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Your NCERT photosynthesis chapter has two halves: light reactions that split water and release oxygen, and the Calvin cycle that fixes carbon dioxide. That split now looks obvious. In the 1930s it was not, and one of the experiments that made it visible was done by a shy Cambridge biochemist using chloroplasts separated from leaves. This post covers who Robert Hill was, what the Hill reaction showed, a worked stoichiometry example, how his 1960 idea became the Z scheme, and what students can take from him.
Who was Robert Hill?
His Royal Society biographical memoir gives his dates as 2 April 1899 to 15 March 1991. The memoir's opening describes a man almost always known as Robin, trained as a chemist, who helped establish biochemistry as an independent discipline and spent his whole working life in Cambridge. He grew the plants for his experiments at his farm home, made his own pigments from dye plants such as madder and woad, and painted watercolours. The memoir notes that he never held a university post: he lived on a series of fellowships until the Agricultural Research Council took him on to its external staff in 1943.
What problem was Hill trying to solve?
In his 1939 paper, Oxygen produced by isolated chloroplasts, Hill set out the problem. For animal respiration, biochemists already knew a lot about the chemical machinery inside cells. For photosynthesis, there was still no direct sign of any single chemical mechanism. Earlier workers had detected small amounts of oxygen from broken plant cells, but only with bacteria sensitive to traces of the gas, and the effect was tiny compared with a living leaf.
His idea was simple. Respiration research had used oxygen uptake, apart from carbon dioxide release, as a guide. So an oxygen output, apart from carbon dioxide uptake, might reveal a mechanism special to photosynthesis.
What did the Hill reaction show?
Hill reported his first result in Nature in 1937 as Oxygen evolved by isolated chloroplasts. Melvin Calvin, in his 1961 Nobel lecture, summed up its importance: the light-driven production of oxygen "had been separated chemically and physically from the reduction of carbon dioxide by the demonstration of oxygen evolution by illuminated chloroplasts. This was done by Robert Hill using ferric iron as oxidant in the place of carbon dioxide."
In other words, chloroplasts outside the cell, lit up and given a substance they could hand electrons to, released oxygen without any carbon dioxide being fixed. Substances that play this role are known as Hill oxidants, a term used in a 2002 appreciation of Hill and his reaction in Photosynthesis Research.
A worked example: Hill reaction stoichiometry
NCERT's Class 11 chapter on photosynthesis in higher plants writes the splitting of water as:
2H2O → 4H+ + O2 + 4e−
In a Hill reaction with ferric iron, each Fe3+ ion accepts one electron and becomes Fe2+. Four electrons are released per O2, so four Fe3+ are needed:
4Fe3+ + 2H2O → 4Fe2+ + 4H+ + O2
Check: iron atoms 4 = 4; hydrogen 4 = 4; oxygen 2 = 2; charge +12 on the left, +8 + 4 = +12 on the right.
Now a sample problem. A chloroplast suspension in light reduces 2.0 mmol of Fe3+. How much oxygen is released?
Mole ratio from the equation: 4 mol Fe3+ to 1 mol O2.
O2 formed = 2.0 / 4 = 0.50 mmol.
Mass: 0.50 × 10^-3 mol × 32 g/mol = 0.016 g, or 16 mg.
Common mistakes:
Using a 1:1 ratio. Each Fe3+ takes one electron, but each O2 releases four.
Forgetting the H+. The protons must appear on the right, or the charge will not balance.
Saying the oxygen came from the iron salt. It came from water; the iron salt only accepts electrons.
How did Hill help create the Z scheme?
In 1960 Hill and his colleague F. Bendall published a short paper in Nature, Function of the two cytochrome components in chloroplasts: a working hypothesis. It placed the cytochromes of the chloroplast in a chain of electron carriers. Peter Mitchell, in his 1978 Nobel lecture, refers to "the abstract Z scheme introduced by Hill & Bendall in 1960".
NCERT describes the result: electrons move from PS II uphill to an acceptor, down an electron transport chain to PS I, are excited again, and pass downhill to NADP+. The pattern looks like a Z when the carriers are placed on a redox scale. The calculation with Hill oxidants above is the first step of that chain: water giving up electrons at PS II.
What can a student take from Hill's way of working?
Split a big process into testable parts. Hill separated oxygen release from carbon fixation, and each part could then be studied on its own.
Use a stand-in when the real partner is missing. Ferric iron replaced the cell's own electron acceptor, so the reaction could run in a flask.
Call a hypothesis a hypothesis. The 1960 paper was titled "a working hypothesis", and later work tested it.
For the NEET chapter, read our guide to photosynthesis. The carbon half of the story belongs to Melvin Calvin and his cycle. The Z scheme is hard to picture from text alone, and the Fermi Simulation Lab has interactive simulations for the ideas that are easier to see than to read.
Try it now: if a Hill reaction releases 0.25 mmol of O2, how many millimoles of Fe3+ were reduced?
