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When you write "oxygen acts as the final hydrogen acceptor" in a respiration answer, you are using a result that took decades to establish. Something in every cell has to make inert oxygen from the air react quickly at body temperature. Otto Warburg showed what that something is, and he did it without ever purifying it. This post explains his method, the numbers behind it, and how it connects to your syllabus.
Who was Otto Warburg?
The Nobel Prize biography records that Otto Heinrich Warburg was born on 8 October 1883 in Freiburg. His father, Emil Warburg, was a physicist. Otto studied chemistry under Emil Fischer, earning a doctorate in Berlin in 1906, and then a medical degree in Heidelberg in 1911. From 1918 he was a professor at the Kaiser Wilhelm Institute for Biology in Berlin-Dahlem, and from 1931 he directed the new Kaiser Wilhelm Institute for Cell Physiology. The Nobel facts page gives his death as 1 August 1970 in West Berlin.
Why couldn't chemists simply isolate the enzyme?
Food burns in air only at high temperatures, yet cells burn it at body temperature. The 1931 presentation speech explains that the catalyst forms less than about a millionth of the weight of the cells, is firmly bound to them, and is destroyed by the methods that might free it.
In his Nobel lecture, Warburg put the concentration of the enzyme's iron at about 1 g in 10 million g of cell substance. You can feel how small that is:
1 kg of cells is 1,000 g.
Iron in it: 1,000 ÷ 10,000,000 = 0.0001 g, which is 0.1 mg.
No ordinary chemical test of his time could hope to purify and study that directly.
How did Warburg study an enzyme he could not see?
He used what he called the inhibition technique. Instead of extracting the enzyme, he looked for substances that switch respiration off specifically and reversibly, then reasoned about what they must be reacting with. He measured respiration as the oxygen consumption of living, breathing cells.
Two inhibitors mattered. Prussic acid (hydrogen cyanide), whose effect on respiration had been found by Claude Bernard, reacts with iron in its trivalent form. Carbon monoxide competes with oxygen for the iron in its bivalent form, so its effect depends on how much oxygen is present. Both behaved the way iron catalysts behave.
Why was light the decisive clue?
Carbon monoxide compounds of iron have an unusual property: light splits the carbon monoxide off. Warburg's lecture describes the result, found in 1926 with Fritz Kubowitz. If cells breathe a mixture of carbon monoxide and oxygen, respiration stops in the dark and resumes when light falls on them. Alternating light and darkness switches cell respiration off and on.
With Erwin Negelein he then shone single colours of light of known intensity on the inhibited cells and measured how much each colour restored respiration. A colour the enzyme absorbs strongly frees it quickly. Plotting the effect against wavelength gave the enzyme's absorption spectrum, at first from 16 wavelengths and later from 31. The bands pointed to an iron compound related to the blood pigments. The Nobel facts page summarises the outcome: by 1928 he concluded the respiratory enzyme was a red ferrous pigment related to haemoglobin.
Where does this enzyme sit in your syllabus?
A review of mitochondrial history by Pagliarini and Rutter describes how David Keilin named the cytochromes in 1925, and how Keilin, Warburg and others built the idea of a respiratory chain with dehydrogenases at one end and Warburg's oxygen-reducing enzyme at the other.
In NCERT Class 11, Chapter 12 (Reprint 2026-27), the end of the electron transport system is complex IV, the cytochrome c oxidase complex, which contains cytochromes a and a3 and two copper centres, and oxygen acts as the final hydrogen acceptor. The electron transport system is in the NEET-UG 2026 syllabus; check the current year's version.
Common mistakes to avoid:
Saying carbon monoxide poisoning in humans works by blocking this enzyme. Warburg's lecture states the toxic effect in people comes from carbon monoxide binding blood iron, at pressures far below those that inhibit cell respiration.
Confusing the two inhibitors. Cyanide acts on the trivalent iron; carbon monoxide acts on the bivalent iron and competes with oxygen.
Thinking Warburg purified the enzyme. He characterised it inside living cells.
What else did he find?
His Nobel biography credits him with showing that flavins and nicotinamide are the active groups of hydrogen-transferring enzymes, and with work on tumour metabolism showing that cancer cells can live and develop even without oxygen. At Heidelberg he also drew a young doctor, Otto Meyerhof, into cell physiology.
What can a student take from Warburg?
The presentation speech praised his "rare perfection in the art of exact measurement". His approach was to turn a question he could not answer directly into one he could measure: how much does respiration return under this colour of light? When a concept feels invisible, ask what you can measure about it. If the electron transport chain still blurs together, the Fermi Concept Library covers every concept in biology and chemistry, each with its own lesson and its place in the map. For what happens before the chain, read about Hans Krebs and his cycle.
Next step: draw complex I to IV from memory, mark where carbon monoxide and cyanide act, and check it against NCERT Figure 12.4.
