By the end of this lesson
- Describe guard-cell control of stomata.
- Explain opening and closing using osmosis and turgor.
- Relate aperture to carbon-dioxide uptake and water loss.
BIG QUESTION
How can two cells act as a controllable valve in the leaf surface?
Ion movement changes guard-cell water potential; osmosis changes turgor and unequal wall properties bend the cells.
Guard cells alter aperture
Each stomaA pore in the leaf surface, opened and closed by two guard cells, through which carbon dioxide diffuses in and oxygen and water vapour diffuse out. is bordered by two guard cells. When guard cells gain solutes, their water potential falls and water enters by osmosis; they become turgid.
Their inner walls are thicker and cellulose fibres constrain expansion, so the cells curve apart and open the pore.
Water loss closes the pore
When solutes leave guard cells, water follows by osmosis and turgor falls. The cells become flaccid and the pore narrows or closes.
Closure reduces both carbon-dioxide entry and water-vapour loss.
Stomata respond to several signals
Light and low internal carbon dioxide often promote opening. Water deficit triggers chemical signalling, including abscisic acid, that favours closure.
Stomata therefore do not simply open in daytime and close at night; species, water status, humidity and carbon dioxide all matter.
Open and closed stomata
Link solute movement to water potential, turgor and pore width.
| Feature | Open stoma | Closed stoma |
|---|---|---|
| Guard-cell solute concentration | Higher. | Lower. |
| Water movement | Net entry by osmosis. | Net exit by osmosis. |
| Guard-cell state | Turgid and curved. | Flaccid/less curved. |
| Gas and water movement | Greater exchange and transpiration. | Restricted exchange and reduced transpiration. |
Language that earns precision
- stoma
- A pore in the epidermis through which gases and water vapour diffuse.
- guard cell
- A specialised epidermal cell controlling stomatal aperture.
- turgid
- Firm from water entry and pressure against the cell wall.
- flaccid
- Lacking turgor after water loss.
- abscisic acid
- A plant hormone involved in responses including stomatal closure during water stress.
Common misconceptions
Claim: Guard cells physically push gas through the pore.
Correction: They alter aperture; gases move by diffusion.
Claim: Stomata always open in light.
Correction: Water stress and other signals can override a light response.
Claim: Closing stomata stops all water loss.
Correction: It reduces stomatal loss, but cuticular water loss and incomplete closure remain.
Match the answer to the command word
State the sequence: solutes → water potential → osmosis → turgor → aperture.
Connect opening to both increased CO₂ uptake and increased water loss.
Do not say guard cells pump gases.
Recall, apply, then justify
Explain how guard cells open a stoma.
Hint
Start with solutes and finish with cell shape.
Suggested marking points
- Solute concentration in guard cells increases/water potential falls.
- Water enters by osmosis.
- Guard cells become turgid.
- Unequal wall properties make them curve apart and open the pore.
Explain why stomata close during water stress.
Hint
Link water conservation to a trade-off.
Suggested marking points
- Guard cells lose water and turgor.
- The pore narrows, reducing water-vapour diffusion.
- Carbon-dioxide uptake and photosynthesis may also decrease.
Explain why a mutation preventing stomatal closure could increase growth in mild conditions but reduce survival during drought.
Planning hint
Contrast carbon gain with water conservation.
What a strong design includes
- Persistently open pores permit high carbon-dioxide entry.
- Photosynthesis and growth may increase when water is abundant.
- Open pores also sustain high transpiration.
- During drought, water loss can exceed uptake.
- Cells lose turgor and xylem transport can fail.
- The same mutation therefore has environment-dependent effects.