What happens in each stage of cellular respiration?

Glycolysis is the first step and takes place in the cytoplasm, the jelly-like substance filling the cell. The word glycolysis literally means 'sugar splitting.' Here, a six-carbon glucose molecule is chopped into two three-carbon molecules called pyruvate. This process produces a very small amount of ATP and strips a few high-energy electrons from the sugar, handing them off to a carrier molecule called NADH. Because it does not require oxygen, glycolysis is considered an anaerobic process.

If oxygen is present, the pyruvate molecules move into the mitochondria, the powerhouse of the cell. Before entering the next cycle, pyruvate is tweaked into a molecule called Acetyl-CoA. Then, in the mitochondrial matrix, it enters the Krebs cycle (also called the Citric Acid Cycle). The main goal of this stage is not to make ATP directly. Instead, it acts like a furnace, completely breaking down the remains of the glucose molecule to extract as many high-energy electrons as possible. These electrons are loaded onto carrier molecules (NADH and FADH2), while the leftover carbon atoms are released as carbon dioxide gas.

The final stage takes place on the inner membrane of the mitochondria. All those electron carriers (NADH and FADH2) drop off their high-energy electrons at the electron transport chain. As the electrons bounce down the chain, their energy is used to pump protons across the membrane, creating a pressurized gradient. Finally, these protons rush back through a specialized enzyme called ATP synthase, spinning it like a tiny turbine to churn out massive amounts of ATP. Oxygen sits at the very end of this chain to catch the used electrons, combining with protons to form water.

A common mistake is confusing where the carbon goes versus where the energy goes. Remember that the carbon atoms from your food do not turn into energy. The carbon is simply the structure holding the energy (the electrons). By the end of the Krebs cycle, all the carbon from the original glucose molecule has been breathed out as carbon dioxide. The actual energy payout comes later, in the electron transport chain, powered by the electrons that were stripped away from that carbon.