From Sugar to Energy: Glycolysis & the Krebs Cycle

Every bite of bread or spoon of honey ends the same way: a single glucose molecule — six carbons of stored sunlight — is taken apart, one careful step at a time, and its energy is repackaged into ATP, the cell's rechargeable battery. Watch glucose split in the cytoplasm through glycolysis (no oxygen needed), watch pyruvate slip inside the mitochondrion, and watch the Krebs cycle turn like a molecular water-wheel — each rotation flinging off carbon dioxide you breathe out and loading up the electron carriers NADH and FADH₂ that feed the final ATP factory.

Try this: press play on Normal, then switch to No oxygen and watch pyruvate turn to lactate as the ATP yield crashes from ~30 down to just 2 — the exact reason a hard sprint burns.

The Science in Plain Language

Three stages stand between sugar and your battery

Turning food into usable energy is not one reaction but a relay. First comes glycolysis in the watery cytoplasm, which splits sugar and needs no oxygen. Then the link reaction (pyruvate dehydrogenase) and the Krebs cycle run inside the mitochondrion, harvesting high-energy electrons. Finally the electron transport chain (ETC) cashes those electrons in for the bulk of the ATP. This page animates the first two stages — the sugar-splitting and the wheel — and hands the electron carriers off to the ETC, which you can explore on the Mitochondria & ATP page.

Glycolysis: splitting sugar in ten steps, no oxygen required

One molecule of glucose (six carbons, formula C₆H₁₂O₆) is walked through ten enzyme steps and split into two pyruvate molecules (three carbons each). The first steps actually cost two ATP — the cell spends money to make money — but the payoff steps generate four, for a net gain of 2 ATP. Along the way, the enzyme GAPDH loads two molecules of NADH. Key players include hexokinase, the rate-setting phosphofructokinase-1 (PFK-1) that commits the sugar to the pathway, and pyruvate kinase at the finish. Because glycolysis needs no oxygen, it is the emergency generator every cell can run — instantly, anywhere.

The pyruvate crossroads: mitochondria or lactate

Pyruvate now faces a fork. When oxygen is available, it is ferried into the mitochondrion by the mitochondrial pyruvate carrier (MPC) to be fully burned. When oxygen runs short — a sprint, a lifted barbell — pyruvate is instead converted to lactate by lactate dehydrogenase. That step regenerates the NAD⁺ that glycolysis needs to keep spinning, so ATP keeps coming fast, but the yield is only the 2 ATP from glycolysis. Switch the animation to No oxygen and you will see pyruvate turn red and the yield collapse. Contrary to old gym lore, lactate is not a waste toxin and does not cause next-day soreness; it is a useful fuel the heart and liver happily recycle.

The link reaction: PDH, and why vitamin B1 matters

Inside the mitochondrion, the pyruvate dehydrogenase (PDH) complex chops one carbon off each pyruvate — released as CO₂ — and attaches the remaining two-carbon fragment to coenzyme A, making acetyl-CoA and one NADH. PDH cannot work without a cofactor called thiamine pyrophosphate (TPP), which your body builds from vitamin B1 (thiamine). This is not trivia: in B1 deficiency, PDH stalls, pyruvate backs up into lactate, and cells starve for energy despite plenty of sugar. Switch to B1 deficiency to watch the gateway freeze and the yield fall toward glycolysis-only levels.

The Krebs cycle: a molecular water-wheel

Acetyl-CoA (2 carbons) joins oxaloacetate (4 carbons) to form citrate (6 carbons) — the first step, run by citrate synthase. The wheel then turns through isocitrate, α-ketoglutarate, succinyl-CoA, succinate, fumarate and malate, arriving back at oxaloacetate ready to grab the next acetyl-CoA. Per turn it releases 2 CO₂ and harvests 3 NADH, 1 FADH₂ and 1 GTP (which is interchangeable with ATP). Because each glucose yields two acetyl-CoA, the cycle turns twice per glucose. Every carbon that entered as sugar leaves as the carbon dioxide you exhale — you literally breathe out yesterday's lunch.

Handing electrons to the ETC — where most ATP is actually made

Here is the twist most people miss: glycolysis and the Krebs cycle make surprisingly little ATP directly. Their real product is the loaded electron carriers NADH and FADH₂. Those carriers drift to the folded inner membrane and dump their electrons into the electron transport chain, which uses the energy to pump protons and spin ATP synthase — and that is where roughly 26 of a glucose's ~30 ATP come from. This is also why oxygen matters: it is the final electron catcher at the end of the chain. No oxygen, no chain, no big payoff — which is exactly the anaerobic scenario in the animation. The full ETC is shown on the Mitochondria & ATP page.

Myth-correction: it was never really "38 ATP"

Generations of textbooks taught that one glucose yields 36–38 ATP. That number is outdated. It assumed every NADH buys exactly 3 ATP and every FADH₂ buys 2, with no losses. Careful measurement shows the true “exchange rate” is closer to 2.5 ATP per NADH and 1.5 per FADH₂, and the mitochondrion spends some of that energy just importing the raw materials. Add it up honestly and a glucose nets about 30–32 ATP — the figure this animation uses. It is a smaller number, but it is the real one, and it is still an astonishing return on a single sugar.

Cofactors you can actually eat

This whole machine runs on vitamins and minerals working as tiny tools. Vitamin B1 (thiamine) forms TPP for PDH and α-ketoglutarate dehydrogenase. Vitamin B2 (riboflavin) becomes FAD, the core of FADH₂. Vitamin B3 (niacin) becomes NAD⁺, the backbone of every NADH. Vitamin B5 (pantothenate) builds coenzyme A. And magnesium is the silent partner of every ATP step: the enzymes do not actually bind ATP, they bind Mg²⁺–ATP. Press Remove Mg²⁺ and glycolysis freezes at its very first phosphate-transfer step — a vivid reminder of why magnesium status quietly shapes energy, and why deficiencies of these nutrients show up as fatigue. Learn more on the Vitamin B1 and Magnesium pages.

When the pathway breaks — and what it explains

Severe thiamine deficiency causes beriberi and, in people with heavy alcohol use or after prolonged vomiting, Wernicke–Korsakoff syndrome — which is why emergency rooms give thiamine before glucose. A stalled Krebs pathway can also produce lactic acidosis, measurable as a high blood lactate. And many cancers deliberately favor glycolysis-to-lactate even when oxygen is plentiful — the Warburg effect — because fast, sloppy energy plus building-block carbons suits rapid growth. Understanding this simple wheel, in other words, quietly underpins a surprising amount of medicine.

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Connections

• All Interactive Visualizations
• Mitochondria & ATP
• Fat Burning & Ketosis
• Magnesium & ATP
• Vitamin B1 (Thiamine)
• Magnesium