2.5.3Unit 2 — Multicellular Organisms

The Xylem and Phloem — SQA National 5 Biology Revision Notes & Topical Questions

5 min read 5 parts Lesson 22 of 39

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Part 1 of 5The Xylem

Function

Xylem transports water and mineral ions from the roots to the stem and leaves of the plant.

Dead cells

Xylem vessels are made of dead cells with no contents, so each forms an empty hollow tube.

Lignin

The walls contain lignin, a strong material that reinforces the vessels and prevents them from collapsing or breaking.

Water entry

Water enters through root hairs, crosses the root and then moves up the stem through the xylem vessels.

Function

Phloem transports sugars and amino acids up and down the plant, from where they are made to where they are needed.

Living cells

End plates

The end walls of the cells have holes in them, which let dissolved sugars pass from one cell to the next.

Direction

Sugars made in the leaves are carried to growing parts and storage organs, so movement can be in either direction.

Comparing Xylem and Phloem

Substances

Xylem carries water and mineral ions, whereas phloem carries sugars and amino acids dissolved in water.

Cells

Xylem tissue is made of dead cells without contents, but phloem tissue is made of living cells.

Direction

Water in xylem moves upwards from the roots, while the contents of phloem can move up or down.

Support

Lignin in xylem walls gives strength and helps to support the plant as well as carrying water.

Feature	Xylem	Phloem
Transports	Water and mineral ions	Sugars and amino acids
Cell type	Dead, hollow, lignified	Living, with perforated end plates
Direction	Roots to leaves	Up and down the plant

Water and minerals are taken in by root hair cells, which have a large surface area for absorption.

The water then passes into the xylem vessels in the root and moves upwards through the stem.

In the leaf the xylem branches into the veins, delivering water to the photosynthesising cells.

Photosynthesis in the leaf makes sugars, which enter the phloem in the same veins.

The phloem carries the sugars to roots, shoots, fruits and storage organs where they are used or stored.

Xylem Tissue of dead, lignified vessels that carries water and mineral ions upwards.
Phloem Tissue of living cells that carries sugars and amino acids.
Lignin A strong substance in xylem walls that prevents the vessels collapsing.
Mineral ions Dissolved nutrients such as nitrate taken up from the soil with water.
End plate The perforated end wall between phloem cells that lets sugars pass.

Saying that phloem is made of dead cells; only xylem vessels are dead.
Stating that xylem carries sugars; it carries water and mineral ions.
Saying that all transport in a plant is upwards; sugars in phloem move up and down.
Mixing up the roles of lignin (strength in xylem) and the perforated end plates (sugar passage in phloem).

Last updated: 28 October 2026