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Movement of water

Water potential describes the tendency for water to move from one area to another. It is often represented by the Greek letter $$\psi$$ (pronounced "psi").

Water will always move from an area of high water potential to an area of low water potential .

Water with high potential is more likely (has more potential) to move than water with low potential.

The water potential of pure water is zero. This is the highest possible water potential. Any solution will have a lower (i.e. negative) water potential than zero.

The higher the concentration of solvents in a solution, the lower its water potential is.

Water enters the roots because the cytoplasm of root hair cells is more concentrated than the water in the surrounding soil. Similarly, water moves from cell A to cell B when: $$$\psi\text{ cell A is greater than } \psi\text{ cell B}$$$

 Water moves between cells in the roots due to a difference in water potential.
Water moves between cells in the roots due to a difference in water potential.

The root hair cell is a specialised cell in plant roots that absorbs water and mineral ions from the surrounding soil.

The root hair cell is well-adapted for water and ion uptake:

  • Finger-like projection increases surface area of the root hair cell, providing more space for absorption.
  • Lower water potential than the soil, so the cell draws in water from the soil.
  • Active transport uses energy to move ions into the cell. This helps to maintain a low water potential in addition to providing useful minerals for the plant.

Like all root structures, the root hair cell contains no chloroplasts.

Transpiration is the loss of water from the leaves of a plant into the atmosphere via evaporation.

Transpiration accounts for the majority of water loss by plants. Water use in photosynthesis is minimal in comparison.

The movement of water during transpiration involves multiple processes:

  • Water travels between mesophyll cells by osmosis.
  • Water evaporates (liquid to gas) into air spaces of the leaf.
  • Water diffuses through stomata into the outside air.

Transpiration is a result of gas exchange between leaves and the outside air.

Stomata open to allow gases, including water vapour resulting from evaporation, in and out of the leaf. If leaves did not have stomata, transpiration would be minimal.

The loss of water from the leaves drives the uptake of water by the roots. This creates a tension in the xylem vessels that helps the stem to stay upright.

Transpiration provides plants with a mechanism for transporting dissolved minerals.
Transpiration provides plants with a mechanism for transporting dissolved minerals.

Environmental factors can affect the transpiration rate and the uptake of water by roots.

Environmental factor Effect of increasing factor on transpiration rate
Temperature Increases. High temperatures increase evaporation.
Humidity Decreases. Reduces water vapour diffusion through stomata.
Wind Increases. Wind removes the humid air around stomata, increasing water vapour diffusion.
Light Increases. Stomata open when light intensity is high, increasing water vapour diffusion.

If the rate of transpiration is not matched by the rate of water uptake, plants wilt. Cells become flaccid and the plant has reduced structural stability.

Wilting in its early stage can be reversed if the plant is given sufficient water.

The petals of a wilting plant droop down.
The petals of a wilting plant droop down.

A number of passive processes (not requiring energy from the plant) drive water movement up xylem vessels.

  1. Root pressure originates from the movement of water from the roots into the xylem. This force pushes water up xylem vessels from below.
  2. Water moves into the xylem because the water potential in the roots is higher than in the xylem vessels.

  3. Within the xylem vessels, water is dragged upwards by capillary action. Capillary action is possible because xylem vessels are very narrow.
  4. Transpiration pull drives water movement from above. Water lost through transpiration is constantly replaced by water from the xylem vessels. Transpiration pull is the most important mechanism for water transport in plants.
  5. Transpiration pull is similar to sucking water through a straw. As water is lost at the top of the straw, it is drawn up from the bottom.

 Three mechanisms contribute to movement of water through xylem vessels
Three mechanisms contribute to movement of water through xylem vessels