The amount of ATP (energy) your cells can produce depends heavily on whether oxygen is available.
In aerobic conditions (with oxygen):
The pathway that produces the most ATP is the electron transport chain, which is part of aerobic respiration and takes place in the mitochondria. Here’s how it works:
- First, glucose is broken down during glycolysis in the cytoplasm, producing a small amount of ATP and some molecules called NADH.
- Then, if oxygen is present, the products of glycolysis move into the mitochondria and enter the citric acid cycle (Krebs cycle), generating even more NADH and another molecule called FADH₂.
- These NADH and FADH₂ molecules carry high-energy electrons to the electron transport chain, where the real ATP payoff happens.
- As the electrons move through the chain, they help pump protons across the mitochondrial membrane, creating a gradient that powers ATP synthase, the enzyme that makes ATP.
- Oxygen is crucial here because it acts as the final electron acceptor—without it, the chain would stop.
This whole process can produce up to 36–38 ATP per glucose molecule, most of it coming from the electron transport chain. So in the presence of oxygen, this is by far the most efficient way your cells make energy.
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In anaerobic conditions (without oxygen):
The cell can’t use the electron transport chain without oxygen, so it has to rely on glycolysis alone.
- Glycolysis still happens in the cytoplasm and breaks glucose into pyruvate, producing just 2 ATP per glucose.
- Without oxygen, pyruvate can't go into the citric acid cycle or the electron transport chain.
- Instead, the cell uses fermentation to convert pyruvate into lactate (in animal cells) or other products (like alcohol in yeast), and this process regenerates NAD⁺ so glycolysis can keep going.
Even though it’s much less efficient, this is the only way to keep making ATP without oxygen. So in anaerobic conditions, glycolysis is the main (and only) source of ATP.
