Glycolysis
Glycolysis is the metabolic pathway every living cell on Earth relies on to extract energy from glucose. It is a sequence of exactly ten reactions, each one driven by a specific enzyme, converting a single molecule of glucose into two molecules of pyruvate. The free energy released along the way builds two kinds of molecular currency: ATP, the cell's primary power unit, and NADH, a carrier of energetic electrons. What makes glycolysis remarkable is not complexity but age. The reactions can take place in the oxygen-free conditions that characterized the ancient Archean oceans, even without enzymes, catalyzed simply by metal ions. That suggests glycolysis may have existed before life as we know it. How did scientists piece together a pathway so fundamental, and so fast that its intermediates last only fractions of a second? And what happens when cancer hijacks it?
Louis Pasteur, working in France during the 1850s, was the first to connect fermentation to living organisms. The French wine industry needed to know why wine spoiled rather than fermenting into alcohol. Pasteur showed that alcohol fermentation is driven by the action of living yeasts, and that glucose consumption drops under aerobic conditions, an observation now called the Pasteur effect.
Eduard Buchner upended that understanding in the 1890s. His non-cellular fermentation experiments showed that a lifeless extract of yeast, containing only enzymes, could still convert glucose to ethanol. That result transformed biochemistry, because it made controlled laboratory analysis of individual steps possible. Between 1905 and 1911, Arthur Harden and William Young built on Buchner's work. They discovered that ATP regulates glucose consumption during fermentation, and they identified fructose 1,6-bisphosphate as a glycolytic intermediate. Harden and Young, working later with Nick Sheppard, further showed that fermentation requires two separate fractions: a heat-sensitive high-molecular-weight fraction carrying the enzymes, and a heat-insensitive low-molecular-weight fraction carrying cofactors such as ADP, ATP, and NAD+.
In the 1920s, Otto Meyerhof linked the individual discoveries into a connected sequence. He extracted glycolytic enzymes from muscle tissue and combined them to recreate the pathway from glycogen to lactic acid outside the body. Working with Renate Junowicz-Kockolaty, Meyerhof also ruled out 1,3-diphosphoglyceraldehyde as an intermediate in the splitting of fructose 1,6-diphosphate. By the 1930s, Gustav Embden proposed the step-by-step outline that scientists now call glycolysis. The biggest obstacle had been the vanishingly short lifetime and low concentration of the fast-reaction intermediates. By the 1940s, Meyerhof, Embden, and many colleagues had finally closed the puzzle.
The ten steps of glycolysis split cleanly into two halves. The first five form the preparatory phase, in which the cell spends ATP to convert glucose into two three-carbon sugar phosphates. The first step has hexokinase using one ATP molecule to phosphorylate glucose into glucose-6-phosphate. That phosphorylation does double duty: it traps glucose inside the cell, since the membrane has no transporter for the charged product, and it keeps intracellular glucose concentration low, which continuously pulls more glucose in from the blood.
Phosphofructokinase drives step three, spending a second ATP to convert fructose-6-phosphate into fructose-1,6-bisphosphate. This is an irreversible step, which makes it a key control point. Aldolase then splits that six-carbon molecule into two three-carbon sugars, dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. Triose phosphate isomerase rapidly converts the former into the latter, so both fragments feed into the second half.
The pay-off phase runs steps six through ten and nets the cell 2 NADH molecules and 4 ATP molecules per glucose. Since 2 ATP were spent in the preparatory phase, the net gain is 2 ATP and 2 NADH per glucose molecule. Step seven, catalyzed by phosphoglycerate kinase, is where glycolysis breaks even: 2 ATP consumed, 2 newly synthesized. The final step has pyruvate kinase generating one more ATP per triose, producing pyruvate, the molecule that feeds downstream pathways. Three of the ten steps carry large negative free energy changes and are effectively irreversible; measurements on red blood cells show the other seven run close to equilibrium in living cells.
Three enzymes govern the rate of glycolysis: hexokinase (or glucokinase in the liver), phosphofructokinase, and pyruvate kinase. Each sits at a point where the pathway becomes irreversible, and each responds to different signals.
Phosphofructokinase is a particularly sensitive switch. Its most potent activator is fructose 2,6-bisphosphate, synthesized by a second enzyme called PFK2. When blood sugar falls and glucagon rises, protein kinase A phosphorylates PFK2, inactivating it and simultaneously activating a phosphatase domain on the same protein. That phosphatase converts fructose 2,6-bisphosphate back to fructose-6-phosphate, reducing phosphofructokinase activity and slowing glycolysis. The liver then shifts toward releasing glucose rather than consuming it. AMP also activates phosphofructokinase, and a 10% drop in ATP concentration causes a roughly 6-fold rise in AMP, making AMP a sensitive indicator of the cell's energy state.
In the liver, pyruvate kinase is regulated by insulin and glucagon operating through opposing phosphorylation cascades. During fasting, glucagon activates protein kinase A, which phosphorylates pyruvate kinase to inhibit it. After a meal, insulin triggers protein phosphatase 1, which dephosphorylates and reactivates the enzyme. Muscle pyruvate kinase, by contrast, does not respond to protein kinase A, so glycolysis in muscle continues even during fasting. An additional layer of control comes from TIGAR, a p53-induced enzyme encoded by the C12orf5 gene. TIGAR reduces fructose 2,6-bisphosphate levels, slowing glycolysis and shunting carbon into the pentose phosphate pathway, a response linked to protection against oxidative stress.
Glycolysis produces pyruvate, but the pathway stalls if NADH cannot be re-oxidized back to NAD+. In the presence of oxygen, electrons from NADH travel through the mitochondrial electron transport chain, ultimately producing about 2.5 ATP for every NADH oxidized. Under low oxygen, organisms use fermentation instead.
In lactic acid fermentation, pyruvate accepts electrons from NADH directly, forming lactate and regenerating NAD+. This is the process used by bacteria that make yogurt and by human muscle cells during intense exercise. The burning sensation in muscles during hard effort traces to hydrogen ions released as glucose fermentation replaces aerobic oxidation. Anoxic NAD+ regeneration in vertebrate muscle can sustain maximal effort for a period ranging from 10 seconds to 2 minutes. The ATP yield is only 2 molecules per glucose, roughly 5% of the 38 molecules that aerobic respiration can extract in bacteria. However, the rate of ATP production by this route is about 100 times faster than oxidative phosphorylation, which explains why sprint athletes depend on it.
Yeast take a different route: ethanol fermentation. Pyruvate is first converted to acetaldehyde and carbon dioxide, then to ethanol. Both fermentation routes allow single-celled organisms to run glycolysis as their sole energy source. The liver handles excess lactate by converting it back to pyruvate in aerobic conditions, a recovery loop called the Cori cycle. Glycolysis itself, strictly defined, ends with pyruvate regardless of what follows.
Malignant tumor cells run glycolysis at a rate ten times faster than their healthy counterparts. Otto Warburg first described this phenomenon in 1930. Tumors often face limited blood supply, producing hypoxia. Under low oxygen, they depend on glycolysis for ATP. Some tumor cells overexpress isoforms of glycolytic enzymes that resist normal feedback inhibition, sustaining high glycolytic flux. The Warburg hypothesis goes further, proposing that cancer arises primarily from dysfunctional mitochondrial metabolism rather than from uncontrolled cell growth alone.
The clinical consequence is practical: the high glucose uptake by tumors is exploited in positron emission tomography. A radioactive modified hexokinase substrate, 2-18F-2-deoxyglucose (FDG), accumulates in glycolytically active tissue. PET imaging of FDG uptake allows clinicians to locate tumors and track how they respond to treatment. Ongoing research explores reducing glycolytic flux in tumors by dietary means, including ketogenic diets, as a way to limit the energy available to cancer cells.
Inherited mutations in glycolytic enzymes are rare, because disrupting this pathway usually kills the cell early. One documented exception is pyruvate kinase deficiency, which causes chronic hemolytic anemia. Another is combined malonic and methylmalonic aciduria caused by ACSF3 deficiency; in that condition, glycolysis is reduced by 50%, caused by impaired lipoylation of the pyruvate dehydrogenase complex and the alpha-ketoglutarate dehydrogenase complex. Diabetes provides yet another angle: when insulin is absent or resisted, glucose cannot be taken up and broken down normally, and the liver adds to the problem through gluconeogenesis, driving blood sugar higher. The connection between glycolytic control in hepatocytes and systemic blood glucose regulation sits at the center of diabetes research.
Common questions
How many steps does glycolysis have?
Glycolysis consists of exactly ten reactions, each catalyzed by a specific enzyme. The first five form the preparatory phase that consumes ATP, and the final five form the pay-off phase that generates more ATP than was spent.
What does glycolysis produce from one molecule of glucose?
Each glucose molecule yields two pyruvate molecules, two NADH molecules, and a net gain of two ATP molecules. The two ATP gained represent the difference between four ATP produced and two ATP consumed in the early steps.
Who discovered glycolysis?
Understanding glycolysis took nearly 100 years of combined effort. Louis Pasteur linked fermentation to living organisms in the 1850s. Eduard Buchner showed that enzymes alone could drive glucose breakdown in the 1890s. Arthur Harden and William Young identified key intermediates and cofactors between 1905 and 1911. Otto Meyerhof connected the steps in the 1920s, and by the 1940s the full pathway, now called the Embden-Meyerhof-Parnas pathway, was complete.
Why can glycolysis happen without oxygen?
Glycolysis itself does not use oxygen. It operates entirely in the cytosol, converting glucose to pyruvate without any oxygen-dependent steps. Oxygen is needed only in downstream processes such as the citric acid cycle and electron transport chain. Under low oxygen, organisms use fermentation to regenerate the NAD+ that glycolysis needs to continue.
What is the Warburg effect?
The Warburg effect, first described by Otto Warburg in 1930, is the observation that malignant tumor cells perform glycolysis at a rate about ten times faster than normal cells, even when oxygen is available. This elevated glycolytic activity is exploited clinically: a radioactive glucose analog called FDG accumulates in tumors and is detected by PET scanning to locate and monitor cancers.
How ancient is glycolysis?
Glycolysis is considered an ancient metabolic pathway. The reactions that make up glycolysis can take place in oxygen-free conditions resembling the Archean oceans, and can proceed without enzymes, driven only by metal ions. This makes glycolysis a plausible prebiotic pathway that may have originated before complex life existed.