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NCERT tells you that the first product of carbon fixation in C3 plants is 3-phosphoglyceric acid, or PGA. How could anyone know which compound comes first, when a leaf turns CO₂ into dozens of compounds within moments? Melvin Calvin's answer was to race the plant with a radioactive label. This post covers Calvin's life, how the experiment was designed, the light-off test that found where PGA comes from, a worked carbon count of the cycle, and what students can learn from his method.
Who was Melvin Calvin?
Calvin was born in St. Paul, Minnesota, on 8 April 1911, according to his Nobel facts page. The NCERT Class 11 chapter Photosynthesis in Higher Plants carries a short profile: he received his PhD in chemistry from the University of Minnesota and served as Professor of Chemistry at the University of California, Berkeley. He won the 1961 Nobel Prize in Chemistry alone, "for his research on the carbon dioxide assimilation in plants". He died on 8 January 1997.
Why did Calvin use carbon-14?
In his Nobel lecture, Calvin explains the central difficulty. The machinery that turns CO₂ into sugar and the material it works on are both made of carbon, so ordinary chemical analysis cannot tell them apart. The long-lived isotope carbon-14, discovered by Samuel Ruben and Martin Kamen in 1940, solved this: newly fixed carbon would be radioactive, and everything already in the cell would not.
In 1945, he writes, it became clear that carbon-14 would be available cheaply and in large amounts from the nuclear reactors that had been built. With the support of Ernest Lawrence, director of the Radiation Laboratory at Berkeley, the group took it up as their main tool. The idea was simple: shorten the time the plant has with the label so much that the radioactivity is caught passing through the first compounds on its way to sugar.
How did the experiment work?
The group mostly used Chlorella, a single-celled green alga that could be grown in a highly reproducible way. Calvin notes that the main features of the cycle were later shown in organisms from bacteria to higher plants.
The steps:
Algae photosynthesised in a glass vessel nicknamed the "Lollipop" for its shape.
The team injected ¹⁴CO₂ into the gas stream for a set time, from fractions of a second to many minutes.
At the end of that time the cells were killed, stopping all reactions.
The extract was spread on filter paper by two-dimensional paper chromatography, and the paper was laid on film so that radioactive spots showed up.
After 30 seconds, the label was already in many compounds. After 5 seconds, sugar phosphates dominated. In fractions of a second, a single compound held over 80 or 90 per cent of the fixed radioactive carbon: phosphoglyceric acid. To show PGA was really present in living cells and not made during killing, the team checked that many different killing methods gave the same result.
Where does PGA come from?
Finding PGA first raised a new question: what does CO₂ combine with to make it? The answer came from turning the light off.
The lecture describes data from Peter Massini, refined by James Bassham. When the light went off, the PGA pool rose sharply while the pool of ribulose diphosphate (RuBP) fell. In the dark, PGA could no longer be reduced to sugar, but RuBP kept reacting with CO₂ to make PGA, and no new RuBP was being made. That pointed to RuBP + CO₂ giving PGA. The model then predicted the reverse: cut CO₂ while the light is on, and RuBP should pile up while PGA falls. Calvin's colleagues, starting with Andrew Benson, did much of this work with him.
Worked example: counting carbons in the Calvin cycle
NCERT gives the inputs: for every CO₂ entering the cycle, 3 ATP and 2 NADPH are needed, and making one glucose takes 6 turns.
Fixation, 6 turns: 6 CO₂ (6 C) + 6 RuBP (6 × 5 = 30 C) = 36 C.
Product: RuBisCO makes 2 PGA per CO₂, so 12 PGA (12 × 3 = 36 C). Carbon balances.
Output: one glucose takes 6 C, leaving 30 C to rebuild 6 RuBP.
Energy: 6 × 3 = 18 ATP and 6 × 2 = 12 NADPH, which matches NCERT's table.
Common mistakes:
One turn per glucose. Each turn fixes one CO₂, so a 6-carbon sugar needs six.
Thinking "dark reaction" means night. NCERT itself asks whether the name is a misnomer: the cycle depends on ATP and NADPH from the light reactions and stops soon after the light goes off, exactly as Calvin's pool data showed.
Forgetting regeneration. Most of the carbon goes back into RuBP, not into sugar.
Our guide to photosynthesis for NEET covers the full chapter, including C4 plants. For the physics of carbon-14 itself, see our post on radioactive decay and half-life.
What can a student take from Calvin's way of working?
Calvin's method had three moves. He found a tool that could separate new carbon from old. He shortened the time window step by step until one compound stood out. Then he disturbed the system, by switching off the light or cutting the CO₂, and watched which pools rose and fell. Each prediction was checked by experiment.
Use the same moves on a pathway you are learning:
Trace one atom. Follow a single carbon from CO₂ to glucose, writing each compound it passes through.
Ask what happens if you block a step. For each enzyme, say which compound would build up and which would drop.
Check against the textbook. Compare your predictions with NCERT's figures and correct them.
When a pathway diagram feels like a maze, a moving picture helps. The Fermi Simulation Lab has interactive simulations for ideas that are easier to see than to read.
Tonight, predict what happens to PGA and RuBP when CO₂ is cut off in light, then check it against Calvin's result above.
