Glycolysis — the ancient cytoplasmic engine of glucose.
Before oxygen-rich skies, before mitochondria became elegant power stations, life still needed a way to pull useful work from carbon. Glycolysis is that old engine. It happens in the cytoplasm, does not directly require oxygen, and turns one six-carbon glucose into two three-carbon pyruvates while making ATP and NADH. The real story is not “sugar burns.” The real story is carbon routing, phosphate tagging, electron capture, redox pressure, and a set of metabolic doors that open depending on what the cell needs.
Click the pathway like a theatre of carbon.
Every step has reveal panels for mechanism, carbon fate, phosphate/charge, enzyme logic, and pathway consequences. The offshoot section lets you click a branch and see what happens to glucose carbon when it leaves the main road.
The pills are not decoration — they are the map key.
Same colour, same meaning. The learner can scan the pathway and instantly see the enzyme worker, molecule traveller, ATP cost, ATP gain, electron movement, charge trapping, control gates, reversible reactions, and branch routes.
Protein catalyst. The worker shaping the reaction’s probability and direction.
The traveller/intermediate being chemically transformed.
Energy investment. ATP is spent to trap or activate carbon.
Energy payout. ATP is made by direct phosphate transfer to ADP.
Electron harvest. NAD⁺ catches reducing power and becomes NADH.
Phosphate adds negative charge, making molecules less membrane-permeable.
Control point. These steps need bypass routes in gluconeogenesis.
Side route. Carbon leaves glycolysis to build, store, defend, or signal.
The accounting before the theatre begins.
Glycolysis has two moods. First it spends ATP to trap and prepare glucose. Then it splits the molecule and earns ATP and NADH back from two three-carbon streams. It is less like burning a log and more like carefully dismantling a six-carbon necklace into reusable energetic beads.
Carbon flow overview
Net result per glucose
Ten steps, but not ten dead facts.
Each reaction is a little machine. Some steps spend ATP to add charge. Some rearrange the skeleton. One harvests electrons. Two make ATP directly. Three act as control gates. Click through the panels and the pathway stops being a list; it becomes a working circuit.
Click a branch: where do glucose carbons go?
Glucose is not a one-way tunnel to pyruvate. It can become antioxidant currency, ribose for genetic material, glycogen storage, lipid backbones, amino acid skeletons, lactate, alanine, acetyl-CoA, oxaloacetate, cholesterol, and steroid precursors. This section shows the carbon itinerary — the metabolic passport stamps.
The irreversible gates are where the cell votes.
A reversible step is like a swinging door: concentration pressure can push it either way. An irreversible step is more like a one-way turnstile: it commits energy, releases heat, and becomes a regulatory decision point. That is why gluconeogenesis does not simply run glycolysis backward; it has to build detours around the one-way gates.
1 · Hexokinase / Glucokinase
This step traps glucose inside the cell by adding a negatively charged phosphate. Hexokinase is product-inhibited by G6P; glucokinase in liver and pancreatic β-cells behaves more like a high-glucose sensor/storage gate.
3 · PFK-1
This is the main throttle. AMP and fructose-2,6-bisphosphate say “run glycolysis.” ATP and citrate say “slow down, energy/carbon is backed up.” PFK-1 is the parliament where energy status argues with carbon abundance.
10 · Pyruvate Kinase
The final ATP payout commits PEP to pyruvate. It is feed-forward activated by fructose-1,6-bisphosphate, so when the upper pathway is flowing, the lower pathway gets told to clear the runway.
The compact exam-and-mechanism table.
Use this like a quick recovery map. It gives the enzyme, reaction role, energy/electron accounting, reversibility, and the reason the step exists.