Gas Exchange in PlantsOxford AQA IGCSE Biology: Revision notes
Section 1
What are stomata and where are they located?
Stomata (singular: stoma) are tiny pores found primarily on the lower epidermis of leaves, though some are present on the upper surface. They allow gas exchange between the plant and the atmosphere. Each stoma is surrounded by two guard cells which control whether the pore opens or closes.
- Stomata are too small to see with the naked eye but visible under a microscope
- The number of stomata varies by plant species and leaf position
- They serve as the main route for carbon dioxide uptake (needed for photosynthesis) and oxygen release
- They also provide a pathway for water vapour to escape during transpiration
Examiners expect you to state that stomata are on the lower epidermis of leaves. Be precise about location—this is a frequently tested detail.
Think of stomata as tiny doors in the leaf: guard cells are the doormen that open and close them to control what enters and leaves the plant.
Section 2
How do guard cells control the opening and closing of stomata?
Guard cells regulate stoma opening through turgor pressure changes caused by the movement of ions and water. This mechanism involves the following process:
When stomata open:
- Guard cells actively transport potassium ions (K⁺) into their cytoplasm using ATP
- Water molecules move into guard cells by osmosis (following the water potential gradient)
- Increased water uptake increases turgor pressure inside guard cells
- The guard cells become turgid and rigid, pulling the stoma open
When stomata close:
- Potassium ions are actively transported out of guard cells
- Water leaves the guard cells by osmosis as water potential becomes more negative outside the cells
- Guard cells lose turgor pressure and become flaccid (limp)
- The stoma closes
Key structural feature: Guard cells are kidney-shaped with unevenly distributed cellulose fibres in their cell walls, meaning when turgid they bow outward, pulling the stoma open.
Examiners want to see that you link ion movement (particularly K⁺) to water movement by osmosis, and then connect this to changes in turgor pressure. Don't just say 'water moves in'—explain the mechanism.
Students often confuse active transport with passive transport. Remember: guard cells ACTIVELY transport ions using ATP and energy. This is NOT diffusion.
During the day, guard cells absorb K⁺ ions using ATP energy. Water follows by osmosis because the cell now has a lower (more negative) water potential. Turgor increases, the cell becomes turgid, and the stoma opens to allow photosynthesis.
Section 3
What is the relationship between stomatal opening and photosynthesis?
Stomata open during the day in response to light, optimising gas exchange for photosynthesis:
- Light triggers K⁺ uptake: Light energy promotes the active transport of potassium ions into guard cells
- Carbon dioxide enters: When stomata are open, CO₂ diffuses into the leaf for photosynthesis
- Oxygen exits: Oxygen produced during photosynthesis diffuses out through open stomata
- Stomata close at night: In darkness, potassium ions leave guard cells, water leaves by osmosis, and stomata close
This daily rhythm is called the stomatal cycle and is controlled by the plant's circadian rhythm (internal 24-hour clock). Some plants also open stomata slightly at night for CAM photosynthesis (a specialised pathway in desert plants).
Link stomatal opening to the plant's needs: during the day, the plant needs CO₂ for photosynthesis, so stomata open. This is purposeful behaviour, not random.
Section 4
How do stomata balance gas exchange with water loss?
Stomata present a fundamental trade-off for plants: they must remain open to obtain carbon dioxide for photosynthesis, but this opening allows water vapour to escape.
Stomata and transpiration:
- Transpiration occurs primarily through stomata (up to 90% of water loss in most plants)
- The greater the stomatal opening, the faster the rate of both CO₂ uptake and water loss
- Plants must balance the gain (photosynthesis) against the loss (dehydration)
Environmental factors affecting this balance:
| Factor | Effect on Stomata | Effect on Water Loss |
|---|---|---|
| High temperature | Stomata open to cool leaf via transpiration | Increased transpiration |
| Low humidity | Stomata may partially close to conserve water | Reduced transpiration |
| Wind | Increases rate of water vapour removal from leaf surface | Increased transpiration |
| Light | Stomata open for photosynthesis | Increases transpiration opportunity |
| Drought/water stress | Stomata close to prevent desiccation | Severely reduced transpiration |
Xerophytic adaptations: Desert plants have evolved reduced stomatal numbers, smaller leaves, and waxy cuticles to minimise this water loss problem.
Examiners test whether you understand this is a BALANCE, not a simple cause-and-effect. Show that plants face conflicting demands: they need stomata open for photosynthesis but this increases water loss.
A stoma is like a shop window: you need it open to attract customers (CO₂), but this lets heat (water) escape. The owner must choose between exposure and energy bills.
Section 5
How is stomatal opening regulated by water availability?
Plants sense water stress and respond by closing stomata to prevent excessive water loss during drought conditions. This regulatory system involves the hormone abscisic acid (ABA):
Under normal water conditions:
- Guard cells maintain K⁺ and remain open (or open in response to light)
- The plant prioritises photosynthesis and growth
Under water stress (drought):
- Root cells detect low water potential in the soil
- ABA is synthesised and transported to the leaf via the xylem
- ABA binds to receptors on guard cell membranes
- Guard cells respond by pumping out K⁺ ions (and sometimes Cl⁻ ions)
- Water leaves by osmosis, turgor decreases, and stomata close
- Transpiration is reduced, conserving water for survival
This is a protective mechanism that sacrifices some photosynthesis to ensure the plant survives. Once water becomes available again, ABA levels decrease and stomata can reopen.
Examiners expect you to name ABA and explain the sequence: water shortage detected → ABA released → guard cells respond → ions exit → water leaves → stomata close. Show the mechanism step-by-step.
During a drought, soil water potential becomes very negative. Root cells detect this and produce ABA, which travels to leaves. ABA causes guard cells to expel K⁺, so water leaves the cells by osmosis. Turgor pressure drops, and stomata close to prevent the plant drying out completely.
Must Know
- Stomata are pores on leaves (mainly lower epidermis) surrounded by guard cells that control gas exchange and water vapour loss (transpiration)
- Guard cells regulate stomatal opening by controlling turgor pressure: potassium ions are actively transported IN (using ATP) to open stomata, and OUT to close them; water follows by osmosis
- Stomata open in light to allow CO₂ uptake for photosynthesis and close in darkness; this cycle is controlled by circadian rhythm and light signals
- Plants face a trade-off: stomata must be open for photosynthesis but this causes water loss through transpiration; they balance these competing demands based on environmental conditions
- Water stress triggers stomatal closure via ABA: when soil water is low, roots produce abscisic acid (ABA), which signals guard cells to close stomata and conserve water for survival
That's the notes covered.
Carry on to the next subtopic.