Stomata

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 

  1. What is the role of guard cells?
  2. Why do stomata open in the day?
  3. What happens to stomata on a hot day?
  4. What does a potometer measure?
  5. 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.