Photosynthesis gives off O2. Respiration gives off CO2.
- The idea
- Predict
- See it
- Why
- Check
The idea
“A plant doing photosynthesis uses up O2, gives off CO2, or breaks down sugar for energy.”
Plants do both processes, and both use CO2, water, sugar and O2. A plant in the dark really does give off CO2, so it's easy to pin that on photosynthesis.
The short version
The mistake: A plant doing photosynthesis uses up O2, gives off CO2, or breaks down sugar for energy.
What's actually true: Photosynthesis builds sugar from CO2 and water and gives off O2. Respiration breaks sugar down with O2, giving off CO2 and water.
- Photosynthesis builds sugar. In the chloroplast, light energy splits water, which releases O2. Then the Calvin cycle fixes CO2 into sugar.
- Cellular respiration breaks sugar down to make ATP and releases CO2. In the mitochondria, O2 accepts the electrons at the end of the electron transport chain.
- Plant cells respire all the time. In bright light, photosynthesis outpaces it, so a leaf takes in CO2 and gives off O2. In the dark, only respiration runs.
- So when a plant gives off CO2 or uses up O2, that's respiration.
On the exam: Photosynthesis takes in CO2 and gives off O2. Respiration takes in O2 and gives off CO2. Plants do both.