Transpiration and Stomata

Question 1

What are the four main things transpiration rate is affected by?

Answer 1

1. Light intensity - brighter light = greater transpiration rate. Stomata begin to close as it gets darker. – Photosynthesis can’t happen in the dark, so they don’t need to be open to let carbon dioxide in. When the stomata are closed, very little water can escape.
2. Temperature - warmer = faster transpiration.
   When it’s warm the water particles have more energy to evaporate and diffuse out of the stomata.
3. Airflow - better airflow (e.g. stronger wind) = greater transpiration rate.
   If airflow around a leaf is poor, the water vapour just surrounds the leaf and doesn’t move away. This means there’s a high concentration of water particles outside the leaf as well as inside it, so diffusion doesn’t happen as quickly. If there’s good airflow, the water vapour is swept away, maintaining a low concentration of water in the air outside the leaf. Diffusion then happens quickly, from an area of higher concentration to an area of lower concentration.
4. Humidity - drier the air around a leaf = faster transpiration.
   If the air is humid there’s a lot of water in it already, so there’s not much of a difference between the inside and outside of the leaf. Diffusion happens fastest if there’s a really high concentration in one place and a really low concentration in the other.

Question 2

How are guard cells adapted for gas exchange and controlling water loss within a leaf?

Answer 2

1. They have a kidney shape which opens and closes the stomata in a leaf.
2. When the plant has lots of water the guard cells fill with it and go plump and turgid. This makes the stomata open so gases can be exchanged for photosynthesis.
3. When the plant is short of water, the guard cells lose water and become flaccid, making the stomata close. This helps stop too much water vapour escaping.
4. Thin outer walls and thickened inner walls make the opening and closing work.
5. They’re sensitive to light and close at night to save water without losing out on photosynthesis.
6. You’ll usually find more stomata on the undersides of leaves than on the top. The lower surface area is shaded and cooler - so less water is lost through the stomata than if they were on the upper surface.