Put photosynthesis and cellular respiration side by side, and a pattern appears: each one uses what the other makes.
Key idea
The outputs of photosynthesis (sugar and oxygen) are the inputs of respiration, and the outputs of respiration (carbon dioxide and water) are the inputs of photosynthesis. Matter cycles; energy flows in as light and is used up as the cell works.
| Photosynthesis | Cellular respiration | |
|---|---|---|
| Where | Chloroplasts | Mitochondria |
| Needs light? | Yes | No: day and night |
| Who | Plants, algae, some bacteria | Nearly all living things |
| Uses | Carbon dioxide, water | Sugar, oxygen |
| Makes | Sugar, oxygen | Carbon dioxide, water, ATP |
Why it works
The atoms are reused, but the energy is not. Light energy is stored in sugar, then released by respiration to do work, and finally lost as heat. That is why life needs a steady supply of sunlight while the same carbon and oxygen atoms go around and around.
Worked examples
Worked example: Plant under a lamp
A water plant sits in a sealed tube under a bright lamp. What happens to the carbon dioxide in the water?
- In bright light the plant photosynthesizes faster than it respires.
- So it uses more carbon dioxide than it gives off.
Answer: It goes down.
Worked example: Snail alone
A pond snail sits in a sealed tube under the same lamp, with no plant. What happens?
- The snail only respires; light does not change that.
Answer: The carbon dioxide goes up.
Common mistake
Thinking the two are exact opposites happening in the same place. They form a cycle of matter, but photosynthesis happens in chloroplasts and respiration in mitochondria, and only photosynthesis needs light. Check: at night, which one still happens? Respiration.