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B2 T) The Rate of Transpiration
B2 T) The Rate of Transpiration
Transpiration is the loss of water from a plant through the stomata by evaporation and diffusion. The rate of transpiration is affected by 4 different factors; light intensity, temperature, air flow and humidity.
Light Intensity
A greater light intensity increases the rate of transpiration. This is because a greater light intensity increases the rate of photosynthesis, which requires more carbon dioxide in leaves. The stomata increase in size/ open to allow more carbon dioxide to enter leaves. The stomata being open means that a greater quantity of water diffuses out of leaves through the stomata. Therefore, a greater light intensity results in a greater rate of transpiration.
Light intensity changes throughout the day. There is no light at night, which means that no photosynthesis takes place. This means that carbon dioxide does not need to enter leaves, so the stomata close. The stomata being closed means that very little water exits the leaves through the stomata. Therefore, the rate of transpiration is low at night. As the sun rises, light intensity increases, which results in more photosynthesis reactions taking place, which means that the leaves will be using lots of carbon dioxide. In order to ensure that enough carbon dioxide is present, the stomata open very wide to allow lots of carbon dioxide to enter the leaves. The stomata being open means that lots of water exits the leaves through the stomata. Therefore, the rate of transpiration is high during the day.
So, a greater light intensity increases the rate of transpiration. The rate of transpiration is high during the day because light intensity is high, and the rate of transpiration is low at night because light intensity is low.
A greater light intensity increases the rate of transpiration. This is because a greater light intensity increases the rate of photosynthesis, which requires more carbon dioxide in leaves. The stomata increase in size/ open to allow more carbon dioxide to enter leaves. The stomata being open means that a greater quantity of water diffuses out of leaves through the stomata. Therefore, a greater light intensity results in a greater rate of transpiration.
Light intensity changes throughout the day. There is no light at night, which means that no photosynthesis takes place. This means that carbon dioxide does not need to enter leaves, so the stomata close. The stomata being closed means that very little water exits the leaves through the stomata. Therefore, the rate of transpiration is low at night. As the sun rises, light intensity increases, which results in more photosynthesis reactions taking place, which means that the leaves will be using lots of carbon dioxide. In order to ensure that enough carbon dioxide is present, the stomata open very wide to allow lots of carbon dioxide to enter the leaves. The stomata being open means that lots of water exits the leaves through the stomata. Therefore, the rate of transpiration is high during the day.
So, a greater light intensity increases the rate of transpiration. The rate of transpiration is high during the day because light intensity is high, and the rate of transpiration is low at night because light intensity is low.
Temperature
Another factor that affects the rate of transpiration is temperature, whereby a greater temperature increases the rate of transpiration. This is because a greater temperature gives the water molecules more kinetic energy. This means that the water molecules move around faster and therefore diffuse out of the leaves through the stomata quicker, thus meaning that the rate of transpiration is high.
When it is cooler, the water molecules have less kinetic energy and therefore move around more slowly. This results in water molecules diffusing out of the leaves through the stomata more slowly, thus meaning that the rate of transpiration is low.
Another factor that affects the rate of transpiration is temperature, whereby a greater temperature increases the rate of transpiration. This is because a greater temperature gives the water molecules more kinetic energy. This means that the water molecules move around faster and therefore diffuse out of the leaves through the stomata quicker, thus meaning that the rate of transpiration is high.
When it is cooler, the water molecules have less kinetic energy and therefore move around more slowly. This results in water molecules diffusing out of the leaves through the stomata more slowly, thus meaning that the rate of transpiration is low.
Air Flow
Transpiration is also affected by air flow (wind) whereby a greater air flow increases the rate of transpiration.
During transpiration, water vapour diffuses through the stomata to the outside of the leaf. If there is no wind (low air flow), the water vapour will stay around the outside of the leaf. This results in there being a very small difference between the concentration of water molecules inside and outside the leaf, which means that the rate of diffusion of water from inside to outside the leaf is low. Therefore, when it is not windy (low air flow), the rate of transpiration is low.
If it is windy (high air flow), the water vapour that comes out of the leaf is blown away, which results in a low concentration of water outside the leaf. This means that there is a large difference in the concentration of water between the high concentration inside the leaf and the very low concentration outside the leaf. This high concentration gradient/ difference for water means that the rate of diffusion out of the leaf is high. Therefore, when it is windy (high air flow), the rate of transpiration is high.
Transpiration is also affected by air flow (wind) whereby a greater air flow increases the rate of transpiration.
During transpiration, water vapour diffuses through the stomata to the outside of the leaf. If there is no wind (low air flow), the water vapour will stay around the outside of the leaf. This results in there being a very small difference between the concentration of water molecules inside and outside the leaf, which means that the rate of diffusion of water from inside to outside the leaf is low. Therefore, when it is not windy (low air flow), the rate of transpiration is low.
If it is windy (high air flow), the water vapour that comes out of the leaf is blown away, which results in a low concentration of water outside the leaf. This means that there is a large difference in the concentration of water between the high concentration inside the leaf and the very low concentration outside the leaf. This high concentration gradient/ difference for water means that the rate of diffusion out of the leaf is high. Therefore, when it is windy (high air flow), the rate of transpiration is high.
Humidity
A final factor that affects the rate of transpiration is humidity. Humidity measures the amount of water vapour in the atmosphere. A high humidity means that there is lots of water vapour in the atmosphere. A greater humidity decreases the rate of transpiration.
If humidity is high, there is a high concentration of water vapour outside the leaf. This means that the concentration of water molecules inside and outside the leaf are very similar. This small concentration gradient means that there is a low rate of diffusion of water from inside to outside the leaf, thus meaning that the rate of transpiration is low.
A final factor that affects the rate of transpiration is humidity. Humidity measures the amount of water vapour in the atmosphere. A high humidity means that there is lots of water vapour in the atmosphere. A greater humidity decreases the rate of transpiration.
If humidity is high, there is a high concentration of water vapour outside the leaf. This means that the concentration of water molecules inside and outside the leaf are very similar. This small concentration gradient means that there is a low rate of diffusion of water from inside to outside the leaf, thus meaning that the rate of transpiration is low.
A low humidity (dry air) means that there will be a greater difference in the concentration of water inside and outside the leaf (a high concentration gradient). This large concentration gradient results in a greater rate of diffusion of water from inside to outside the leaf, thus meaning a greater rate of transpiration.
So, a high humidity results in a low rate of transpiration. And, a low humidity results in a high rate of transpiration.
Summary
Here is a summary of the effects that the different factors have:
Here is a summary of the effects that the different factors have:
- Light intensity – a greater light intensity increases the rate of transpiration
- Temperature – a greater temperature increases the rate of transpiration
- Air flow (wind) – a greater air flow increases the rate of transpiration
- Humidity – a greater humidity decreases the rate of transpiration