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🔋 Cellular Respiration — Krebs Cycle

Animate the citric acid cycle's eight intermediates turning acetyl-CoA into CO2, NADH, FADH2 and ATP; block oxygen to see the cycle stall without a working electron transport chain.

Biology & Life2DEasy60 FPS

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.

Speed factor: v = acetylRate · animSpeed · (pool / poolMax)
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.

About this simulation

A single glowing flux packet chases its own tail around eight named intermediates, releasing tiny "CO₂" labels at the two decarboxylation steps and quietly draining a NAD+/FAD pool bar every time it passes a carrier-producing enzyme — flip the oxygen switch off and watch that bar refuse to refill, the packet slow to a crawl, and the whole cycle stall exactly like a mitochondrion starved of its electron acceptor.

🔬 What it shows

Eight Krebs-cycle intermediates arranged in a ring with a moving flux packet that triggers real per-step stoichiometry: 3 NADH, 1 FADH2 and 1 ATP per full turn, plus two CO2 release events.

🎮 How to use

Adjust Acetyl-CoA Input Rate and Cycle Animation Speed to change how fast the packet moves, tune the NAD+/FAD Pool Size, and toggle Oxygen Present off to see the pool deplete and the cycle stall; P pauses, R resets.

💡 Did you know?

The Krebs cycle itself consumes no oxygen at all — it is entirely the electron transport chain's need for oxygen as a final electron acceptor that indirectly forces the citric acid cycle to halt during anaerobic conditions.

Frequently asked questions