Xylem and phloem

Structure and functions in living organisms · The need for transport and transport in plants · note 2 of 5

Spec 2.53, 2.54Download all The need for transport and transport in plants notes (PDF)Download all notes (PDF)9 pages

Xylem and phloemSpec 2.53, 2.54

Xylem and phloem are the two transport tissues in a flowering plant. They run together through the roots, stem and leaves.

Xylem

The xylem carries water and mineral ions from the roots to the other parts of the plant, such as the stem and leaves. It is made of dead cells joined end to end.

The cells are dead with no cytoplasm, and the end walls between them have broken down. This makes a continuous hollow tube that water can flow through easily.
The walls are strengthened with lignin. This is strong and waterproof, so it stops the tubes collapsing and helps to support the plant.

Phloem

The phloem carries sucrose and amino acids between the leaves and other parts of the plant. Sucrose is made from the glucose produced in photosynthesis. Phloem is made of living cells, and the substances can move up or down the plant, depending on where they are needed.

Sucrose and amino acids travel from the leaves to growing tips, roots, flowers, fruits and storage organs.
The phloem tubes (sieve tubes) are made of living cells joined end to end. Their end walls have small holes (sieve plates), so the solution can flow from one cell to the next.

Xylem and phloem compared
	Xylem	Phloem
Substances carried	Water and mineral ions	Sucrose and amino acids
Direction	Roots to the rest of the plant (upwards)	Between the leaves and other parts, up or down
Cells	Dead, hollow tubes	Living cells
Cell walls	Strengthened with lignin	Sieve plates in end walls

A stem cross-section with a ring of vascular bundles, phloem on the outer side and xylem on the inner side; a xylem vessel of dead cells forming a hollow tube with no end walls and lignin in the walls, with water and mineral ions moving up from the roots; and a phloem sieve tube of living cells with sieve plates, with sucrose and amino acids moving up and down. — **Xylem**: dead, hollow tubes strengthened with lignin. **Phloem**: living sieve tubes with sieve plates.

A plant with blue xylem arrows carrying water and mineral ions up from the roots through the stem to the leaves, and amber phloem arrows carrying sucrose and amino acids from the leaves, where sucrose is made by photosynthesis, up to the growing shoot tip and down to a fruit, the roots and a storage organ. — Xylem carries water and mineral ions up the plant. Phloem carries sucrose and amino acids up or down to where they are needed.

Common mistake:

Do not mix the two up. Xylem = water and mineral ions, dead cells. Phloem = sucrose and amino acids, living cells.