Plants Refill the Air
How oxygen and carbon dioxide stay in balance.
What this lesson covers
Why it matters
Every breath you take uses up oxygen and breathes out carbon dioxide. So why does the air never run out of oxygen? Green plants quietly refill it.
Predict first
A green water plant is left in sunlight in a jar of water. Tiny bubbles rise from its leaves. Which gas is the plant giving off?
In sunlight, green plants carry out photosynthesis: 6CO₂ + 6H₂O → glucose + 6O₂. The bubbles are oxygen.
- Carbon dioxide
- Oxygen — correct
- Nitrogen
- Hydrogen
What you do
Give the green plant carbon dioxide, water and sunlight and watch oxygen bubble out of its leaves. This is photosynthesis in action.
Check yourself
What does photosynthesis do to the gases in the air?
Green plants take in carbon dioxide and give out oxygen during photosynthesis, which keeps refilling the oxygen in the air.
Which processes do the OPPOSITE of photosynthesis, using up oxygen and releasing carbon dioxide?
Respiration in living things and the burning of fuels both take in oxygen and give out carbon dioxide, the reverse of photosynthesis.
How is the balance of oxygen and carbon dioxide in the air kept steady?
Plants make oxygen and use carbon dioxide, while animals and fires do the reverse. These opposite processes together keep the oxygen and carbon dioxide balanced. This is the oxygen cycle.
- It makes oxygen and uses up carbon dioxide — correct
- It makes carbon dioxide and uses up oxygen
- It makes nitrogen and uses up oxygen
- It uses up both oxygen and carbon dioxide
- Respiration and burning — correct
- Photosynthesis and evaporation
- Condensation and melting
- Rusting and dissolving
- Photosynthesis balances respiration and burning, so the two gases stay in balance — correct
- Oxygen and carbon dioxide never change in amount at all
- Rain washes the extra gases out of the sky
- The Sun makes fresh oxygen directly