertiliser may be damaged, because the solution outside the root becomes more concentrated than the cell contents.

In a diagram of a root cross-section, xylem usually appears at the centre in an X or star shape, surrounded by phloem, with root hairs on the outside.

Part 7 of 7Exam-style questions

Describe how water moves from the soil into a root hair cell. [2]2 marks
Explain how the structure of xylem vessels is adapted for transporting water. [3]3 marks
Explain why a plant growing in waterlogged soil may show poor mineral uptake. [3]3 marks

Key terms

Term	Meaning
Xylem	Plant tissue that transports water and mineral ions upward from the roots.
Phloem	Plant tissue that transports sugars around the plant.
Lignin	A strong, waterproof substance that strengthens xylem walls.
Root hair cell	A root epidermal cell with an extension for absorbing water and ions.
Transpiration stream	The continuous flow of water from roots to leaves.
Active transport	Movement of ions against a concentration gradient using energy from respiration.
Cohesion	Attraction between water molecules that keeps the water column together.

Avoid these mistakes

Saying water enters roots by active transport; it enters by osmosis.

Saying xylem carries sugars; that is phloem.

Describing xylem cells as living; mature xylem is dead.

Forgetting to link mineral uptake to active transport and respiration.

Saying that the roots "suck" water up; it is pulled up by transpiration from the leaves.

Last updated: 20 October 2026