In this topic of IBDP Biology, we will learn about the first part of the passage of water through a plant.

• Highly branching system.
• Penetrate deep into the soil.
• Absorb water and anchor the plant.
• When it comes to IBDP Biology, xylem is arranged as a central tetrarch (X shaped) in the root.
• This enables the root to resist forces exerted when penetrating soil.
• Xylem is surrounded by cortex cells – simple packing tissue.
• The outer layer tissue layer is made of epidermal cells – called the piliferous layer.
• At the ends of the roots, this is permeable to water.
• Many cells have fine extensions called root hairs –root hair cells.

• Conducts water with dissolved mineral salts from the roots to all parts of the plant.
• When it comes to IBDP Biology, it supports the plant.
• Mature xylem consists of dead cylindrical cells with their adjoining cross walls are broken down.
• They are called xylem vessels.

Adaptations:

• Narrow lumen:
  • Increases capillarity.
• The cellulose thoroughly impregnated with lignin – it is lignified.
  • Lignin is laid down in rings or spirals to allow for longitudinal expansion.
  • Lignin is impermeable to water
  • When it comes to IBDP Biology, it provides rigidity.
  • In mature vessels it forms an irregular, continuous mesh.
  • It prevents xylem vessels collapsing.
  • Lignin increases adhesion of water molecules.
• Pits are present in the walls, where lignin is not laid down:
  • This allows for sideways movement of water:
    • between adjacent xylem vessels, and
    • from xylem to other tissues
• Long open tubes:
  • Allows water to flow in an uninterrupted continuous column.

• Herbaceous plants are made up of 90% water
• They use it in metabolism:
  • Hydrolysis reactions eg hydrolysis of starch into maltose.
  • All biochemical reactions take place in solution in water.
  • It enables diffusion of solutes.
  • It is a photosynthetic substrate.
• It is also used:
  • To transport solutes in xylem and phloem
  • To maintain turgor pressure
  • To control temperature through evapotranspiration.

• When it comes to IBDP Biology, most water is absorbed close to the root tip (0.5 to 10cm).
• Absorbed across piliferous layer through root hair cells.
• Root hair cells have a large surface area.
• Root hair cells are in intimate contact with the soil particles and surrounding water.

That's the end of Part 1. You can go to part 2~