Xylem and Phloem Notes
Cambridge IGCSE Biology: Revision notes
Key facts
- Xylem transports water and mineral ions upwards and supports the plant.
- Phloem transports sucrose and amino acids in both directions, from sources to sinks.
- Xylem and phloem lie together in vascular bundles.
- Xylem vessels have thick lignified walls, no cell contents and no cross walls.
- Water passes from root hair cells to root cortex cells, then into the xylem and on to the mesophyll cells.
Functions
Xylem carries water and mineral ions up; phloem carries sucrose and amino acids to wherever they are needed.
Xylem moves substances only upwards, from roots to leaves, and also gives support. Phloem can move substances in either direction, depending on where the sources and sinks are.
- 1
Roots
absorb water and mineral ions
- 2
Xylem
carries them up to the leaves
- 3
Leaves
make sucrose and amino acids
- 4
Phloem
carries them to wherever they are needed
| Xylem | Phloem | |
|---|---|---|
| Transports | Water and mineral ions; also support | Sucrose and amino acids |
| Direction | Upwards, from roots to leaves | Both directions, from sources to sinks |
Xylem
- Transports:
- Water and mineral ions; also support
- Direction:
- Upwards, from roots to leaves
Phloem
- Transports:
- Sucrose and amino acids
- Direction:
- Both directions, from sources to sinks
Which tissue moves substances in both directions?
Position
They lie together in vascular bundles in every organ, arranged differently in the root, stem and leaf.
In the root the bundles are towards the centre. In a dicot stem they form a ring around the outer region. In the leaf they run in the veins (midrib and smaller veins).
- 1
Root
towards the centre, resists being pulled
- 2
Stem
around the outer region, supports against bending
- 3
Leaf
in the veins, a short route to every cell
| Root | Stem | Leaf | |
|---|---|---|---|
| Position | Towards the centre | Around the outer region | In the veins |
| Why it suits | Helps the root resist being pulled | Gives support against bending | Short route to every cell |
Root
- Position:
- Towards the centre
- Why it suits:
- Helps the root resist being pulled
Stem
- Position:
- Around the outer region
- Why it suits:
- Gives support against bending
Leaf
- Position:
- In the veins
- Why it suits:
- Short route to every cell
Where are the vascular bundles in a dicot stem?
Xylem structure
Xylem vessels are strong, hollow, continuous tubes.
Always link each feature to its function. Lignin in the thick walls gives strength and support. No cell contents leaves an unobstructed passage for water. No cross walls make a continuous tube so water flows easily.
- 1
Thick walls with lignin
strength and support
- 2
No cell contents
unobstructed passage for water
- 3
No cross walls
a continuous tube for easy flow
Lignin in thick walls
- Strength and support
No cell contents
- Unobstructed passage for water
No cross walls
- Continuous tube for easy flow
Why do xylem vessels have no cross walls?
Water pathway
Water enters root hair cells, crosses the root cortex, enters the xylem and travels up to the leaf mesophyll cells.
A root hair cell is long and thin, which increases the surface area for absorbing water and mineral ions. Water then crosses the root cortex cells, enters the xylem and moves upwards through the root and stem to the mesophyll cells in the leaf.
- 1
Root hair cells
Absorb water from the soil
- 2
Root cortex cells
Water passes across them
- 3
Xylem
Water moves upwards through the root and stem
- 4
Mesophyll cells
In the leaf; much of the water evaporates here
Which comes straight after the root cortex cells in the water pathway?
Try an exam question
Describe the features of xylem vessels and explain how each helps the vessel to do its job.
[4 marks]
- [1]Thick walls containing lignin, which give strength and support.
- [1]No cell contents, so water can pass without obstruction.
- [1]No cross walls, forming a continuous tube.
- [1]So water flows easily up the plant.
That's the notes covered.
Carry on to the next subtopic.