The O2 plants release comes from water
- The idea
- Predict
- See it
- Why
- Check
The idea
“Plants take in CO2 and give off O2, so the O2 must be oxygen pulled off the CO2.”
CO2 goes in and O2 comes out, so it looks like the plant just strips the carbon off. Follow the oxygen atoms and they go somewhere else.
The short version
The mistake: Plants take in CO2 and give off O2, so the O2 must be oxygen pulled off the CO2.
What's actually true: The O2 comes from water split in the light reactions. The oxygen in CO2 ends up in sugar and new water.
- Photosystem II pulls electrons from water to replace the ones light knocked loose. That splits the water, and the leftover oxygen atoms pair up and leave as O2.
- The electrons and H+ go on to make NADPH and ATP. The Calvin cycle spends those to fix CO2 into sugar, so CO2's oxygen lands in sugar and new water.
- Heavy-oxygen tracers showed this. Tag water's oxygen and the O2 comes out tagged. Tag CO2's and it doesn't.
On the exam: O2 is the leftover oxygen from split water. CO2's oxygen goes into sugar and new water.