How it Works
The Krebs cycle (citric acid cycle) is a closed loop of eight enzyme-catalyzed reactions that occur in the mitochondrial matrix. Each turn begins when acetyl-CoA condenses with oxaloacetate to form citrate, and proceeds through isocitrate, α-ketoglutarate, succinyl-CoA, succinate, fumarate and malate before regenerating oxaloacetate to start again. Along the way, the carbons of the acetyl group are progressively oxidized, releasing two molecules of CO2 at the isocitrate→α-ketoglutarate and α-ketoglutarate→succinyl-CoA steps, while high-energy electrons are captured by the carriers NAD+ and FAD.
The cycle cannot run faster than its supply of oxidized carriers allows: each lap consumes NAD+ and FAD, converting them to NADH and FADH2. In real cells, the electron transport chain (ETC) reoxidizes these carriers by passing their electrons to oxygen, regenerating NAD+/FAD so the cycle can continue. This simulation represents that dependency directly — when oxygen is present, the carrier pool regenerates every frame; when oxygen is switched off, the pool only depletes as it is used, the flux packet slows in proportion to the remaining pool fraction, and the cycle visibly grinds to a halt, mirroring how anaerobic conditions stop oxidative metabolism even though glycolysis can continue independently.
Pool use: pool -= carrierCost at each NADH/FADH2-producing step
Pool regen (O2 on): pool += regenRate · (poolMax - pool)
Pool regen (O2 off): 0
Per full turn: +3 NADH, +1 FADH2, +1 ATP(GTP), +2 CO2
Frequently Asked Questions
What is the Krebs cycle?
The Krebs cycle (citric acid cycle) is a series of eight enzyme-catalyzed reactions in the mitochondrial matrix that oxidize acetyl-CoA to carbon dioxide, capturing the released energy as NADH, FADH2 and one ATP (via GTP) per turn.
How much ATP, NADH and FADH2 does one turn produce?
Each turn of the cycle produces 3 NADH, 1 FADH2 and 1 ATP (as GTP) directly, along with 2 molecules of CO2 released during the two decarboxylation steps.
Why does the cycle need oxygen to keep running?
The Krebs cycle itself does not use oxygen directly, but it depends on a steady supply of NAD+ and FAD. These carriers are only regenerated when the electron transport chain passes electrons to oxygen, so without oxygen the NAD+/FAD pool is not replenished and the cycle stalls.
Where do the two CO2 molecules come from?
CO2 is released at two decarboxylation steps: when isocitrate is converted to alpha-ketoglutarate by isocitrate dehydrogenase, and when alpha-ketoglutarate is converted to succinyl-CoA by the alpha-ketoglutarate dehydrogenase complex.
What happens to oxaloacetate at the end of the cycle?
Oxaloacetate is regenerated when malate is oxidized in the final step. It then condenses with a new acetyl-CoA molecule to form citrate again, allowing the cycle to repeat indefinitely as long as acetyl-CoA and oxidized carriers are available.
Why is the Krebs cycle considered the hub of metabolism?
The Krebs cycle intermediates are shared with amino acid, fatty acid and nucleotide metabolism, so the cycle both harvests energy from acetyl-CoA and supplies precursor molecules for biosynthesis, making it central to overall cellular metabolism.