Articles about photosynthesis in the popular press or online often make me cringe.
Because sometimes they lead people to think that the oxygen (O2) produced by photosynthesis is derived from carbon dioxide (CO2).
Some even further compound their mistake by stating that plants actually convert CO2 into O2 at night!
This is simply NOT true!!
Please allow me to explain…
The Oxygen You Breathe Comes from Water
Yes, that’s correct, water… H2O
Here are how, where, and when this works in green plants:
How: Photosynthesis is basically a two-step process, and the first step is when water is converted into oxygen.
The first step directly requires light energy, which is captured by the photosynthetic pigments, mainly chlorophyll. The chlorophyll converts light energy (photons) into chemical energy, in the form of high-energy electrons.
2H20 –> 4 e- + 4 H+ + O2
The first step, described above, takes place in the thylakoid membranes (see Figure 1 above).
When: Since the splitting of water to form oxygen requires light energy, this only occurs naturally during the daytime.
Where Does the CO2 Come In?
The chemical energy captured in step one above is used in step two of photosynthesis, that is, to convert CO2 into carbohydrates (sugars). This is called carbon fixation, a.k.a., the Calvin cycle, which takes place in the chloroplast stroma. (see Figure 1 above)
What is the scientific evidence that O2 isn’t derived from CO2 in photosynthesis?
Well, one way to test this is to use water or CO2 containing the radionuclide, a.k.a., radioactive isotope, of oxygen (e.g., oxygen-18 = O18) in photosynthesis and see which one, H2O18 or CO218, produces radioactive O218. Turns out, it’s the water.
Cyanobacteria, Green Algae and Plants All Do This
So, where does the oxygen you enjoy breathing mostly come from?
For a probable answer, see here.
Bottom line: Green plants DO NOT convert carbon dioxide (CO2) into oxygen (O2). The oxygen comes from water. Green plants DO, however, convert atmospheric CO2 into sugars. So, the oxygen atoms in the CO2 wind up in the sugars (e.g., glucose = C6H12O6).
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