Why Plants Need to Exchange Gases
Like all living organisms, plants need to exchange gases with their surroundings. For photosynthesis, plants require carbon dioxide (CO₂) from the air and release oxygen (O₂) as a by-product. For cellular respiration (which happens in all plant cells, all the time), they require oxygen and release carbon dioxide — exactly the reverse.
During daylight hours, photosynthesis usually proceeds faster than respiration, so the net effect is that plants absorb CO₂ and release O₂. At night, only respiration continues, so plants take in O₂ and release CO₂.
Stomata: The Gas Exchange Pores
Gas exchange in leaves takes place mainly through stomata (singular: stoma). These are microscopic pores, or openings, found mostly on the underside of leaves. Each stoma is surrounded by a pair of curved cells called guard cells.
The guard cells control the size of the opening:
- When guard cells are full of water (turgid), they curve apart and the stoma opens.
- When guard cells lose water, they become limp and the stoma closes.
This allows the plant to regulate gas exchange and, crucially, to control water loss.
A single leaf may have up to 300,000 stomata per square centimetre. Most are on the underside to reduce water loss, since the upper surface faces direct sunlight and would evaporate more water if stomata were there.
Water Vapour and Transpiration
When stomata open to let in CO₂, water vapour also escapes from the moist cells inside the leaf. This loss of water vapour is called transpiration. Plants lose a surprising amount of water this way — a large tree can transpire hundreds of litres of water per day.
Transpiration has benefits:
- It creates a transpiration stream — a flow of water and dissolved minerals upward through the plant's xylem vessels.
- It cools the leaf surface (similar to sweating in animals).
If water is scarce, guard cells close the stomata to reduce water loss, even though this also reduces CO₂ uptake and slows photosynthesis.
A common misconception: plants do NOT "breathe" the way animals do. Plants do not have lungs or a respiratory system. Gas exchange happens by diffusion through stomata — no active pumping is involved.
Gas Exchange in Other Plant Parts
Leaves are the main site of gas exchange, but other plant parts also exchange gases:
- Stems — Woody stems have small pores called lenticels through which gas exchange occurs.
- Roots — Root cells respire and exchange gases with air pockets in soil through their surfaces.
The Balance Between Photosynthesis and Respiration
At a certain light level called the light compensation point, the rate of photosynthesis exactly equals the rate of respiration — no net gas exchange occurs. Below this level, the plant uses more oxygen than it produces.
Real world: Wilting houseplants on a hot afternoon
On a hot, sunny day, the guard cells in your houseplant's stomata close to prevent water loss. You might notice the plant begins to droop (wilt) by mid-afternoon — even if the soil is moist. The stomata have shut to protect the plant from desiccation, which simultaneously slows photosynthesis because CO₂ can no longer enter.
Real world: Breathing through a forest
In a dense forest, the collective stomata of millions of leaves absorb enormous amounts of CO₂ from the air. Researchers have measured that a single mature oak tree can absorb around 22 kg of CO₂ per year through its stomata — the equivalent of driving a car about 90 km.
Real world: Oxygen in a fish tank
Aquarium owners often add aquatic plants (like Elodea) to their tanks. During daylight hours, these plants open their stomata-equivalent pores and release oxygen bubbles into the water, helping fish breathe. At night, the process reverses — the plants compete with fish for dissolved oxygen.
Practice Problems
What are guard cells, and what role do they play?
Show answer
Why do plants close their stomata during drought conditions, and what is the consequence for photosynthesis?