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Where does the oxygen that plants release come from: carbon dioxide or water? The overall equation does not tell you, because both contain oxygen. Cornelis van Niel answered the question by looking at bacteria that do not release oxygen at all. This post walks through his reasoning, balances the equations so you can see why NCERT writes 12 water molecules, and lists the mistakes students make.
Who was Cornelis van Niel?
The NCERT Class 11 chapter Photosynthesis in Higher Plants (2026-27 reprint) calls van Niel (1897-1985) a microbiologist whose work was a "milestone contribution" to understanding photosynthesis. An encyclopedia entry by Aharon Oren in the Encyclopedia of Life Sciences describes him as a Dutch-born microbiologist who worked in California for most of his life, whose summer courses at Pacific Grove inspired many leading biologists, and who proposed a simple model linking plant and bacterial photosynthesis.
His major early study, On the morphology and physiology of the purple and green sulphur bacteria, filled more than 100 pages of the Archiv für Mikrobiologie in the early 1930s.
What did the sulphur bacteria show?
Purple and green sulphur bacteria photosynthesise: they use light to turn carbon dioxide into carbohydrate. But they use hydrogen sulphide (H2S) instead of water, and they release sulphur, not oxygen.
Van Niel saw a pattern. In both cases light drives a reaction in which hydrogen from some donor reduces carbon dioxide. NCERT writes his general equation as:
2H2A + CO2 → 2A + CH2O + H2O (in light)
Here H2A is the hydrogen donor. For sulphur bacteria, A is sulphur. For green plants, H2A is water, so A is oxygen. A 2019 review in Annals of Botany states the same equation and credits it to van Niel's work in 1931 and 1941.
A worked example: balance the equations
Check that each version balances.
Sulphur bacteria: 2H2S + CO2 → 2S + CH2O + H2O
Hydrogen: left 4; right 2 (in CH2O) + 2 (in H2O) = 4.
Sulphur: left 2; right 2.
Oxygen: left 2 (in CO2); right 1 (in CH2O) + 1 (in H2O) = 2.
Green plants: 2H2O + CO2 → O2 + CH2O + H2O
Oxygen: left 2 (from water) + 2 (from CO2) = 4; right 2 (in O2) + 1 + 1 = 4.
The two oxygen atoms in O2 match the two oxygen atoms from the water molecules on the left.
Multiply the plant equation by 6:
6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2
Now count. The 6 O2 molecules hold 12 oxygen atoms, exactly the 12 oxygen atoms in 12 water molecules. That is why NCERT writes 12 water molecules on the left and 6 on the right, rather than the shorter 6CO2 + 6H2O. The short version balances too, but it hides the fact that all the released oxygen can come from water.
How was the idea tested?
An inference is not proof. In 1941 Samuel Ruben, Merle Randall, Martin Kamen and James Hyde published Heavy Oxygen (O18) as a Tracer in the Study of Photosynthesis in the Journal of the American Chemical Society. The Annals of Botany review notes that the origin of photosynthetic oxygen from water was established using 18O-labelled water. Oxygen-18 is a heavier, stable isotope, so the oxygen given off could be traced to its source. (NCERT describes this as "radioisotopic techniques".)
Other work pointed the same way. The same review describes Robert Hill's 1937 finding that isolated chloroplasts can release oxygen when given an artificial electron acceptor, even in the absence of carbon dioxide: the light reactions could run on their own.
Common mistakes in exam answers
Saying oxygen comes from CO2. Van Niel's inference and the isotope evidence both point to water.
Writing 6H2O on the left in the full equation. NCERT's "correct equation" has 12H2O on the left and 6H2O on the right.
Calling all photosynthetic bacteria oxygen producers. Sulphur bacteria release sulphur or sulphate, not oxygen.
Forgetting what A stands for. In 2H2A, A is whatever is left after hydrogen is removed: sulphur for H2S, oxygen for H2O.
For the full chapter, see our guide to photosynthesis for NEET. The story continues with Robert Emerson and the enhancement effect, which led to the idea of two photosystems.
What can a student take from van Niel?
He learned something about plants by studying bacteria. Comparing a familiar case with an unusual one often shows which parts are essential. When a process seems hard to pin down, look for a variant that differs in one way and ask what changes. If you get stuck at the step of choosing which principle applies to an equation or numerical, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
Next step: balance 2H2A + CO2 → 2A + CH2O + H2O for H2S and H2O without looking, then check against this post.
