By Kevin Beck – Updated March 24, 2022
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Glucose is a six‑carbon sugar that can be ingested, infused, or produced endogenously through the metabolism of carbohydrates, proteins, and fats. It serves as the primary fuel for cellular respiration, enabling the synthesis of glycogen or the generation of ATP, the cell’s energy currency. The conversion of glucose into usable energy unfolds in four distinct, sequential phases.
Glycolysis takes place in the cytoplasm and is an anaerobic process. In eight enzymatic steps, one glucose molecule is split into two pyruvate molecules, consuming two ATP and generating four ATP for a net gain of two ATP. Additionally, two water molecules are produced. Glycolysis supplies the substrates for the next mitochondrial stages.
In the mitochondrial matrix, each pyruvate is converted to acetyl‑CoA. This single‑step reaction releases two CO₂ and attaches a coenzyme A, yielding two acetyl‑CoA molecules that will enter the citric acid cycle.
Still within the mitochondrial matrix, the two acetyl‑CoA molecules undergo a series of enzymatic reactions, producing two ATP (or GTP), four CO₂, and high‑energy carriers NADH and FADH₂. These carriers are essential for the subsequent electron transport chain.
On the inner mitochondrial membrane, electrons from NADH and FADH₂ flow through protein complexes, pumping protons into the intermembrane space. Oxygen acts as the final electron acceptor, forming water. The resulting proton gradient drives ATP synthase, generating approximately 34 ATP molecules per glucose. The overall reaction is:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP
While glycolysis and the citric acid cycle together produce four ATP per glucose, the electron transport chain accounts for the bulk of the yield—about 34 ATP. This explains why oxygen deprivation quickly halts cellular function and why sustained high‑intensity, anaerobic exercise is limited to short bursts.
