The Oxygen and Carbon Dioxide Balance
≈ 20 minPhotosynthesis and respiration are opposites, and together they keep the gases of the atmosphere in balance.
| Photosynthesis | Respiration | |
|---|---|---|
| Where | Green parts of plants only | Every living cell |
| When | Only in the light | All the time, day and night |
| Uses | Carbon dioxide + water | Glucose + oxygen |
| Produces | Glucose + oxygen | Carbon dioxide + water |
| Energy | Stores light energy in food | Releases energy from food |
In daylight a healthy plant photosynthesises much faster than it respires, so overall it takes in carbon dioxide and gives out oxygen. At night only respiration continues, so the plant takes in oxygen and gives out carbon dioxide, just like you do.
This is what interdependence means: animals depend on plants for oxygen and food; plants depend on the carbon dioxide that animals (and decay, and fires) return to the air.
Worked example. A sealed glass jar holds a healthy pot plant and a small grasshopper.
- Left on a sunny windowsill: the plant photosynthesises, releasing oxygen for the grasshopper; the grasshopper respires, releasing carbon dioxide for the plant. Both can survive for a long time.
- Locked in a dark cupboard: photosynthesis stops. Now both organisms are respiring and neither is making oxygen. The oxygen runs out and the carbon dioxide builds up.
The jar makes visible what the whole atmosphere is doing all the time.
Name the process that removes carbon dioxide from the air and stores its energy in food.
Which statement about a healthy pot plant standing in a dark room at night is correct?
A school estimates that each indigenous tree it plants removes about 22 kg of carbon dioxide from the air per year. The school plants 150 trees. How many kilograms of carbon dioxide will they remove in one year?
A sealed jar containing a pot plant and a grasshopper is kept on a sunny windowsill, and an identical jar is locked in a dark cupboard. Which jar keeps both organisms alive for longer, and why?