Put Photosynthesis and Cellular Respiration side by side and they look like mirror images. The products of one are the raw materials of the other, and together they move carbon around the planet.
Key idea
Photosynthesis pulls CO₂ from the air into sugars. Respiration by plants, animals and decomposers returns it. Their balance, along with burning fuels and exchange with the ocean, sets the amount of carbon dioxide in the air.
Why it works
Plants have mitochondria too, and they respire all day and all night to power their own cells. In the light, a growing plant photosynthesizes faster than it respires, so overall it takes in CO₂ and gives off O₂. In the dark it only respires, so it gives off CO₂. What a gas sensor measures in the light is the net rate: $$\text{net uptake} = \text{gross photosynthesis} - \text{respiration}$$ Over a whole day, a growing plant takes in more CO₂ than it gives off; the extra carbon becomes its new leaves, stems and roots.
Worked example: Gross from net
In the light, a plant takes up 40 mg of CO₂ per hour (net). In the dark it gives off 10 mg per hour. What is its gross rate of photosynthesis?
- Respiration runs in the light too, at about the dark rate: 10 mg per hour.
- Gross = net + respiration $= 40 + 10 = 50$ mg per hour.
Answer: $50$ mg of CO₂ per hour
Worked example: A whole day
The same plant gets 12 hours of light and 12 hours of dark. What is its net CO₂ uptake for the day?
- Light: $12 \times 40 = 480$ mg taken up.
- Dark: $12 \times 10 = 120$ mg given off.
- Net: $480 - 120 = 360$ mg taken up: the plant grows.
Answer: $360$ mg taken up
Common mistake
Thinking plants only photosynthesize and animals only respire. Every living cell respires, plant cells included, day and night. Plants simply photosynthesize faster than they respire when the light is good, so their net exchange in the light is CO₂ in and O₂ out.