Science · Cells & organisms
Photosynthesis & respiration
How plants and animals swap gases.
The Great Gas Swap
Plants and animals are perfect partners. Plants take in carbon dioxide and release oxygen during photosynthesis. Animals breathe in oxygen and breathe out carbon dioxide during respiration. This gas swap keeps both alive. Photosynthesis happens in green plant parts, especially leaves. Respiration happens in all living cells, both plant and animal.
Photosynthesis: Making Food from Light
Photosynthesis is the process plants use to make their own food (glucose). They need three ingredients: carbon dioxide from air, water from soil, and light energy from the sun. Chlorophyll (the green pigment in leaves) captures light energy. The plant makes glucose and releases oxygen as a waste product. The word equation is: carbon dioxide + water → glucose + oxygen.
Respiration: Releasing Energy from Food
Respiration is how all living things release energy from food. Cells break down glucose using oxygen. This releases energy for movement, growth, and staying warm. Carbon dioxide and water are waste products. The word equation is: glucose + oxygen → carbon dioxide + water. Notice respiration is the opposite of photosynthesis!
Where the Gases Travel
In plants, carbon dioxide enters leaves through tiny holes called stomata (singular: stoma). Oxygen exits through the same holes. In animals, oxygen enters the lungs when we breathe in. It travels to cells through blood. Carbon dioxide travels from cells back to lungs, then we breathe it out. The circulatory system transports these gases around the body.
Plants Do Both Processes
Plants photosynthesise during the day when light is available. They also respire all the time, day and night. During daytime, photosynthesis is faster than respiration, so plants take in more carbon dioxide than they release. At night, plants only respire, so they take in oxygen and release carbon dioxide, just like animals.
Worked examples
1. A plant is placed in bright sunlight. Which gas will it take in more of, and which will it release?
- In bright sunlight, the plant will photosynthesise rapidly.
- Photosynthesis takes in carbon dioxide and releases oxygen.
- The plant takes in carbon dioxide and releases oxygen.
2. Explain why a sealed room full of plants and animals can maintain its oxygen levels during the day.
- Animals breathe in oxygen and breathe out carbon dioxide during respiration.
- Plants take in that carbon dioxide and release oxygen during photosynthesis.
- The oxygen released by plants replaces the oxygen used by animals, maintaining balance.
3. At night, would you find more oxygen or carbon dioxide building up in a greenhouse full of plants? Why?
- At night, there is no light for photosynthesis, so plants only respire.
- Respiration uses oxygen and produces carbon dioxide.
- Carbon dioxide would build up because plants are only respiring, not photosynthesising.
Try it yourself
A mouse and a plant are sealed in a glass jar in sunlight. Explain how they help each other survive.
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