Transpiration

Transpiration

Loss of water from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata

What are the 4 factors affecting transpiration?

Wind

Humidity

Light intensity

Temperature

How does wind affect transpiration

High winds increase the rate of transpiration

Good airflow removes water vapour from the air surrounding the leaf which sets up a conceneration gradient between the leaf and the air, increasing water loss

How does humidity affect transpiration

High humidity decreases the rate of transpiration

Humidity is a measure of moisture in the aur, when air is saturated with water vapour, the concentration gradient is weaker so less water is lost

How does light intensity affect transpiration?

High light intensity increases the rate of transpiration

Guard cells are responsive to light intensity, when it’s high they are turgid and the stomata open allowing more water to be lost.

How does temperature affect transpiration?

High temperature increases the rate of transpiration

At higher temperatures, particles have more kinetic energy so transpiration occurs at a faster rate as water molecules evaporate from the mesophyll and diffuse away faster than at lower temperatures

How can we investigate the effect of factors on transpiration?

Using a bubble potometer or a mass potometer

What does a bubble potometer show?

Amount of water taken up by measuring movement of bubble in a capillary tube

What does a mass potometer show?

Weighed to determine mass of water lost during transpiration

Why do plants need water?

• Water with minerals enters roots to replace water lost through leaves

• Nitrates and phosphates are vital for healthy plant growth

• Water is needed for turgidity and support in plants, without water plants wilt

• Water is needed for photosynthesis and translocation

• Loss of water helps cool a plant down

What is the transpiration stream?

Transpiration produces a pull on the water in the xylem vessels by leaves, water molecules are cohesive so water is pulled up through the plant by the transpiration stream

What is the function of the transpiration stream?

• Transporting mineral ions

• Providing water to keep cells turgid by filling vacuoles in order to support the plant and stop it wilting

• Provide water to leaf cells for photosynthesis

• Keeping leaves cool by evaporation

What is the stomata?

A structure that can be opened or closed depending on the plant conditions by guard cells

What is the role of the stomata?

Control gas exchange and water loss

Guard cells at day

Water enters guard cell from surrounding cells by osmosis and the vacuole swells, guard cell swells and becomes turgid, this pulls guard cells apart and the stomata opens

Guard cells at night

Water leaves guard cell to surrounding cells by osmosis and the vacuole shrinks, guard cells are floppy, flaccid, the guard cells collapse together and the stomata closes

Why are guard cells predominantly distributed on the underside of the leaf?

Cooler and shadier, lower light intensity. This leads to less transpiration and therefore less water loss

Marram grass stomata

Curled to trap moisture and increase humidity

Cacti stoamta

Fewer stomata to minimise water loss

Water lilies stomata

Stomata are upwards because it is underwater

Grasses grow vertically stomata

Stomata all over because it is vertical

Heather grown in windy place stomata

Protection of stomata, sunken into plant as wind will blow water vapour out