In this IBDP Biology Blog Post, we will look at the structure of xylem and how it impacts the transpiration stream process, let's go!
Transport Systems: Xylem
Xylem: Carries water and minerals from the roots around the plant
- Long continuous tubes running through the plant, stems and roots
- Walls are thickened with lignin in the form of wood, rings or spirals
- Lignin prevents the walls of the xylem vessels from collapsing
- Adds strength to the woody material of older plants
Structure:
- No plasma membranes so water can move in and out freely
- Lumen is filled with sap as the nuclei of the original cells break down. End walls also break down forming a continuous tube
- Cell wall is impregrated with lignin in the form or helical or ring-shaped cellulose thickenings - resist inward pressures
- Pores in the outer cellulose walls conduct water out of xylem into cell walls of adjacent leaves
How does Water flow through a Plant?
1. Water enters through the roots
2. Root hair cells have hair-like projections growing between the soil particles to absorb water
3. Water goes up the steam till it reaches the leaves through a transpiration stream
Transpiration stream: Flow of water in xylem from roots to leaves to replace the water lost by Transpiration
How does the Transpiration Stream happen?
- Tension
- Adhesion
- Cohesion
How does the Transpiration Stream happen?
1) Main movement of water in the xylem is from the roots to the leaves to replace the water lost by Transpiration
2) Tension is generated in the leaves by Transpiration due to the adhesive property of water to the cell wall
3) Tension causes the water to move up to the leaves
4) When water evaporates from the mesophyll cell walls, more water is drawn from the xylem through cellulose pores in the cell walls to replace it, this generates tension which is transmitted from one water molecule to the next due to the cohesive property of water
5) Tension generated in the leaves is transmitted down the coloumn of water in the xylem to the roots
What if there is maximum Transpiration?
Pressure in xylem is very low
- Side walls are very strong preventing inward collapse
- The walls are strengthened by depositing more cellulose and impregnanting the thickening with lignin
- Thickened cell walls with lignin are much harder and woody
That's all! Good job!
References:
- https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.shalom-education.com%2Fcourses%2Fgcse-biology%2Flessons%2Ftransport-systems%2Ftopic%2Fxylem%2F&psig=AOvVaw0WYi-DA6ZCzzK9ct5JcOCX&ust=1626972488006000&source=images&cd=vfe&ved=0CAsQjRxqFwoTCJjaof7O9PECFQAAAAAdAAAAABAO
Drafted by Venetia (Biology)