Not all photosynthesis gives off oxygen
- The idea
- Predict
- See it
- Why
- Check
The idea
“Anything that does photosynthesis splits water and releases oxygen.”
Plants are the first photosynthesizers you learn about, and they give off oxygen. Some bacteria also run on light, but they get their electrons from something other than water.
The short version
The mistake: Anything that does photosynthesis splits water and releases oxygen.
What's actually true: Only photosynthesis that takes its electrons from water releases O2. Bacteria that use H2S or other donors release none.
- A photosystem needs a steady supply of electrons. The molecule that hands them over is called the electron donor.
- Plants, algae and cyanobacteria use water. Photosystem II splits it, and the oxygen left over is the O2 they release.
- Some bacteria use H2S, H2 or iron instead. They never split water, so they make no O2. This is anoxygenic photosynthesis.
On the exam: Find the electron donor. Water leaves O2 behind. H2S leaves sulfur. No water split means no O2.