Transpiration

Transpiration is the evaporation of water at the surfaces of the mesophyll cells, followed by the diffusion of water vapour through stomata to the atmosphere.

Factors affecting transpiration rate

Factor	Effect on rate	Explanation
Increasing temp.	Increases	Water molecules gain more kinetic energy, increasing the rate of evaporation from mesophyll cells. Increases molecular movement, accelerating diffusion out of stomata.
High wind speed	Increases	Moving air blows away the escaped water vapour accumulating just outside the leaf surface. This maintains a steep concentration gradient between the inside and outside of the leaf.
Humid	Decreases	High humidity means the air outside the leaf is already saturated with water vapour, flattening/reducing the concentration gradient, slowing down diffusion.

A potometer is used to measure the rate of water uptake (which is closely equal to the rate of transpiration).
- Function: As the leafy shoot transpires, it draws water from the capillary tube. This moves an air bubble along a scale. By measuring the distance the bubble travels in a set amount of time, you can calculate the rate of transpiration.
- Varying conditions in an experiment:
  - Testing temp.: Place the apparatus under a heat lamp or in rooms of different temps.
  - Testing wind speed: Place a fan at varying distances or speed settings next to a plant.

Large internal surface area: The spongy mesophyll layer contains a vast network of interconnecting air sppaces. This creates a highly maximized internal surface area for water to evaporate into, accelerating the rate of water vapour accumulation.
Size and number of stomata: Leaves with more stomata or wider stomatal openings provide more exit pathways, increasing the rate of diffusion out of the leaf.

Transpiration pull is when water moves upwards in the xylem and this pull draws up a column of water molecules, held together by forces of attraction between water molecules.

Cohesion: Water molecules tend to attract each other, sticking together and moving upwards together.
Adhesion: The water molecules tend to stick to the inside of the xylem vessel so reducing the chance that the columns of water will be broken.
Continuous column: Since water molecules cohere strongly, they form an unbroken, continuous column of water stretching all the way from the roots, up through the xylem in the stem, and into the leaves.

If water loss > water uptake, plants wilt because of lack of turgor in cells in stems and leaves.

The water loss (or transpiration) > water uptake causes a water deficit which then causes wilting.
Water moves out of vacuoles of plant cells. These cells lose their internal fluid pressure and change from being turgid to flaccid.
Without this turgor pressure pushing against the rigid cell walls, the cells can no longer mechanically support the plant’s weight. The leaves and stem droop downward.

Wilting is a survival mechanism.
- When the leaves droop downward, their exposed surface area decreases and the stomata often close, which cuts down further water loss.