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B2 V) Stomata & Guard Cells
B2 V) Stomata & Guard Cells
Stomata are tiny pores/ openings/ holes on the bottom of leaves. Stomata facilitate gas exchange in leaves; carbon dioxide enters the leaves through the stomata where it is used for photosynthesis, and oxygen (the waste gas of photosynthesis) exits the leaves through the stomata. Stoma is the singular of stomata (plural). Each of the stomata are surrounded by two guard cells. There are two diagrams below of the stomata and the surrounding guard cells; the first diagram is the side view of a leaf, and the second diagram is the underside of a leaf.
The stomata can be open or closed. When the stomata are open, carbon dioxide enters the leaves, and oxygen & water vapour exit the leaves. The stomata are usually found on the bottom of leaves rather than on the top of leaves. This is because the bottom of leaves are cooler compared to the top of leaves as the bottom of leaves are exposed to less sunlight. A cooler temperature means that the water molecules have less kinetic energy, which means that the diffusion of water out of leaves (transpiration) happens at a lower rate on the bottom of leaves compared to the top of leaves. So, the stomata being on the bottom of leaves reduces the amount of water loss/ transpiration.
Turgid or Flaccid
All of the stomata are surrounded by two guard cells. The guard cells have thin outer walls and thicker inner walls which allows them to change their shape, thus opening and closing the stomata. The guard cells are shaped like a kidney and they can be flaccid or turgid.
All of the stomata are surrounded by two guard cells. The guard cells have thin outer walls and thicker inner walls which allows them to change their shape, thus opening and closing the stomata. The guard cells are shaped like a kidney and they can be flaccid or turgid.
- The guard cells are turgid when they have lots of water and this leads to the guard cells being plump, which results in the stomata opening. When the stomata are open, lots of carbon dioxide enters the leaf, and lots of oxygen and water vapour exits the leaf.
- The guard cells will be flaccid when the plant is short of water. Flaccid guard cells result in the stomata being closed, which means that less carbon dioxide enters the leaf, and less oxygen and water vapour exits the leaf. This is useful because if the plant is short of water, the stomata being closed reduces water loss through the stomata (it slows the rate of transpiration), which saves the plant water.
Sunlight
The size of the stomata also depends on sunlight because the guard cells are sensitive to light; a greater light intensity increases the size of the stomata. This happens because a greater light intensity results in more photosynthesis reactions taking place in the leaves, which requires a greater quantity of carbon dioxide. Therefore, the guard cells change shape to increase the size of the stomata to allow more carbon dioxide into the leaves. So, when it is light, the stomata are open.
There is no light at night, which means that photosynthesis does not take place at night. As no photosynthesis takes place at night, there is no need for carbon dioxide to enter the leaves through the stomata at night, and therefore no reason for the stomata to be open. The closing of the stomata at night allows the plant to reduce the amount of water lost by transpiration whilst ensuring that the plant does not lose out on photosynthesis.
So, when it is light the stomata are open. And, when it is dark the stomata are closed.
The size of the stomata also depends on sunlight because the guard cells are sensitive to light; a greater light intensity increases the size of the stomata. This happens because a greater light intensity results in more photosynthesis reactions taking place in the leaves, which requires a greater quantity of carbon dioxide. Therefore, the guard cells change shape to increase the size of the stomata to allow more carbon dioxide into the leaves. So, when it is light, the stomata are open.
There is no light at night, which means that photosynthesis does not take place at night. As no photosynthesis takes place at night, there is no need for carbon dioxide to enter the leaves through the stomata at night, and therefore no reason for the stomata to be open. The closing of the stomata at night allows the plant to reduce the amount of water lost by transpiration whilst ensuring that the plant does not lose out on photosynthesis.
So, when it is light the stomata are open. And, when it is dark the stomata are closed.