Homeostasis in Plants (Cambridge (CIE) A Level Biology): Flashcards
Exam code: 9700
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- FrontStomata
What is a stoma (plural stomata)?
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What is a stoma (plural stomata)?
A stoma is a pore in the epidermis of a leaf, bordered by two guard cells, that allows gas exchange and water vapour loss.
Why must the stomatal aperture be regulated?
It is a compromise: stomata must be open enough to allow carbon dioxide uptake for photosynthesis, but closed enough to minimise water loss by transpiration.
When are stomata typically open and closed over a day?
Stomata show a daily rhythm.
They are usually open during the day (for photosynthesis) and closed at night, when there is no photosynthesis.
Stomata open and close following daily .
Stomata open and close following daily rhythms.
True or False?
Closing the stomata reduces water loss without any drawback for the plant.
False.
Closing stomata reduces water loss, but it also reduces carbon dioxide uptake, slowing photosynthesis.
What are guard cells?
Guard cells are pairs of specialised epidermal cells that surround each stoma and control its opening and closing by changing their turgor.
How do guard cells cause a stoma to open?
Potassium ions (K^+^) are actively pumped into the guard cells.
This lowers their water potential, so water enters by osmosis, making the cells turgid and bowing apart to open the stoma.
How do guard cells cause a stoma to close?
K^+^ leaves the guard cells, raising their water potential.
Water leaves by osmosis, the cells become flaccid, and the stoma closes.
Why does the shape of a guard cell change so it bends when turgid?
The inner wall (next to the pore) is thicker and less elastic than the outer wall.
When turgid, the thin outer wall stretches more, so the cell curves, pulling the stoma open.
A stoma opens when its guard cells take in water and become .
A stoma opens when its guard cells take in water and become turgid.
Which ion is actively transported to change the water potential of guard cells?
Potassium ions (K^+^).
Their movement into or out of the guard cells drives water in or out by osmosis.
What is abscisic acid (ABA)?
Abscisic acid (ABA) is a plant hormone that triggers the closure of stomata during times of water stress.
When is abscisic acid produced, and what does it do?
ABA is produced when the plant is under water stress (short of water).
It binds to receptors on the guard cells and triggers the stomata to close, reducing further water loss.
What acts as the second messenger in ABA-triggered stomatal closure?
Calcium ions (Ca^2+^) act as the second messenger inside the guard cells.
Describe how ABA causes a stoma to close.
ABA binds to receptors on the guard cell surface membrane.
This raises calcium ions (Ca^2+^) inside the cell as a second messenger, opening channels so K^+^ leaves.
Water then leaves by osmosis, the guard cells become flaccid, and the stoma closes.
In ABA signalling, ions act as a second messenger inside guard cells.
In ABA signalling, calcium ions act as a second messenger inside guard cells.
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