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 an isotope of oxygen (e.g., oxygen-18 = O18) in photosynthesis and see which one, H2O18 or CO218, produces O218.
In 1941, Ruben, et al. (see Ref. 1 below) reported that they used an isotope of oxygen, O18, to find out where the oxygen atoms went in photosynthesis. They fed plants water containing O18, but because O18 is not a radioactive isotope of the most common form of oxygen, O16, they used a mass spectrometer to determine the fate of the O18. The O18 was found in the oxygen gas produced by the plant, but was NOT found in the sugars formed during photosynthesis.
This and other experiments have provided clear evidence that the oxygen produced by photosynthesis is derived from 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 produced during photosynthesis comes from water. During photosynthesis, green plants DO, however, convert atmospheric CO2 into sugars. And we know now that one half the oxygen atoms in the CO2 wind up in the sugars (e.g., glucose = C6H12O6) and the other half wind up in phosphate byproducts of the Calvin Cycle (see Addendum below).
1. Ruben, S., M. Randall, M. D. Kamen, and J. L. Hyde. (1941) “Heavy oxygen (O18) as a tracer in the study of photosynthesis.” Journal of the American Chemical Society, Vol. 63, pp. 877–879. (PDF)
Addendum: During the so-called “Reduction Phase” of the Calvin Cycle, ATP is used to phosphorylate the 3-PGAs, and then NADPH is used to reduce (add electrons) the 3-carbon compound to produce GAP. In doing so, one of the phosphate (PO43-) groups is removed, containing one of the original oxygens from the CO2.
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