Articles about photosynthesis in the popular press or online often make me cringe.

Why?

Because sometimes they lead people to think that the oxygen (O 2 ) produced by photosynthesis is derived from carbon dioxide (CO 2 ).

Some even further compound their mistake by stating that plants actually convert CO 2 into O 2 at night!

Ack!

This is simply NOT true!!

Please allow me to explain…

The Oxygen You Breathe Comes from Water

Yes, that’s correct, water… H 2 O

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.

This chemical energy is used in the photosynthetic reaction centers to split 2 water molecules, producing 4 electrons, 4 protons, and 2 oxygen atoms, which combine to form oxygen gas (O 2 ).

2H 2 0 –> 4 e– + 4 H+ + O 2

Where: In green plants, photosynthesis occurs in chloroplasts, about two to four dozen of which float around in the cytoplasm of photosynthetic plant cells.

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 CO 2 Come In?

The chemical energy captured in step one above is used in step two of photosynthesis, that is, to convert CO 2 into carbohydrates (sugars). This is called carbon fixation, a.k.a., the Calvin cycle, which takes place in the chloroplast stroma. (Please see Fig. 1 above and Ref. 2 below.)

What is the scientific evidence that O 2 isn’t derived from CO 2 in photosynthesis?

Well, one way to test this is to use water or CO 2 containing an isotope of oxygen (e.g., oxygen-18 = O18) in photosynthesis and see which one, H 2 O18 or CO 2 18, produces O 2 18.

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 supplied plants with 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 scientific experiments have provided clear evidence that the oxygen produced by photosynthesis is derived from water.

Cyanobacteria, Green Algae and Plants All Do This

All of the photosynthetic organisms – plants, green algae (e.g., phytoplankton in the oceans), and cyanobacteria – that use water as an electron source 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 (CO 2 ) into oxygen (O 2 ). The oxygen produced during photosynthesis comes from water. During photosynthesis, green plants DO, however, convert atmospheric CO 2 into sugars. And we know now that one half the oxygen atoms in the CO 2 wind up in the sugars (e.g., glucose = C 6 H 12 O 6 ) and the other half wind up in phosphate byproducts of the Calvin Cycle (see Addendum below).

References

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)

2. Bassham, J. A. and M. Calvin (1960) “The Path of Carbon in Photosynthesis.” from: The Biogenesis of Natural Substances, M. Gates (ed.) (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 (PO 4 3-) groups is removed, containing one of the original oxygens from the CO 2 .

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