Water transport in plants

How does water travel across the leaf and then out

  • Water moves out of the xylem and across mesophyll cells by osmosis, from a higher water potential to a lower one.
  • The water then evaporates from the surface of the spongey mesophyll cells into the airspace.
  • The water vapour then diffuses out of the leaf through the stomata, down a water vapour potential.
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How does water move up the xylem?

  • The active transport of ions into the xylem causes water to enter the xylem by osmosis. The force of water entering the xylem creates a pressure and this moves water up the xylem by a few centimetres.
  • Cohesive forces between the water molecules and adhesive forces between the molecules and the wall of the xylem helps to pull water up the column.
  • The evaporation of water through the leaves creates a pulling force through the leaf and the xylem that drags water molecules up the xylem. This is aided by adhesion and cohesion.
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How is the xylem adapted to its function?

  • They have no end walls between their cells and so the transport of water and whatnot is uninterrupted.
  • Their walls are thickened by lignin which supports the vessels and stops them from collapsing inwards.
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How does water move across the root?

  • The symplast pathway --> Water moves through the cytoplasm. The cytoplasm's of adjacent cells are joined through plasmodesmata. Water moves through the symplast pathway by osmosis. The root hair cell has a higher water potential than the cell next along as a result of water diffusing in from the soil. This enables water to diffuse into the next cell (which has a lower water potential), and this continues from cell to cell. As water leaves the root by osmosis the water potential of the cytoplasm falls again, maintaining the steep water potential.
  • The apoplast pathway --> Water moves through the cell wall. The cell walls are really absorbent so water can easily diffuse into them and pass through the spaces between them. As water molecules progress through the cell wall more are pulled along behind them due to the cohesive forces between the molecules. When the pathway reaches the endodermis it comes to the casparian strip which stops the pathway because of the presence of waterproof suberin within the strip. This forces the pathway to join to the symplast pathway and move through the cytoplasm.
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How does water enter root hair cells?

  • Water is drawn into the roots by osmosis.
    • Moving from an area of higher water potential (the soil) to an area of lower water potential (the root hair cell) It moves down the water potential gradient.
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