Homeostasis in plants
Homeostasis in plants: stomata
- Stomata are pores in a leaf.
- The plant opens and closes them to balance two needs.
- A drought hormone can shut them to save water.
Practice
A plant opens and closes its stomata to balance:
Open stomata let CO₂ in for photosynthesis but also let water escape, so the plant must balance the two.
A balancing act
- Open stomata let in carbon dioxide for photosynthesis.
- But open stomata also lose water by transpiration.
- So stomata usually open by day and close at night, on a daily rhythm.
Practice
Guard cells open a stoma by:
Taking in ions lowers their water potential; water enters, the guard cells swell turgid and bend apart.
How guard cells work
- Each stoma is opened and closed by two guard cells:
- to open: guard cells pump in ions → water potential falls → water enters by osmosis → they swell turgid and bend apart, opening the pore.
- to close: ions leave → water follows out → guard cells go floppy → the pore shuts.
Practice
When a plant is short of water, abscisic acid (ABA):
ABA is released under water stress and triggers stomatal closure to conserve water.
Practice
Inside the guard cells, the second messenger in ABA signalling is:
Calcium ions act as the second messenger that brings about stomatal closure in response to ABA.
Water stress and ABA
- When the plant is short of water (water stress), it releases the hormone abscisic acid (ABA).
- ABA makes the guard cells lose ions and water, so the stomata close and save water.
- Inside the guard cells, calcium ions act as a second messenger.
You've got it
Key idea
- stomata balance CO₂ in (photosynthesis) vs water loss (transpiration)
- guard cells open the pore by taking in ions + water (turgid); close it by losing them (floppy)
- under water stress, ABA makes stomata close to save water
- calcium ions act as the second messenger in the guard cells