Water and Sugar Transport in Plants pt. 5 PDF

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Water and Sugar Transport in Plants pt. 5

The Cohesion–Tension Theory ● The leading hypothesis to explain water movement in vascular plants is the cohesion - tension theory ● This theory involves three steps: 1. Open stomata create water potential gradient between the leaf interior and its surroundings 2. Water evaporates from the walls of the parenchyma cells 3. Menisci form at the air - water interface, creating a negative force pulling water from roots to leaf The Role of Surface Tension in Water Transport ● In cohesion - tension theory: ➔ Negative force or pull (tension) generated at air - water interface is transmitted through water outside of leaf cells - To water in xylem - To water in vascular tissue of roots - And finally to water in soil The Role of Surface Tension in Water Transport ● Transmission of pulling force from leaf surface to root is possible because: - Xylem forms continuous network from roots to leaves - All of the water molecules are held together through hydrogen bonds (cohesion) Creating a Water Potential Gradient ● Water in a column of xylem cells moves by bulk flow: - Mass movement along a pressure gradient ● Tension at each meniscus is small, but there are millions of menisci in the entire plant: - Tension creates steep enough water-potential gradient between leaves and roots to overcome force of gravity and pull water up long distances - Secondary cell walls of tracheids and vessel elements in xylem allow cells to withstand extreme negative pressure without collapsing...