Movement of water through the xylem.

1. Movement of water through the xylem. Objective... Be able to explain how root pressure, cohesion tension theory and transpiration contribute to the movement of water through the xylem.

2. Structure of the xylem What are the adaptations of the xylem for the transport of water? • Cell walls of adjoining cells are lost to form a continuous column. • Cells are dead at maturity to give the column an empty lumen. • Cell walls are strengthened by lignin to prevent collapse.

3. There are 3 parts to the movement of water through the xylem.... • At the roots-root pressure • In the xylem-cohesion tension theory • At the leaves-transpiration

5. Root Pressure. Casparian strip What is it? What does it do? Causes water to move through the endodermal cells into the xylem via the symplast pathway. Great! Symplast pathway = through the cytoplasm. Mineral ions found in the cytoplasm are pulled into the base of the xylem by the water.

6. Root Pressure So, we have an increase in the concentration of mineral ions in the base of the xylem. What will this do to the water potential? What affect will this have? Water floods into the xylem. This increases the pressure of the water at the base of the xylem.

7. Cohesion Tension Theory Water molecules form hydrogen bonds both with each other and with the sides of the xylem. This forms a continuous column of water.

8. Issues with cohesion-tension? What if the column of water is broken?

9. Transpiration Water is lost from the leaves though stomata. Stomata = pores on the underside of a leaf. This loss of water pulls the column of water up through the xylem.

10. There are 3 parts to the movement of water through the xylem.... • At the roots-root pressure • In the xylem-cohesion tension theory • At the leaves-transpiration

11. Notice negative pressures. Water in xylem under tension therefore lower than atmospheric pressure.