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When you sprint, your muscles make lactic acid. Where does it go, and how does your body turn stored glycogen into usable sugar in the first place? Gerty and Carl Cori spent their careers on these questions. This post covers their lives, the Cori ester and the Cori cycle, a worked example linking their work to the NCERT respiration chapter, and what a student can learn from how they worked.
Who were Gerty and Carl Cori?
Both were born in Prague in 1896: Gerty Radnitz on 15 August and Carl Cori on 5 December. Their Nobel biography records that they studied medicine together at the German University of Prague, graduated as doctors in 1920 and married the same year. They emigrated to the United States, where Carl took a position in Buffalo, New York, in 1922, and Gerty joined him. In 1931 Carl became Professor of Pharmacology at Washington University in St. Louis. Gerty joined him there as a research associate and was made Professor of Biochemistry in 1947.
The biography says they collaborated on most of their research, starting in their student days.
What did the Coris win the Nobel Prize for?
The 1947 Nobel Prize in Physiology or Medicine went to Carl and Gerty Cori "for their discovery of the course of the catalytic conversion of glycogen". They shared it with Bernardo Houssay, who was honoured for separate work on the pituitary. The Nobel Prize's list of women laureates shows Gerty Cori as the first woman to receive the prize in Physiology or Medicine. She died on 26 October 1957.
What is the Cori ester?
Glycogen is the chain of glucose units that animals store in liver and muscle. The presentation speech at the 1947 ceremony describes how the Coris found a new sugar phosphate in washed muscle. By crystallising it and working out its structure, they showed that its phosphate was attached to the first carbon of glucose instead of the sixth. This glucose-1-phosphate became known as the Cori ester.
It appeared only in washed muscle because washing removed an enzyme that moves the phosphate from position 1 to position 6. The enzyme that forms the Cori ester, phosphorylase, can run in both directions: it splits glycogen into glucose-1-phosphate using phosphate, or builds glycogen from the Cori ester. Which way it goes depends on the relative amounts of the components, and building glycogen needs a small amount already present to build on. The biography adds that this made it possible to synthesise glycogen and starch in a test tube.
What is the Cori cycle?
In 1929 the Coris published Glycogen formation in the liver from d- and l-lactic acid in the Journal of Biological Chemistry. That line of work is behind what is now called the Cori cycle. A 2021 review in Clinical Nutrition describes it this way: glucose is broken down in muscle to pyruvate and lactate, and the lactate is reused to make glucose in the liver and kidney, which completes the Cori cycle.
The muscle half is in your syllabus. The NCERT Class 11 chapter Respiration in Plants explains that in muscles during exercise, when oxygen is inadequate, pyruvic acid is reduced to lactic acid by lactate dehydrogenase, with NADH reoxidised to NAD⁺. NCERT does not name the Cori cycle or the Cori ester, so treat them as background that helps the chapter make sense.
Worked example: why send lactate back to the liver?
NCERT gives the numbers you need.
In lactic acid fermentation, NCERT says "less than seven per cent of the energy in glucose is released", and the net gain is only two ATP per glucose.
In aerobic respiration, NCERT's balance sheet gives a net gain of 38 ATP per glucose, on stated assumptions.
Compare: 2 / 38 ≈ 0.053, so fermentation yields about 5 per cent of the ATP the full pathway could.
So most of the energy is still locked in the lactate. Throwing it away would waste fuel. Shipping it to the liver, where it can be turned back into glucose, keeps the carbon in use.
Common mistakes:
Calling lactic acid "waste". It is a partly used fuel, which is the point of the cycle.
Mixing up the phosphates. The Cori ester is glucose-1-phosphate; glycolysis uses glucose-6-phosphate.
Treating 38 ATP as exact. NCERT itself says the balance sheet is a theoretical exercise built on assumptions that are not really valid in a living system.
The next stage of the story, what happens to pyruvate when oxygen is available, is in our post on Hans Krebs and the citric acid cycle. For the full NEET chapter, see our guide to respiration in plants.
What can a student take from the Coris' way of working?
Their biography describes a steady narrowing of the question: from whole animals, to isolated tissues, to tissue extracts, and finally to isolated enzymes, some in crystalline form. Each step removed a layer of complication. The Cori ester itself came from noticing that washed muscle behaved differently, and asking why.
A routine built on that approach:
Narrow the question. When a whole chapter feels confusing, pick one reaction and learn it fully before moving to the pathway.
Notice odd results. When a practice answer surprises you, write down what was different about that question.
Study with a partner. Explain one step each and check each other, the way the Coris divided and shared their work.
When one step in a pathway will not stick, it helps to see it as a concept on its own. The Fermi Concept Library has every concept in biology and chemistry with its own lesson, so you can learn a single reaction and return to the chapter.
Tonight, draw the Cori cycle as two boxes, muscle and liver, and label what moves between them.
