Stomata
What Are Stomata?
Stomata are tiny pores found mostly on the underside of leaves.
They allow gas exchange and control water loss through transpiration.
💡 Key Concept: Stomata open and close to balance the plant’s need for gas exchange with minimising water loss.
Structure of a Stoma
Each stoma is flanked by two guard cells that:
✔ Control whether the stoma is open or closed
✔ Swell with water to open the pore
✔ Lose water and shrink to close the pore
Functions of Stomata
Function | Why It Matters |
Gas exchange | Allows CO₂ in for photosynthesis and O₂ out |
Transpiration | Water vapour exits the leaf – helps pull water up from roots |
Regulation | Stomata close to reduce water loss during hot or dry conditions |
Factors That Affect Stomatal Opening
Factor | Effect on Stomata |
Light | Stomata open in light to allow CO₂ in for photosynthesis |
High temperature | Can cause stomata to close to prevent water loss |
Humidity | High humidity = stomata stay open; low humidity = stomata may close |
Wind | Increases transpiration → stomata may close to conserve water |
Required Practical: Investigating Transpiration (Stomatal Activity)
🔬 Method: Using a Potometer
A potometer measures the rate of water uptake, which is used to estimate the rate of transpiration.
Steps:
1️ Set up a leafy shoot in a potometer
2️ Seal all joins to make it airtight
3️ Introduce an air bubble into the capillary tube
4️ Measure the distance the bubble moves over time
👉 Change one factor at a time (light, wind, humidity, temp) and keep others constant
Limitations of the Potometer
❌ It doesn’t measure water loss directly – just uptake
❌ Some water is used for photosynthesis or stored in tissues
✅ Still a reliable estimate for comparing different conditions
Questions
- What is the role of guard cells?
- Why do stomata open in the day?
- What happens to stomata on a hot day?
- What does a potometer measure?
- Why are most stomata found on the underside of the leaf?
Summary
- Stomata are vital for gas exchange and water regulation in plants.
- Their opening is controlled by guard cells, responding to environmental conditions.
- Transpiration can be investigated using a potometer, helping us understand plant water use.