Van Niel's Experiment and the Hydrogen Donor Concept
Biology · Photosynthesis in Higher Plants · NEET
C.B. Van Niel studied purple and green sulphur bacteria and showed that photosynthesis splits a hydrogen donor to reduce CO2. In green plants the hydrogen donor is water (H2O), so the by-product is oxygen (O2). In sulphur bacteria the donor is H2S, so they release sulphur, not O2. Memory hook: "Whatever you split, that is what you release" — split water, get oxygen; split H2S, get sulphur.
The same general equation gives O2 when water is the hydrogen donor (green plants) but sulphur when H2S is the donor (sulphur bacteria) — proving the released oxygen comes from the split donor, i.e. from water.
Your doubts, answered
Does the O2 released in photosynthesis come from water or from CO2?
It comes from water (H2O), not from CO2. This is the whole point of Van Niel's work. He compared green plants (donor = H2O, release O2) with sulphur bacteria (donor = H2S, release sulphur) and inferred the oxygen must come from the hydrogen donor, which is water. This was later confirmed with radioisotope techniques (heavy oxygen, 18O).
What exactly is a 'hydrogen donor' in photosynthesis?
A hydrogen donor is the compound that is split to give hydrogen (H), and this hydrogen is used to reduce CO2 into carbohydrate. In green plants, cyanobacteria and algae the donor is water (H2O). In purple and green sulphur bacteria the donor is hydrogen sulphide (H2S). The general form written in NCERT is CO2 + 2H2A → carbohydrate + H2O + 2A, where H2A is the hydrogen donor and 2A is the oxidised by-product.
Why do green and purple sulphur bacteria not release oxygen?
Because their hydrogen donor is H2S, not H2O. When H2S is split the oxidation product is sulphur (S) or sulphate, not oxygen. This is called anoxygenic photosynthesis. So oxygen release depends only on WHAT is used as the hydrogen donor, not on the CO2.
How did Van Niel reach his conclusion if he could not see the oxygen atoms?
He used comparison and logic. He noticed that both green plants and sulphur bacteria run a light-dependent reaction that reduces CO2, but only water-users release O2. Since the only difference was the hydrogen donor, he inferred the O2 must come from the donor (water). Direct proof came later using radioisotopic (18O) tracing.
Is Van Niel's equation the same as the normal photosynthesis equation?
It is the general form. Van Niel wrote CO2 + 2H2A → [CH2O] + H2O + 2A. Putting H2A = H2O (green plants) gives 6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2. NEET expects you to know that 12 water molecules are written on the left (not 6) precisely because water is the hydrogen donor being split.
⚠️ The NEET trap ✗ Oxygen released in photosynthesis comes from carbon dioxide. ✓ Oxygen comes from water (H2O), the hydrogen donor that is split; CO2 is only reduced to carbohydrate. 🧠 NTA loves flipping the source of O2. Remember: O2 comes from H2O, NOT from CO2. Van Niel's comparison with H2S-using bacteria (which give sulphur) is the proof.
Real NEET questions
NEET 2018
Oxygen is not produced during photosynthesis by
A · Cycas
B · Nostoc
C · Green sulphur bacteria ✓
D · Chara
Solution: Oxygen is released only when water (H2O) is the hydrogen donor, as in Cycas, the cyanobacterium Nostoc and the alga Chara. Green (and purple) sulphur bacteria use H2S as the hydrogen donor, so their oxidation product is sulphur or sulphate, not O2 (anoxygenic photosynthesis). Hence green sulphur bacteria do not produce oxygen.
ReNEET 2026
Select the correct sequence of experiments that led to a gradual understanding of photosynthesis in green plants.
A · Absorption spectra of chlorophyll a and b → production of glucose → release of oxygen → role of air
B · Role of air → release of oxygen → production of glucose → absorption spectra of chlorophyll a and b ✓
C · Release of oxygen → production of glucose → absorption spectra → role of air
D · Production of glucose → role of air → release of oxygen → absorption spectra
Solution: The historical chain runs: Priestley (role of air) → Ingenhousz (release of oxygen by green parts in light) → von Sachs (glucose/starch production) → Engelmann (absorption/action spectrum of chlorophyll). Van Niel's hydrogen donor insight sits within this same line of reasoning about where the released O2 originates, so understanding the experiment sequence is directly tested.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Cornelius B. Van Niel (1897-1985) was a microbiologist who studied purple and green sulphur bacteria. From their photosynthesis he generalised that light splits a hydrogen donor to reduce CO2 into carbohydrate.
What is the single most important conclusion of Van Niel's experiment for NEET?
That the oxygen released in green-plant photosynthesis comes from water (the hydrogen donor), not from carbon dioxide. This was later confirmed by radioisotope (18O) experiments.
What is the general equation Van Niel proposed?
CO2 + 2H2A → [CH2O] + H2O + 2A, in the presence of light. Here H2A is the hydrogen donor; in green plants H2A = H2O and 2A = O2, while in sulphur bacteria H2A = H2S and 2A = sulphur.
How was Van Niel's conclusion finally proved?
By radioisotopic techniques using heavy oxygen (18O). When water was labelled, the released O2 carried the label, proving oxygen originates from water and not from CO2.
Why is oxygen-releasing photosynthesis called oxygenic and the bacterial type anoxygenic?
Oxygenic photosynthesis (green plants, cyanobacteria, algae) uses water as donor and releases O2. Anoxygenic photosynthesis (purple and green sulphur bacteria) uses H2S and releases sulphur, so no oxygen is produced.
