Cell BiologySimulator20 minLesson 6 of 7

Cellular Respiration

Every contraction, thought, and cell division runs on ATP. Your cells make it with a three-stage pipeline where every step depends on the next. Start by breaking it — take away oxygen, add a poison — and watch what fails. Then step through how it works.

What if?

Break the pipeline

The map shows one glucose molecule's path through a working cell. Flip a switch to remove oxygen or add a drug, then follow the red mark to see where the pipeline jams — and how the problem spreads backward to the other stages.

CYTOPLASMMITOCHONDRIONGlucoseGlycolysis+2 ATPPyruvateFermentation→ lactateCO₂Krebs+2 ATPNADHIIIIIVETCO₂→ H₂OH⁺H⁺H⁺H⁺H⁺ATP synthase+32 ATP
ATP per glucose36
O₂ use100%
H⁺ gradientnormal
CO₂ released100%
Heatnormal
Lactatenone

Change the conditions

What's happening
  • Everything is running. O₂ accepts electrons at the end of the chain, the H⁺ gradient drives ATP synthase, and one glucose yields about 36 ATP.

Challenges · 0 / 3

  • Not yet done: Stop the electron transport chain while oxygen is still available.

  • Not yet done: Make the cell use MORE oxygen while making LESS ATP.

  • Not yet done: Push the H⁺ gradient to its maximum.

How it works

Step through the three stages

CYTOPLASMMITOCHONDRIONMATRIXC₆H₁₂O₆GlucoseATP0NADH0FADH₂0running total
drag to explore stages

← drag right to advance stages →

Glucose
Electron Transport Chain

Click a stage tab or drag the diagram to explore

Stage 1 of 4

Glucose

The starting material

One glucose molecule (C₆H₁₂O₆) enters cellular respiration. All of its chemical energy will be extracted across three sequential stages. By the end, all 6 carbons will have been released as CO₂, and ~36 ATP will have been produced.

  • Glucose contains 6 carbons and significant stored chemical energy
  • Overall equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~36–38 ATP
  • All three stages are needed to extract the full ATP yield — glycolysis alone gives just 2
Predict before you continue

Where in the cell do you think glucose starts to be broken down?

Three things to keep straight

  • Most ATP comes last

    Glycolysis yields just 2 ATP. The Krebs cycle adds 2 more. The electron transport chain produces ~32 — about 89% of the total. The first two stages matter mainly because they generate NADH and FADH₂ to power the ETC.

  • Location = stage

    Glycolysis happens in the cytoplasm (no organelle needed). The Krebs cycle runs in the mitochondrial matrix. The ETC sits on the inner mitochondrial membrane. Each stage feeds the next.

  • Oxygen is the finish line

    O₂ doesn't participate until the very last step — accepting electrons at Complex IV. Without it, the entire ETC backs up. ATP production drops from ~36 to just 2, forcing the cell into fermentation.

At a glance

StageLocationATP
GlycolysisCytoplasm2 net
Krebs CycleMitochondrial matrix2
ETCInner mito. membrane~32
Total—~36
Without oxygen

What happens without oxygen? Fermentation.

When O₂ runs out, the ETC and Krebs cycle shut down. Cells fall back on fermentation — a shortcut that regenerates NAD⁺ so glycolysis can keep running. In muscle cells this produces lactic acid (the burn during intense exercise). In yeast it produces ethanol and CO₂. Either way, yield drops to just 2 ATP per glucose — 18× less than aerobic respiration.

Quick recap

  1. 1

    Glycolysis — Splits glucose (C₆) into 2 pyruvate (C₃) in the cytoplasm. Net 2 ATP + 2 NADH. No oxygen needed.

  2. 2

    Pyruvate oxidation — Each pyruvate loses one carbon as CO₂ and becomes Acetyl-CoA. Produces 1 NADH per pyruvate. Happens twice.

  3. 3

    Krebs cycle — Acetyl-CoA enters the cycle in the matrix. Per glucose: 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂. All carbon released.

  4. 4

    ETC — NADH and FADH₂ electrons drive H⁺ pumping. H⁺ gradient spins ATP synthase (chemiosmosis). ~32 ATP.

  5. 5

    Oxygen's role — Final electron acceptor at Complex IV. O₂ + 4H⁺ + 4e⁻ → 2H₂O. Without O₂, the chain backs up and stops.