The oldest way a cell makes ATP takes a six-carbon sugar apart into two three-carbon pieces and nets two ATP from the process, which turns out to be a strikingly poor return.
The previous lesson established that ATP is a currency held far from equilibrium by continuous resupply, worth about 50 kJ/mol inside a cell. This lesson is the first supply route: glycolysis, ten enzymatic steps in the cytosol, present in bacteria, archaea and eukaryotes alike, and requiring no oxygen, no membrane and no organelle. Its universality is itself an argument that it is very old.
The shape of the pathway
Glycolysis converts one glucose into two pyruvate, and it does so in two halves that behave quite differently.
The first five steps are a preparatory phase, and they cost money. Hexokinase phosphorylates glucose using one ATP. An isomerase converts glucose 6-phosphate to fructose 6-phosphate. Phosphofructokinase spends a second ATP to make fructose 1,6-bisphosphate. Aldolase then cuts that six-carbon molecule in half, and a final isomerase makes the two halves identical, so the cell now holds two molecules of glyceraldehyde 3-phosphate and is two ATP poorer than when it started.
The last five steps are the payoff phase, and everything after the split happens twice. Glyceraldehyde 3-phosphate dehydrogenase oxidises the aldehyde and captures the energy released as a phosphate bond, producing 1,3-bisphosphoglycerate and reducing NAD⁺ to NADH. Phosphoglycerate kinase hands that phosphate to ADP, making the first ATP. Two rearrangements follow, a mutase and then enolase, which produce phosphoenolpyruvate. Pyruvate kinase hands its phosphate to ADP as well, making the second ATP, and leaves pyruvate.
Two ATP made per three-carbon fragment, twice over, is four; minus the two invested, the net is two ATP and two NADH per glucose. That is the entire yield.
Why spend ATP to get ATP
Investing two ATP before earning any looks perverse and is doing three jobs at once.
It traps the substrate. Glucose enters a cell through a transporter that works in either direction, and glucose could leave the same way. Glucose 6-phosphate carries a charge and cannot cross a membrane or pass back through GLUT, so phosphorylation is a one-way door. It also keeps the internal free glucose concentration low, which keeps the transporter running inward.
It commits the molecule. The phosphofructokinase step is the pathway's true commitment point, because everything before it can be diverted to other uses and nothing after it can, and that is exactly where the regulation sits.
And it destabilises the sugar. Putting phosphate groups at both ends of fructose 1,6-bisphosphate sets up a molecule that aldolase can cleave cleanly into two phosphorylated three-carbon pieces. Cutting an unphosphorylated sugar in half would produce fragments with nothing to hold on to and nothing to activate them.
Where the ATP actually comes from
Glycolysis makes ATP by substrate-level phosphorylation: a phosphate group is handed directly from a metabolite to ADP by an enzyme, with no membrane and no gradient involved. For that to work the donor must have a phosphate transfer potential above ATP's, and glycolysis manufactures two such donors.
The first is built in the glyceraldehyde 3-phosphate dehydrogenase step, and it is the cleverest chemistry in the pathway. Oxidising an aldehyde to a carboxylic acid releases roughly 43 kJ/mol, and in a bomb calorimeter that would all become heat. The enzyme instead attacks the aldehyde with a cysteine thiol to form a thioester, oxidises it there with NAD⁺, and then lets inorganic phosphate displace the thioester. The product, 1,3-bisphosphoglycerate, is an acyl phosphate whose hydrolysis is worth kJ/mol, comfortably above ATP's . Phosphoglycerate kinase then transfers that phosphate to ADP with kJ/mol, favourable and effectively irreversible under cellular conditions. The energy of an oxidation has been captured as a phosphate bond rather than lost as heat, and note that the phosphate came from free inorganic phosphate in solution, not from any ATP.
The second donor is phosphoenolpyruvate, whose hydrolysis is worth kJ/mol, twice ATP's. The reason is not the phosphate but what happens after it leaves: the enol that remains immediately tautomerises to the far more stable keto form of pyruvate, and that rearrangement, which has nothing to do with phosphorus, supplies most of the driving force. Enolase's job in the previous step is simply to create a molecule that is trapped in its enol form by the phosphate group, storing the tautomerisation until a kinase can spend it.
Example. Glucose fully burned to carbon dioxide and water releases 2870 kJ/mol. Fermenting it to two lactate releases 196 kJ/mol. Glycolysis captures two ATP. Assess how good the pathway is, using both the tabulated and the cellular value of ATP.
Two ATP at the standard kJ/mol is 61 kJ. Against the 196 kJ that fermentation actually releases, that is per cent captured, which is respectable and comparable to a petrol engine. Against the 2870 kJ that was available in the glucose, it is per cent. Using the cellular value of about 50 kJ per ATP the second figure rises to per cent, and neither version changes the verdict. The two ratios say different things and both are worth holding. Glycolysis is efficient at what it attempts and attempts almost nothing: it takes glucose only as far as pyruvate or lactate, molecules that are still highly reduced and still burnable, so around ninety-six per cent of the chemical energy walks out of the pathway untouched. The reason is visible in the chemistry. Only one oxidation happens in the whole of glycolysis, the GAPDH step, and a carbon atom can be oxidised much further than that. Getting the rest requires stripping every hydrogen off the carbon skeleton, and that needs a terminal electron acceptor the pathway does not have.
Now you. A yeast fermenting sugar to ethanol and a muscle fermenting it to lactate both net exactly two ATP per glucose, despite making completely different products. What does that tell you about where the ATP is made, and what are the two branches actually for?
Answer
It tells you that all the ATP is made upstream of the branch point, which is pyruvate. Everything from glucose to pyruvate is identical in both organisms, and that is where both substrate-level phosphorylation steps sit, so whatever happens to pyruvate afterwards cannot change the yield. The branches therefore exist for some other reason, and the reason is NAD⁺. Both routes take the NADH that GAPDH produced and reoxidise it: lactate dehydrogenase does it in one step by reducing pyruvate, while yeast first removes carbon dioxide to make acetaldehyde and then reduces that to ethanol. Neither branch yields any energy at all, and both in fact throw away the reducing power that NADH represents. That is the correct way to think about fermentation. It is not an energy-producing process, it is a disposal process that exists so the energy-producing steps upstream can keep running, and the ethanol or the lactate is the bill for it.
The bottleneck nobody sees at first
The NAD⁺ requirement is the constraint that shapes the whole of anaerobic metabolism, and its size is easy to underestimate until it is counted.
A cell holds NAD⁺ and NADH together at roughly 0.5 to 1 mM. Glycolysis consumes two NAD⁺ per glucose. A muscle working hard runs glycolysis at something like 1 mM of glucose per second, so it consumes NAD⁺ at 2 mM per second. Against a pool of 0.5 mM, that is complete exhaustion in a quarter of a second.
NAD⁺ is therefore not a reagent but a catalyst that must be turned over hundreds of times a second, and glycolysis stops dead the moment it is all in the reduced form. This is why fermentation exists, and it explains a fact that otherwise looks like waste: the cell discards a molecule, lactate or ethanol, that still contains almost all the energy it started with, purely to get its NAD⁺ back.
Lactate is not a waste product in the ordinary sense. It leaves the muscle, travels in the blood, and is either oxidised by the heart and by other muscle fibres, which take it up and convert it back to pyruvate, or reconverted to glucose by the liver in the Cori cycle at a cost of six ATP per glucose. It is a way of exporting a metabolic problem to a tissue with more oxygen. It is also worth correcting the folklore: lactate does not cause the muscle soreness felt a day or two after exercise, which is inflammation from microscopic muscle damage, and blood lactate is back to baseline within an hour. The burning felt during hard exercise tracks acidification and other metabolites rather than lactate itself, and in fact lactate production consumes a proton rather than producing one.
Example. Yeast fermenting grape juice stops on its own at around 14 to 16 per cent ethanol, which is why fortified wine has to be fortified. Nothing has run out. What has gone wrong?
The yeast has poisoned itself with its own product, and the mechanism is the subject of the second lesson of this course. Ethanol is a small molecule that partitions readily into a lipid bilayer, and at high concentration it disorders the membrane, increases its permeability, and destroys the ion and proton gradients the cell maintains across it. The membrane stops being a barrier, so the cell can no longer hold a composition different from its surroundings, and it dies. This has two consequences worth drawing out. First, a fermentation product is not just a discarded electron sink, it accumulates in the medium and its toxicity sets a hard ceiling on the process, which is exactly the problem industrial ethanol production spends money on. Second, the ceiling varies with strain: wine yeasts tolerate more ethanol than bakers' yeast does, and the difference is largely in membrane lipid composition, including how much ergosterol they can make. A cell's tolerance to a solvent is a property of its membrane, not of its metabolism.
Now you. Cancer cells often ferment glucose to lactate even when oxygen is plentiful, a pattern Otto Warburg described in the 1920s. Fermentation yields fifteen times less ATP per glucose than full oxidation does. Suggest what could make it worth doing anyway.
Answer
Two answers are well supported and a third is speculative, and it is worth being clear which is which. The first is rate rather than yield: glycolysis has few steps, needs no membrane, and can be scaled up simply by making more enzyme, so it can deliver ATP per unit time far faster than oxidative phosphorylation, which is limited by mitochondrial membrane area. A cell competing to divide cares about ATP per second, not ATP per glucose, and glucose is abundant in a tumour. The second is that a dividing cell needs carbon skeletons and reducing power at least as much as it needs ATP: glycolytic intermediates are the precursors of ribose, serine, glycerol and fatty acids, and running high flux through the pathway while diverting intermediates out of it supplies the building blocks for a new cell. Fermenting is partly a way of keeping the flux high without oxidising the carbon away. The third and less settled suggestion is that the exported lactate acidifies the surrounding tissue in ways that favour invasion and suppress immune cells. Warburg's own conclusion, that the mitochondria of cancer cells are damaged and that this causes cancer, was wrong: most tumour cells have functional mitochondria and use them.
Why a bad pathway is universal
Glycolysis captures about three per cent of the energy in its fuel and every organism on Earth still runs it. Several things explain that.
It works without oxygen, and for most of the history of life there was no oxygen to work with. Free atmospheric oxygen appeared only around 2.4 billion years ago, and a pathway that predates it cannot depend on it.
It is fast and simple. Ten soluble enzymes in the cytosol, no membrane, no compartment, no cofactor more exotic than NAD⁺ and ATP. A cell can raise glycolytic flux enormously just by expressing more enzyme, and a muscle fibre in a sprint does exactly that.
It is a supply depot as much as an energy pathway. Glucose 6-phosphate feeds the pentose phosphate pathway, which makes ribose for nucleotides and NADPH for biosynthesis. Dihydroxyacetone phosphate becomes the glycerol backbone of lipids. 3-phosphoglycerate becomes serine, glycine and cysteine. Pyruvate becomes alanine. A cell that shut down glycolysis would lose the raw material for its own construction, not merely some ATP.
And it is the front end of the aerobic route as well. Everything the next lesson describes begins with the pyruvate that glycolysis delivers, so the pathway was never replaced, only extended.
Regulation, and where the control point sits
A pathway with a committed step will be regulated at that step, and phosphofructokinase is the classic case in all of biochemistry.
It is inhibited by ATP, which is the direct feedback: a cell with plenty of ATP does not need to burn sugar. It is activated by AMP, which is a much better signal of energy shortage than ADP because the enzyme adenylate kinase converts two ADP into one ATP and one AMP, so a small percentage fall in ATP produces a large percentage rise in AMP. Reading a small fractional change on a small pool is a general trick for sensitive control. It is also inhibited by citrate, an intermediate of the citric acid cycle, which reports that the downstream pathway is already saturated, and in the liver it is powerfully activated by fructose 2,6-bisphosphate, a molecule that exists for no purpose except signalling and whose concentration is set by hormones.
Example. A muscle holds 5 mM ATP, 0.5 mM ADP and 0.05 mM AMP, and adenylate kinase keeps near 1. Suppose demand causes ATP to fall by ten per cent, with the lost ATP appearing as ADP. What happens to AMP, and why does the enzyme read AMP rather than ATP?
ATP goes from 5 to 4.5 mM, a fall of ten per cent, and ADP goes from 0.5 to 1.0 mM, a doubling. Adenylate kinase then sets mM, against 0.05 mM before. AMP has risen 4.4-fold from a ten per cent fall in ATP. The reason is arithmetic rather than biological: ATP is the large pool and AMP the small one, so the same absolute movement of adenylate is a small fractional change in the first and a large one in the second, and the squared term in the equilibrium amplifies it further. An enzyme trying to detect energy shortage by measuring ATP would be looking for a ten per cent change against a noisy background, while one measuring AMP sees a fourfold signal. This is a general design rule for any sensor: read the species whose fractional change is largest, which is almost always the least abundant member of a conserved pool.
Now you. Metformin, the most widely prescribed drug for type 2 diabetes, mildly inhibits mitochondrial complex I. Given the example above and what a cell does with an AMP signal, sketch the chain from that inhibition to a metabolic effect.
Answer
Partially inhibiting complex I lowers the rate of ATP regeneration, so ATP falls slightly and, by the amplification just derived, AMP rises substantially. The cell has a dedicated reader for that signal, AMP-activated protein kinase, which is switched on by AMP binding and which then acts as a general low-energy alarm: it activates catabolic pathways such as glucose uptake and fatty acid oxidation and inhibits expensive biosynthetic ones such as fatty acid and cholesterol synthesis, and in the liver it suppresses gluconeogenesis, which is the main way metformin lowers blood glucose. The chain is worth noticing for two reasons. The drug never touches glucose metabolism directly; it acts on the respiratory chain and the effect arrives through an energy signal, in the same indirect way digoxin acted through a shared gradient in an earlier lesson. And the honest caveat is that the AMP-activated kinase route is well supported but is probably not the whole story for metformin, since effects have been reported in cells lacking that kinase, so this should be read as the main proposed mechanism rather than a settled one.
Hexokinase is inhibited by its own product, and pyruvate kinase is regulated too, but neither controls the flux the way phosphofructokinase does. The general lesson carries beyond this pathway: control is exerted where commitment happens, and the regulators are chosen so that the enzyme hears both the local state of the pathway and the global state of the cell.
The unavoidable conclusion of this lesson is the accounting. Pyruvate still holds roughly ninety-six per cent of the energy that was in the glucose, sitting in carbon and hydrogen that have not been oxidised. Getting at it means removing every one of those hydrogens and finding somewhere to put the electrons, and doing that with a soluble enzyme handing phosphate to ADP is not possible: no substrate-level step can capture 200 kJ in one bite. The next lesson takes the electrons instead of the phosphate, spends them on a membrane, and turns the resulting gradient back into ATP, which is a mechanism so unlike anything in this lesson that its author was disbelieved for a decade.