Xylem and PhloemCambridge IGCSE Biology: Revision notes

Section 1

Functions of Xylem and Phloem

Plants have two types of transport tissue, each carrying different substances:

Tissue	Transports	Direction
Xylem	Water and mineral ions; also provides support	Upwards, from roots to leaves
Phloem	Sucrose and amino acids	Both directions, from sources to sinks

Key termsxylemphloem

Common mistake

Xylem only ever moves substances one way (up); phloem can move substances in either direction depending on where sources and sinks are.

Section 2

Where Are Xylem and Phloem Found?

Xylem and phloem run throughout non-woody dicotyledonous plants, found together in vascular bundles.

In the root, vascular bundles are usually found towards the centre
In the stem, vascular bundles are arranged around the outer region
In the leaf, xylem and phloem run through the veins (midrib and smaller veins)

Key termsvascular bundle

Section 3

Xylem Structure and Function

Xylem vessels have a structure closely related to their function of transporting water:

Thick walls containing lignin, which strengthens the vessel and helps support the plant
No cell contents (the cells are dead) - this leaves a hollow, unobstructed tube
Cells are joined end to end with no cross walls, forming one long continuous tube for water to flow through easily

Key termslignin

Exam tip

Always link each xylem feature to its function: lignin = strength/support, no cross walls = continuous tube for easy flow, no cell contents = unobstructed passage.

Section 4

Root Hair Cells

Root hair cells are specialised cells on the surface of roots, adapted for taking up water and mineral ions from the soil.

Each root hair cell has a long, thin extension that increases surface area
This allows more water and mineral ions to be absorbed from the soil into the plant
Root hair cells connect to the xylem via the root cortex

Key termsroot hair cell

Section 5

The Pathway of Water Through the Plant

Water travels through the plant along a defined pathway:

Root hair cells absorb water from the soil
Water passes across the root cortex cells
Water enters the xylem and moves upwards through the root and stem
Water reaches the mesophyll cells of the leaf, where much of it evaporates

Must Know

Xylem transports water and mineral ions upwards and supports the plant; phloem transports sucrose and amino acids
Xylem and phloem are found together in vascular bundles
Xylem vessel features: thick lignified walls, no cell contents, no cross walls - forming a continuous tube
Root hair cells have a large surface area for absorbing water and mineral ions
Water pathway: root hair cells to root cortex cells to xylem to mesophyll cells

That's the notes covered.

Carry on to the next subtopic.