Lesson 2 – Gas Exchange in Plants
What is Gas Exchange?
Gas exchange means the swapping of gases—plants take in some gases and release others. Just like humans breathe in oxygen and breathe out carbon dioxide, plants also “breathe.” But their process is a bit different because they carry out two important activities:
- Photosynthesis (during the day)
- Respiration (day and night)
In photosynthesis, plants take in carbon dioxide (CO₂) and give out oxygen (O₂).
In respiration, plants take in oxygen and give out carbon dioxide, just like animals.
Stomata and Their Role in Gaseous Exchange
Let’s begin by understanding stomata (singular: stoma).
What are stomata?
- Stomata are tiny pores or holes found mostly on the surface of leaves and sometimes on stems.
- These pores are the entry and exit points for gases like oxygen and carbon dioxide.
- Think of stomata like tiny mouths on the plant’s surface that open and close to let gases move in and out.
Guard Cells – The Gatekeepers
- Each stoma is surrounded by two guard cells.
- These bean-shaped cells act like tiny valves or gates. They control whether the stoma is open or closed.
- Guard cells are special because they contain chloroplasts, which help them sense light and make energy.
Why are stomata important?
Stomata help in:
- Photosynthesis – by letting in CO₂ needed by the plant to make food.
- Respiration – for gas exchange to release energy.
- Transpiration – which is the loss of water through the stomata in the form of water vapor. This helps the plant cool down and also move water and nutrients from the roots to the leaves.
Stomatal Activity During Day and Night
During the Day:
- Stomata open.
- This allows carbon dioxide to enter the leaf.
- Plants use this CO₂ along with sunlight to make food (glucose) in the process of photosynthesis.
- Guard cells absorb water, become turgid (swollen), and this opens the stomatal pore.
At Night:
- Photosynthesis stops because there is no sunlight.
- Stomata usually close to conserve water.
- But plants still respire, so a small amount of oxygen enters and carbon dioxide exits.
- Guard cells lose water, become flaccid (shrunken), and close the stomata.
So, the opening and closing of stomata depend on light, water availability, and the plant’s needs.