Xylem
Biology: Cambridge International A Level, topic 7.1.2
Structure
The xylem are plant vessels that are responsible for transporting water and mineral ions. The structure of the xylem is specialised for this role.
Vessel elements
- The xylem vessels are long, tubes of cells that run up the stem of plants.
- The cells of the xylem are called vessel elements.
- The vessel elements in the xylem are dead.
End walls
- The vessel elements are stacked on top one another.
- There are no cell walls at the ends of each vessel element.
- This creates a continuous tube for water to flow through.
Lignin
- The walls of the xylem are lined with a waterproof polymer called lignin.
- Lignin reinforces the walls of the vessel elements to provide structural support.
Cohesion-Tension Theory
Water and inorganic ions travel up the xylem through cohesion and tension. The steps involved in this process are:
1) Transpiration
- Some of the water in the leaves is used in photosynthesis.
- Most of the water in the leaves evaporates in a process called transpiration.
2) Tension
- The loss of water from the leaves creates tension in the xylem.
- Tension is the formation of hydrogen bonds between water molecules and the sides of the xylem vessel elements.
- Water in the xylem is pulled upwards by this tension towards the leaves.
3) Cohesion
- Individual water molecules also form hydrogen bonds with each other. This process is called cohesion.
- When water molecules are pulled up the xylem, other molecules of water are also pulled upwards due to cohesion.
- The combination of cohesion and tension together continuously pull water upwards to replace water that has been lost in the leaves by transpiration.
4) Diffusion in the roots
- When water is pulled up the stem, the water potential at the bottom of the plant decreases.
- Water diffuses into the roots via osmosis down its water potential gradient.
- Water in the roots travels via the apoplast or symplast pathway into the xylem.
- In the apoplastic pathway, water moves through the cell walls of adjacent root cells.
- In the symplastic pathway, water moves through the cytoplasm & vacuoles of adjacent root cells via the plasmodesmata.
The Casparian strip
- Near the root xylem vessels, there is an impermeable strip in the cell walls of root cells.
- This is called the Capsarian strip.
- The Casparian strip blocks the apoplast pathway as water can't pass through the strip.
- This means water must pass through the cytoplasm to continue into the xylem.
Test yourself on Xylem
Which of the following statements about the xylem structure are true?
Which of these is NOT a structural feature of xylem vessels?
Steps Involved in Water Transport in the Xylem
Where are the hydrogen bonds formed in cohesion?