Transpiration

• Stomata are gaps in the leaf. which allows the exchange of gases and water between the leaf tissue and the surrounding air.
• Surrounding each stoma are two guard cells. These can become turgid or flaccid to control whether the gap in covered to stop exchanges, or open to allow exchanges to take place.
• At the leaf

  Water arrives at the leaf via the xylem vessels, where it then diffuses into the air spaces between the spongy mesophyll.

  Water will then move into the spongy mesophyll cells via osmosis, provided the water potential inside the cell is low enough.
• At the leaf 2

  The spongy mesophyll cells will lose water by evaporation, due to the sun's heat. This lost water will now be back in the air spaces.

  Water can leave the air spaces by diffusion in the air (as it is in gas state), via the stomata, provided that the guard cells are turgid.
• At the root

  The surrounding soil must have a higher water potential than the cytoplasm of the root for water to enter the root hair cell.

  Water will enter via osmosis, using the aquaporin channels in the cell membrane.
• Root structure

  Outside: epidermis - protective layer of cells.
• Root structure

  Inner layer 1: cortex - cells between the root hair cell and the xylem and phloem vessels.
• Root structure

  Centre 1: xylem - transports water throughout the plant
• Root structure

  Centre 2 - phloem - transports nutrients throughout the plant
• In the root

  Water leaves the root via 2 pathways:

  • apoplastic pathway
  • symplastic pathway
  • Both routes take water to the xylem vessel.
• Apoplastic pathway:

  Water moves from the root hair cell to the xylem by passing through the cell walls, but never entering the cytoplasm.
• Symplastic pathway:

  Water moves from the root hair cell to the xylem by passing through the cytoplasm of each cell. Some plant cells have an extra organelle called the plasmodesmata which helps with this.
• In the root 2

  In the cells next to the xylem vessel, there is a structure known as a Casparian strip in the cell wall. This is a waxy structure that does not allow water to continue via the apoplastic pathway, instead forcing the water into the cytoplasm.

  This means that water uptake by the xylem can be controlled by the cell membranes.
• When cells in the stem divide, some differentiate to form immature xylem cells. As the cells mature, changes take place:

  • Living contents (organelles) of the cell are broken down and removed
  • End walls between cells break down to form a continuous vessel.
• Water flows up xylem vessels without obstruction and as a continuous, unbroken column of water due to the hydrogen bonds between water molecules.
• Cohesion-tension Theory 1

  When the water leaves the leaf via transpiration, a water molecule will be lost.

  When this happens, it pulls another water molecule into the leaf, due to the hydrogen bond between them.

  The entire column of water in the xylem will then also be pulled upwards towards the leaf.
• Cohesion-tension Theory 2

  When the column of water is pulled upwards, pressure is placed on the walls of the xylem vessel, so it feels a tension.

  Due to this, the process of water being pulled up the xylem is known as the cohesion-tension theory.

  Because water is only lost from the leaves, this process is one-directional.
• Xylem Vessel Walls

  The walls of xylem vessels are thickened with rings of a substance known as lignin.

  This allows the vessels to withstand the negative pressures in the xylem that cause water to be 'pulled up' the xylem as a continuous, unbroken column of water (cohesion-tension theory).
• Problems with measuring transpiration:

  • Very difficult to do, as we would need to measure the rate of evaporation.
  • Only way to do this would be to collect all water vapour released from leaves, which would be very inaccurate.
• How we actually measure transpiration

  We measure the rate of uptake of water, as this should be almost directly proportional to the water lost via transpiration.
• Potometer Set-Up

  1. A shoot containing leaves is cut underwater.
  2. The potometer is filled with water, ensuring there are no air bubbles within the apparatus.
  3. The shoot is the fitted to the potometer using a rubber bung.
  4. All joints are sealed by a waterproof jelly.
  5. An air bubble is introduced into the capillary tube, via the reservoir.
• Potometer Experiment

  The bubble in the capillary tube will move along as more water is taken up by the plant.

  There is a scale on the potometer, so we can see how far the bubble has moved. This means we can give a numerical value to our rate of uptake of water.