Homeostasis in Plants

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Explain that stomata respond to changes in environmental conditions by opening and closing and that regulation of stomatal aperture balances the need for carbon dioxide uptake by diffusion with the need to minimise water loss by transpiration

  2. Explain that stomata have daily rhythms of opening and closing

  3. Describe the structure and function of guard cells and explain the mechanism by which they open and close stomata

  4. Describe the role of abscisic acid in the closure of stomata during times of water stress, including the role of calcium ions as a second messenger

Main Content

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What is Homeostasis in Plants?

Just like animals, plants need to maintain a constant internal environment. This is called homeostasis. Plants must carefully control the gases moving in and out of their leaves, and they must balance their need for carbon dioxide with their need to keep water inside their bodies. The tiny pores on leaves, called stomata, are the key structures that help plants achieve this balance.

1. Stomata: Responding to Environmental Changes

What Are Stomata?

Stomata (singular: stoma) are tiny holes or pores found on the surface of leaves. Each stoma is surrounded by two special cells called guard cells. These guard cells control whether the stoma is open or closed.

Think of stomata as gates that can open and close. When they are open, gases can pass through. When they are closed, the gates are shut and gases cannot pass.

The Important Balance

Plants face a difficult challenge. They need carbon dioxide (CO₂) to carry out photosynthesis and make food. But when stomata open to let carbon dioxide in, water vapour escapes from inside the leaf through a process called transpiration (this is water loss through evaporation).

This creates a balancing act:

When stomata are OPEN:

  • Carbon dioxide can diffuse into the leaf for photosynthesis ✓
  • But large amounts of water are lost by transpiration ✗

When stomata are CLOSED:

  • Water is retained inside the leaf ✓
  • But the supply of carbon dioxide is reduced, so photosynthesis slows down ✗

The plant must carefully regulate (control) the opening and closing of its stomata to get enough carbon dioxide while not losing too much water.

Environmental Conditions That Affect Stomata

Stomata respond to various environmental changes by opening or closing. Here are the main factors:

Stomata OPEN in response to:

  • Increasing light intensity – More light means photosynthesis can happen, so the plant needs CO₂
  • Low carbon dioxide concentration inside the leaf – When CO₂ is used up during photosynthesis, more is needed

Stomata CLOSE in response to:

  • Darkness – No photosynthesis can happen at night, so CO₂ is not needed
  • High carbon dioxide concentration inside the leaf – Plenty of CO₂ is available already
  • Low humidity – Dry air outside causes faster water loss, so stomata close to conserve water
  • High temperature – Heat increases the rate of transpiration, so stomata close to prevent too much water loss
  • Water stress – When water supply from the roots is limited or transpiration rates are very high, stomata close to prevent dehydration

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