Stomata (Cambridge (CIE) A Level Biology): Revision Note
Stomata
Plants carry out homeostasis – just like animals they need to maintain a constant internal environment
For example, mesophyll cells in leaves require a constant supply of carbon dioxide for photosynthesis
Stomata (specifically the guard cells) control the diffusion of gases in and out of leaves
This means stomata control the entry of carbon dioxide into leaves
Response of guard cells & stomata table
Environmental stimuli causing stomata to open | Environmental stimuli causing stomata to close |
|---|---|
Increasing light intensity | Darkness |
Low carbon dioxide concentration in the air spaces within the leaf | High carbon dioxide concentrations in the air spaces within the leaf |
| Low humidity |
| High temperature |
| Water stress - when the supply of water from the roots is limited and/or there are high rates of transpiration |
Regulation of stomatal aperture balances the need for carbon dioxide uptake by diffusion with the need to minimise water loss by transpiration
Advantages & disadvantages of stomatal opening & closure table
| Stomata open during the day | Stomata closed during the day |
|---|---|---|
Advantage | Leave gain carbon dioxide for photosynthesis | Water is retained inside the leaf, which is important in times of water stress |
Disadvantage | Leaves lose large amounts of water by transpiration | Supply of carbon dioxide decreases so the rate of photosynthesis decreases |
Examiner Tips and Tricks
A stoma is actually the aperture (hole) between two guard cells, but the term is often used to refer to the whole unit (the two guard cells and the hole between them). Don’t forget – stoma (singular) refers to one of these units, whereas stomata (plural) refers to many!
Opening & Closing of Stomata
Stomata open and close in a daily rhythm
Even when the plant is kept in constant light or constant darkness, the daily rhythm of opening and closing of the stomata continues
Opening of stomata during the day:
Maintains the inward diffusion of carbon dioxide and the outward diffusion of oxygen
Allows the outward diffusion of water vapour in transpiration
Closing of stomata at night when photosynthesis cannot occur:
Reduces the rate of transpiration
Conserves water
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