Water and Mineral Uptake: Study Notes

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Water and mineral uptake by roots, xylem and phloem

Water Movement in Xylem:

Plants rely on an intricate mechanism to move water from the roots to the leaves, ensuring their survival and supporting vital processes. This movement, known as the ascent of sap, is facilitated by the forces of transpiration pull and the unique properties of water, including cohesion and adhesion.

• • Transpiration and Water Loss:Transpiration is the evaporation of water from aerial parts of the plant, primarily the leaves.
  • Only a small portion of absorbed water is used in metabolic activities, with the majority being lost through transpiration.
  • The Role of Xylem in Water Transport:Xylem vessels act as conduits for water transport, moving water upward from the roots to the leaves.
  • The movement of water is sustained by a combination of transpiration pull, adhesion to the vessel walls, and cohesion between water molecules, forming a continuous column.
  • Cohesion-Tension Theory:The cohesive property of water allows it to form a strong, continuous column within the narrow xylem vessels.
  • Transpiration creates a tension that pulls water upwards, while the adhesive force between water and the xylem walls helps counteract gravity.
  • Upward Water Movement: Overcoming Gravity:Despite gravity’s pull, the narrow diameter of xylem vessels and the cohesive force between water molecules help maintain an uninterrupted water column.
  • This process does not require metabolic energy from the plant but is driven by the physical properties of water and the evaporative power of sunlight.

Mechanism of opening and closing of stomata in plants:

The opening and closing of stomata, microscopic pores on plant leaves, are crucial for regulating gas exchange and controlling water loss. This process is governed by the turgidity of the guard cells, which respond to changes in osmotic potential.

• • Guard Cells and Turgidity:Guard cells surrounding each stomatal pore control its opening and closing based on their turgidity (water content).
  • When guard cells are turgid, the stomata open, allowing gases such as carbon dioxide to enter for photosynthesis; when flaccid, the stomata close to minimize water loss.
  • Osmotic Changes and Water Movement:Changes in the osmotic potential of guard cells influence the movement of water in and out, altering their turgidity.
  • Surrounding mesophyll and epidermal cells exchange water and solutes with guard cells, driving the osmotic adjustments that open or close the stomata.
  • Environmental Control of Stomata:Stomatal movements are influenced by environmental factors such as light, humidity, and carbon dioxide levels.
  • The ability of guard cells to react to these factors helps the plant optimize gas exchange while minimizing excessive water loss during dry conditions.

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