Transpiration

Definition= The evaporation of water at the surfaces of the mesophyll cells followed by loss of water from aerial plant parts by diffusion down a water potential gradient, through the stomata. Helping to maintain an adequate mineral uptake.

1. Water is trapped in the soil from sources of precipitation providing a high water potential
2. Water moves by osmosis into root hair cells of young roots through the thin, non-waterproof cuticle (root tip itself covered by tough, impermeable layer)
3. Water moves through the root cortex via 3 mechanisms: Apoplast pathway (~90% move through the spaces in cell walls), Symplast pathway (a small quantity moves between cell cytoplasm via plasmodesmata), Vacuolar pathway from an area of high water potential and low pressure to an area of low water potential and high pressure into the xylem vessels
4. Water moves up the xylem
5. Water leaves the xylem via osmosis to the mesophyll cells.
6. Evaporation from mesophyll cells in the leaf into intercellular spaces
7. Water vapour then diffuses from the intercellular spaces out through the stomata into the atmosphere providing a low water potential
8. Transpiration from the leaf

3 processes move water up a plant:

1. Root pressure
2. Capillarity (due to thin tubes)
3. Cohesion-tension between water molecules 

How these mechanisms work:

Evaporation of water from mesophyll cell wall, this lowers water potential. Water moves down the water potential gradient and due to the cohesion tension between water molecules and the adhesion of water molecules to the vessel walls the water can be pulled up under tension,a transpirational pull. This creates a transpiration stream from high to low hydrostatic pressure.

Factors affecting rate:

1. Relative humidity- Higher relative humidity in the air will decrease the rater of water loss. This is because there will be a smaller water vapour potential gradient between the air spaces in the leaf and the air outside.
2. Wind Speed- Air moving outside the leaf will carry away water vapour that has just diffused out of the leaf. This will maintain a high water vapour potential gradient.
3. Temperature- A higher temperature will increase the rate of water loss in 3 ways: 1) Increase the rate of evaporation from the cell surfaces (increasing the water vapour potential in the leaf) 2) Increase the rate of diffusion through the stomata because the water molecules have more kinetic energy 3) Decrease relative water vapour potential in the air, resulting in more rapid diffusion out of the plant
4. Light Intensity- In light, the stomata open to allow gaseous exchange for photosynthesis.
5. Water supply- If there is little water in the soil, then the plant cannot replace the water that is lost. Water loss in plants is reduced when stomata are closed or when the plants shed leaves in winter.
6. Cuticle- A thick waxy cuticle reduces the amount of water loss
7. Amount of leaves- Many leaves increases the area that transpiration can take place.
8. Amount and position of stomata- Many stomata on the top of the leaf increases the rate of transpiration

Common question:

Explain why transpiration is an inevitable consequence of photosynthesis

During the day plants take up a lot of carbon dioxide, which is used in photosynthesis. The plant must also remove a build up of oxygen which is a by-product of photosynthesis. Therefore the stomata must be open constantly during the day. While the stomata are open there is an easy route for water to be lost.

The difference between evaporation and transpiration:

Evaporation:
Occurs when water becomes vapour from a body of water. Requires energy/heat. The water loss from the cell walls of the mesophyll cells.

Transpiration:
Water vapour loss to an external atmosphere. The diffusion through stomata down a water potential gradient.

Potometer:

A simple instrument used for investigating transpiration rate (water loss per unit time). Everything must be water and air tight and therefore the stem must be cut underwater. Record the position of the meniscus at set time intervals. Plot a graph of distance moved over time. To calculate volume of water produced:



Limitations:

• No roots (roots provide resistance in water uptake)
• If not air tight xylem vessels can seal off
• When Stem is cut some of the xylem vessels can be damaged
• Not a true indication of transpiration (water also used in photosynthesis, structure and to dissolve solutes in phloem vessels
• If not water tight, water can leak out